@node Low-Level Terminal Interface, Syslog, Sockets, Top @c %MENU% How to change the characteristics of a terminal device @chapter Low-Level Terminal Interface This chapter describes functions that are specific to terminal devices. You can use these functions to do things like turn off input echoing; set serial line characteristics such as line speed and flow control; and change which characters are used for end-of-file, command-line editing, sending signals, and similar control functions. Most of the functions in this chapter operate on file descriptors. @xref{Low-Level I/O}, for more information about what a file descriptor is and how to open a file descriptor for a terminal device. @menu * Is It a Terminal:: How to determine if a file is a terminal device, and what its name is. * I/O Queues:: About flow control and typeahead. * Canonical or Not:: Two basic styles of input processing. * Terminal Modes:: How to examine and modify flags controlling details of terminal I/O: echoing, signals, editing. Posix. * BSD Terminal Modes:: BSD compatible terminal mode setting * Line Control:: Sending break sequences, clearing terminal buffers @dots{} * Noncanon Example:: How to read single characters without echo. * Pseudo-Terminals:: How to open a pseudo-terminal. @end menu @node Is It a Terminal @section Identifying Terminals @cindex terminal identification @cindex identifying terminals The functions described in this chapter only work on files that correspond to terminal devices. You can find out whether a file descriptor is associated with a terminal by using the @code{isatty} function. @pindex unistd.h Prototypes for the functions in this section are declared in the header file @file{unistd.h}. @comment unistd.h @comment POSIX.1 @deftypefun int isatty (int @var{filedes}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c isatty ok @c tcgetattr dup ok This function returns @code{1} if @var{filedes} is a file descriptor associated with an open terminal device, and @math{0} otherwise. @end deftypefun If a file descriptor is associated with a terminal, you can get its associated file name using the @code{ttyname} function. See also the @code{ctermid} function, described in @ref{Identifying the Terminal}. @comment unistd.h @comment POSIX.1 @deftypefun {char *} ttyname (int @var{filedes}) @safety{@prelim{}@mtunsafe{@mtasurace{:ttyname}}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsfd{} @acsmem{}}} @c ttyname @mtasurace:ttyname @ascuheap @asulock @aculock @acsmem @acsfd @c isatty dup ok @c fstat dup ok @c memcpy dup ok @c getttyname @mtasurace:ttyname @ascuheap @asulock @aculock @acsmem @acsfd @c opendir @ascuheap @acsmem @acsfd @c readdir ok [protected by exclusive access] @c strcmp dup ok @c free dup @asulock @aculock @acsfd @acsmem @c malloc dup @asulock @aculock @acsfd @acsmem @c closedir @ascuheap @acsmem @acsfd @c mempcpy dup ok @c stat dup ok If the file descriptor @var{filedes} is associated with a terminal device, the @code{ttyname} function returns a pointer to a statically-allocated, null-terminated string containing the file name of the terminal file. The value is a null pointer if the file descriptor isn't associated with a terminal, or the file name cannot be determined. @end deftypefun @comment unistd.h @comment POSIX.1 @deftypefun int ttyname_r (int @var{filedes}, char *@var{buf}, size_t @var{len}) @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{} @acsfd{}}} @c ttyname_r @ascuheap @acsmem @acsfd @c isatty dup ok @c fstat dup ok @c memcpy dup ok @c getttyname_r @ascuheap @acsmem @acsfd @c opendir @ascuheap @acsmem @acsfd @c readdir ok [protected by exclusive access] @c strcmp dup ok @c closedir @ascuheap @acsmem @acsfd @c stpncpy dup ok @c stat dup ok The @code{ttyname_r} function is similar to the @code{ttyname} function except that it places its result into the user-specified buffer starting at @var{buf} with length @var{len}. The normal return value from @code{ttyname_r} is @math{0}. Otherwise an error number is returned to indicate the error. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{filedes} argument is not a valid file descriptor. @item ENOTTY The @var{filedes} is not associated with a terminal. @item ERANGE The buffer length @var{len} is too small to store the string to be returned. @end table @end deftypefun @node I/O Queues @section I/O Queues Many of the remaining functions in this section refer to the input and output queues of a terminal device. These queues implement a form of buffering @emph{within the kernel} independent of the buffering implemented by I/O streams (@pxref{I/O on Streams}). @cindex terminal input queue @cindex typeahead buffer The @dfn{terminal input queue} is also sometimes referred to as its @dfn{typeahead buffer}. It holds the characters that have been received from the terminal but not yet read by any process. The size of the input queue is described by the @code{MAX_INPUT} and @w{@code{_POSIX_MAX_INPUT}} parameters; see @ref{Limits for Files}. You are guaranteed a queue size of at least @code{MAX_INPUT}, but the queue might be larger, and might even dynamically change size. If input flow control is enabled by setting the @code{IXOFF} input mode bit (@pxref{Input Modes}), the terminal driver transmits STOP and START characters to the terminal when necessary to prevent the queue from overflowing. Otherwise, input may be lost if it comes in too fast from the terminal. In canonical mode, all input stays in the queue until a newline character is received, so the terminal input queue can fill up when you type a very long line. @xref{Canonical or Not}. @cindex terminal output queue The @dfn{terminal output queue} is like the input queue, but for output; it contains characters that have been written by processes, but not yet transmitted to the terminal. If output flow control is enabled by setting the @code{IXON} input mode bit (@pxref{Input Modes}), the terminal driver obeys START and STOP characters sent by the terminal to stop and restart transmission of output. @dfn{Clearing} the terminal input queue means discarding any characters that have been received but not yet read. Similarly, clearing the terminal output queue means discarding any characters that have been written but not yet transmitted. @node Canonical or Not @section Two Styles of Input: Canonical or Not POSIX systems support two basic modes of input: canonical and noncanonical. @cindex canonical input processing In @dfn{canonical input processing} mode, terminal input is processed in lines terminated by newline (@code{'\n'}), EOF, or EOL characters. No input can be read until an entire line has been typed by the user, and the @code{read} function (@pxref{I/O Primitives}) returns at most a single line of input, no matter how many bytes are requested. In canonical input mode, the operating system provides input editing facilities: some characters are interpreted specially to perform editing operations within the current line of text, such as ERASE and KILL. @xref{Editing Characters}. The constants @code{_POSIX_MAX_CANON} and @code{MAX_CANON} parameterize the maximum number of bytes which may appear in a single line of canonical input. @xref{Limits for Files}. You are guaranteed a maximum line length of at least @code{MAX_CANON} bytes, but the maximum might be larger, and might even dynamically change size. @cindex noncanonical input processing In @dfn{noncanonical input processing} mode, characters are not grouped into lines, and ERASE and KILL processing is not performed. The granularity with which bytes are read in noncanonical input mode is controlled by the MIN and TIME settings. @xref{Noncanonical Input}. Most programs use canonical input mode, because this gives the user a way to edit input line by line. The usual reason to use noncanonical mode is when the program accepts single-character commands or provides its own editing facilities. The choice of canonical or noncanonical input is controlled by the @code{ICANON} flag in the @code{c_lflag} member of @code{struct termios}. @xref{Local Modes}. @node Terminal Modes @section Terminal Modes @pindex termios.h This section describes the various terminal attributes that control how input and output are done. The functions, data structures, and symbolic constants are all declared in the header file @file{termios.h}. Don't confuse terminal attributes with file attributes. A device special file which is associated with a terminal has file attributes as described in @ref{File Attributes}. These are unrelated to the attributes of the terminal device itself, which are discussed in this section. @menu * Mode Data Types:: The data type @code{struct termios} and related types. * Mode Functions:: Functions to read and set the terminal attributes. * Setting Modes:: The right way to set terminal attributes reliably. * Input Modes:: Flags controlling low-level input handling. * Output Modes:: Flags controlling low-level output handling. * Control Modes:: Flags controlling serial port behavior. * Local Modes:: Flags controlling high-level input handling. * Line Speed:: How to read and set the terminal line speed. * Special Characters:: Characters that have special effects, and how to change them. * Noncanonical Input:: Controlling how long to wait for input. @end menu @node Mode Data Types @subsection Terminal Mode Data Types @cindex terminal mode data types The entire collection of attributes of a terminal is stored in a structure of type @code{struct termios}. This structure is used with the functions @code{tcgetattr} and @code{tcsetattr} to read and set the attributes. @comment termios.h @comment POSIX.1 @deftp {Data Type} {struct termios} Structure that records all the I/O attributes of a terminal. The structure includes at least the following members: @table @code @item tcflag_t c_iflag A bit mask specifying flags for input modes; see @ref{Input Modes}. @item tcflag_t c_oflag A bit mask specifying flags for output modes; see @ref{Output Modes}. @item tcflag_t c_cflag A bit mask specifying flags for control modes; see @ref{Control Modes}. @item tcflag_t c_lflag A bit mask specifying flags for local modes; see @ref{Local Modes}. @item cc_t c_cc[NCCS] An array specifying which characters are associated with various control functions; see @ref{Special Characters}. @end table The @code{struct termios} structure also contains members which encode input and output transmission speeds, but the representation is not specified. @xref{Line Speed}, for how to examine and store the speed values. @end deftp The following sections describe the details of the members of the @code{struct termios} structure. @comment termios.h @comment POSIX.1 @deftp {Data Type} tcflag_t This is an unsigned integer type used to represent the various bit masks for terminal flags. @end deftp @comment termios.h @comment POSIX.1 @deftp {Data Type} cc_t This is an unsigned integer type used to represent characters associated with various terminal control functions. @end deftp @comment termios.h @comment POSIX.1 @deftypevr Macro int NCCS The value of this macro is the number of elements in the @code{c_cc} array. @end deftypevr @node Mode Functions @subsection Terminal Mode Functions @cindex terminal mode functions @comment termios.h @comment POSIX.1 @deftypefun int tcgetattr (int @var{filedes}, struct termios *@var{termios-p}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c Converting the kernel-returned termios data structure to the userland @c format does not ensure atomic or consistent writing. This function is used to examine the attributes of the terminal device with file descriptor @var{filedes}. The attributes are returned in the structure that @var{termios-p} points to. If successful, @code{tcgetattr} returns @math{0}. A return value of @math{-1} indicates an error. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{filedes} argument is not a valid file descriptor. @item ENOTTY The @var{filedes} is not associated with a terminal. @end table @end deftypefun @comment termios.h @comment POSIX.1 @deftypefun int tcsetattr (int @var{filedes}, int @var{when}, const struct termios *@var{termios-p}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c Converting the incoming termios data structure to the kernel format @c does not ensure atomic or consistent reading. This function sets the attributes of the terminal device with file descriptor @var{filedes}. The new attributes are taken from the structure that @var{termios-p} points to. The @var{when} argument specifies how to deal with input and output already queued. It can be one of the following values: @table @code @comment termios.h @comment POSIX.1 @item TCSANOW @vindex TCSANOW Make the change immediately. @comment termios.h @comment POSIX.1 @item TCSADRAIN @vindex TCSADRAIN Make the change after waiting until all queued output has been written. You should usually use this option when changing parameters that affect output. @comment termios.h @comment POSIX.1 @item TCSAFLUSH @vindex TCSAFLUSH This is like @code{TCSADRAIN}, but also discards any queued input. @comment termios.h @comment BSD @item TCSASOFT @vindex TCSASOFT This is a flag bit that you can add to any of the above alternatives. Its meaning is to inhibit alteration of the state of the terminal hardware. It is a BSD extension; it is only supported on BSD systems and @gnuhurdsystems{}. Using @code{TCSASOFT} is exactly the same as setting the @code{CIGNORE} bit in the @code{c_cflag} member of the structure @var{termios-p} points to. @xref{Control Modes}, for a description of @code{CIGNORE}. @end table If this function is called from a background process on its controlling terminal, normally all processes in the process group are sent a @code{SIGTTOU} signal, in the same way as if the process were trying to write to the terminal. The exception is if the calling process itself is ignoring or blocking @code{SIGTTOU} signals, in which case the operation is performed and no signal is sent. @xref{Job Control}. If successful, @code{tcsetattr} returns @math{0}. A return value of @math{-1} indicates an error. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{filedes} argument is not a valid file descriptor. @item ENOTTY The @var{filedes} is not associated with a terminal. @item EINVAL Either the value of the @code{when} argument is not valid, or there is something wrong with the data in the @var{termios-p} argument. @end table @end deftypefun Although @code{tcgetattr} and @code{tcsetattr} specify the terminal device with a file descriptor, the attributes are those of the terminal device itself and not of the file descriptor. This means that the effects of changing terminal attributes are persistent; if another process opens the terminal file later on, it will see the changed attributes even though it doesn't have anything to do with the open file descriptor you originally specified in changing the attributes. Similarly, if a single process has multiple or duplicated file descriptors for the same terminal device, changing the terminal attributes affects input and output to all of these file descriptors. This means, for example, that you can't open one file descriptor or stream to read from a terminal in the normal line-buffered, echoed mode; and simultaneously have another file descriptor for the same terminal that you use to read from it in single-character, non-echoed mode. Instead, you have to explicitly switch the terminal back and forth between the two modes. @node Setting Modes @subsection Setting Terminal Modes Properly When you set terminal modes, you should call @code{tcgetattr} first to get the current modes of the particular terminal device, modify only those modes that you are really interested in, and store the result with @code{tcsetattr}. It's a bad idea to simply initialize a @code{struct termios} structure to a chosen set of attributes and pass it directly to @code{tcsetattr}. Your program may be run years from now, on systems that support members not documented in this manual. The way to avoid setting these members to unreasonable values is to avoid changing them. What's more, different terminal devices may require different mode settings in order to function properly. So you should avoid blindly copying attributes from one terminal device to another. When a member contains a collection of independent flags, as the @code{c_iflag}, @code{c_oflag} and @code{c_cflag} members do, even setting the entire member is a bad idea, because particular operating systems have their own flags. Instead, you should start with the current value of the member and alter only the flags whose values matter in your program, leaving any other flags unchanged. Here is an example of how to set one flag (@code{ISTRIP}) in the @code{struct termios} structure while properly preserving all the other data in the structure: @smallexample @group int set_istrip (int desc, int value) @{ struct termios settings; int result; @end group @group result = tcgetattr (desc, &settings); if (result < 0) @{ perror ("error in tcgetattr"); return 0; @} @end group @group settings.c_iflag &= ~ISTRIP; if (value) settings.c_iflag |= ISTRIP; @end group @group result = tcsetattr (desc, TCSANOW, &settings); if (result < 0) @{ perror ("error in tcsetattr"); return 0; @} return 1; @} @end group @end smallexample @node Input Modes @subsection Input Modes This section describes the terminal attribute flags that control fairly low-level aspects of input processing: handling of parity errors, break signals, flow control, and @key{RET} and @key{LFD} characters. All of these flags are bits in the @code{c_iflag} member of the @code{struct termios} structure. The member is an integer, and you change flags using the operators @code{&}, @code{|} and @code{^}. Don't try to specify the entire value for @code{c_iflag}---instead, change only specific flags and leave the rest untouched (@pxref{Setting Modes}). @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t INPCK @cindex parity checking If this bit is set, input parity checking is enabled. If it is not set, no checking at all is done for parity errors on input; the characters are simply passed through to the application. Parity checking on input processing is independent of whether parity detection and generation on the underlying terminal hardware is enabled; see @ref{Control Modes}. For example, you could clear the @code{INPCK} input mode flag and set the @code{PARENB} control mode flag to ignore parity errors on input, but still generate parity on output. If this bit is set, what happens when a parity error is detected depends on whether the @code{IGNPAR} or @code{PARMRK} bits are set. If neither of these bits are set, a byte with a parity error is passed to the application as a @code{'\0'} character. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t IGNPAR If this bit is set, any byte with a framing or parity error is ignored. This is only useful if @code{INPCK} is also set. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t PARMRK If this bit is set, input bytes with parity or framing errors are marked when passed to the program. This bit is meaningful only when @code{INPCK} is set and @code{IGNPAR} is not set. The way erroneous bytes are marked is with two preceding bytes, @code{377} and @code{0}. Thus, the program actually reads three bytes for one erroneous byte received from the terminal. If a valid byte has the value @code{0377}, and @code{ISTRIP} (see below) is not set, the program might confuse it with the prefix that marks a parity error. So a valid byte @code{0377} is passed to the program as two bytes, @code{0377} @code{0377}, in this case. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ISTRIP If this bit is set, valid input bytes are stripped to seven bits; otherwise, all eight bits are available for programs to read. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t IGNBRK If this bit is set, break conditions are ignored. @cindex break condition, detecting A @dfn{break condition} is defined in the context of asynchronous serial data transmission as a series of zero-value bits longer than a single byte. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t BRKINT If this bit is set and @code{IGNBRK} is not set, a break condition clears the terminal input and output queues and raises a @code{SIGINT} signal for the foreground process group associated with the terminal. If neither @code{BRKINT} nor @code{IGNBRK} are set, a break condition is passed to the application as a single @code{'\0'} character if @code{PARMRK} is not set, or otherwise as a three-character sequence @code{'\377'}, @code{'\0'}, @code{'\0'}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t IGNCR If this bit is set, carriage return characters (@code{'\r'}) are discarded on input. Discarding carriage return may be useful on terminals that send both carriage return and linefeed when you type the @key{RET} key. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ICRNL If this bit is set and @code{IGNCR} is not set, carriage return characters (@code{'\r'}) received as input are passed to the application as newline characters (@code{'\n'}). @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t INLCR If this bit is set, newline characters (@code{'\n'}) received as input are passed to the application as carriage return characters (@code{'\r'}). @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t IXOFF If this bit is set, start/stop control on input is enabled. In other words, the computer sends STOP and START characters as necessary to prevent input from coming in faster than programs are reading it. The idea is that the actual terminal hardware that is generating the input data responds to a STOP character by suspending transmission, and to a START character by resuming transmission. @xref{Start/Stop Characters}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t IXON If this bit is set, start/stop control on output is enabled. In other words, if the computer receives a STOP character, it suspends output until a START character is received. In this case, the STOP and START characters are never passed to the application program. If this bit is not set, then START and STOP can be read as ordinary characters. @xref{Start/Stop Characters}. @c !!! mention this interferes with using C-s and C-q for programs like emacs @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t IXANY If this bit is set, any input character restarts output when output has been suspended with the STOP character. Otherwise, only the START character restarts output. This is a BSD extension; it exists only on BSD systems and @gnulinuxhurdsystems{}. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t IMAXBEL If this bit is set, then filling up the terminal input buffer sends a BEL character (code @code{007}) to the terminal to ring the bell. This is a BSD extension. @end deftypevr @node Output Modes @subsection Output Modes This section describes the terminal flags and fields that control how output characters are translated and padded for display. All of these are contained in the @code{c_oflag} member of the @w{@code{struct termios}} structure. The @code{c_oflag} member itself is an integer, and you change the flags and fields using the operators @code{&}, @code{|}, and @code{^}. Don't try to specify the entire value for @code{c_oflag}---instead, change only specific flags and leave the rest untouched (@pxref{Setting Modes}). @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t OPOST If this bit is set, output data is processed in some unspecified way so that it is displayed appropriately on the terminal device. This typically includes mapping newline characters (@code{'\n'}) onto carriage return and linefeed pairs. If this bit isn't set, the characters are transmitted as-is. @end deftypevr The following three bits are effective only if @code{OPOST} is set. @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ONLCR If this bit is set, convert the newline character on output into a pair of characters, carriage return followed by linefeed. @end deftypevr @comment termios.h (optional) @comment BSD @deftypevr Macro tcflag_t OXTABS If this bit is set, convert tab characters on output into the appropriate number of spaces to emulate a tab stop every eight columns. This bit exists only on BSD systems and @gnuhurdsystems{}; on @gnulinuxsystems{} it is available as @code{XTABS}. @end deftypevr @comment termios.h (optional) @comment BSD @deftypevr Macro tcflag_t ONOEOT If this bit is set, discard @kbd{C-d} characters (code @code{004}) on output. These characters cause many dial-up terminals to disconnect. This bit exists only on BSD systems and @gnuhurdsystems{}. @end deftypevr @node Control Modes @subsection Control Modes This section describes the terminal flags and fields that control parameters usually associated with asynchronous serial data transmission. These flags may not make sense for other kinds of terminal ports (such as a network connection pseudo-terminal). All of these are contained in the @code{c_cflag} member of the @code{struct termios} structure. The @code{c_cflag} member itself is an integer, and you change the flags and fields using the operators @code{&}, @code{|}, and @code{^}. Don't try to specify the entire value for @code{c_cflag}---instead, change only specific flags and leave the rest untouched (@pxref{Setting Modes}). @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t CLOCAL If this bit is set, it indicates that the terminal is connected ``locally'' and that the modem status lines (such as carrier detect) should be ignored. @cindex modem status lines @cindex carrier detect On many systems if this bit is not set and you call @code{open} without the @code{O_NONBLOCK} flag set, @code{open} blocks until a modem connection is established. If this bit is not set and a modem disconnect is detected, a @code{SIGHUP} signal is sent to the controlling process group for the terminal (if it has one). Normally, this causes the process to exit; see @ref{Signal Handling}. Reading from the terminal after a disconnect causes an end-of-file condition, and writing causes an @code{EIO} error to be returned. The terminal device must be closed and reopened to clear the condition. @cindex modem disconnect @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t HUPCL If this bit is set, a modem disconnect is generated when all processes that have the terminal device open have either closed the file or exited. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t CREAD If this bit is set, input can be read from the terminal. Otherwise, input is discarded when it arrives. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t CSTOPB If this bit is set, two stop bits are used. Otherwise, only one stop bit is used. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t PARENB If this bit is set, generation and detection of a parity bit are enabled. @xref{Input Modes}, for information on how input parity errors are handled. If this bit is not set, no parity bit is added to output characters, and input characters are not checked for correct parity. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t PARODD This bit is only useful if @code{PARENB} is set. If @code{PARODD} is set, odd parity is used, otherwise even parity is used. @end deftypevr The control mode flags also includes a field for the number of bits per character. You can use the @code{CSIZE} macro as a mask to extract the value, like this: @code{settings.c_cflag & CSIZE}. @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t CSIZE This is a mask for the number of bits per character. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t CS5 This specifies five bits per byte. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t CS6 This specifies six bits per byte. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t CS7 This specifies seven bits per byte. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t CS8 This specifies eight bits per byte. @end deftypevr The following four bits are BSD extensions; these exist only on BSD systems and @gnuhurdsystems{}. @comment termios.h @comment BSD @deftypevr Macro tcflag_t CCTS_OFLOW If this bit is set, enable flow control of output based on the CTS wire (RS232 protocol). @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t CRTS_IFLOW If this bit is set, enable flow control of input based on the RTS wire (RS232 protocol). @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t MDMBUF If this bit is set, enable carrier-based flow control of output. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t CIGNORE If this bit is set, it says to ignore the control modes and line speed values entirely. This is only meaningful in a call to @code{tcsetattr}. The @code{c_cflag} member and the line speed values returned by @code{cfgetispeed} and @code{cfgetospeed} will be unaffected by the call. @code{CIGNORE} is useful if you want to set all the software modes in the other members, but leave the hardware details in @code{c_cflag} unchanged. (This is how the @code{TCSASOFT} flag to @code{tcsettattr} works.) This bit is never set in the structure filled in by @code{tcgetattr}. @end deftypevr @node Local Modes @subsection Local Modes This section describes the flags for the @code{c_lflag} member of the @code{struct termios} structure. These flags generally control higher-level aspects of input processing than the input modes flags described in @ref{Input Modes}, such as echoing, signals, and the choice of canonical or noncanonical input. The @code{c_lflag} member itself is an integer, and you change the flags and fields using the operators @code{&}, @code{|}, and @code{^}. Don't try to specify the entire value for @code{c_lflag}---instead, change only specific flags and leave the rest untouched (@pxref{Setting Modes}). @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ICANON This bit, if set, enables canonical input processing mode. Otherwise, input is processed in noncanonical mode. @xref{Canonical or Not}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ECHO If this bit is set, echoing of input characters back to the terminal is enabled. @cindex echo of terminal input @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ECHOE If this bit is set, echoing indicates erasure of input with the ERASE character by erasing the last character in the current line from the screen. Otherwise, the character erased is re-echoed to show what has happened (suitable for a printing terminal). This bit only controls the display behavior; the @code{ICANON} bit by itself controls actual recognition of the ERASE character and erasure of input, without which @code{ECHOE} is simply irrelevant. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t ECHOPRT This bit is like @code{ECHOE}, enables display of the ERASE character in a way that is geared to a hardcopy terminal. When you type the ERASE character, a @samp{\} character is printed followed by the first character erased. Typing the ERASE character again just prints the next character erased. Then, the next time you type a normal character, a @samp{/} character is printed before the character echoes. This is a BSD extension, and exists only in BSD systems and @gnulinuxhurdsystems{}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ECHOK This bit enables special display of the KILL character by moving to a new line after echoing the KILL character normally. The behavior of @code{ECHOKE} (below) is nicer to look at. If this bit is not set, the KILL character echoes just as it would if it were not the KILL character. Then it is up to the user to remember that the KILL character has erased the preceding input; there is no indication of this on the screen. This bit only controls the display behavior; the @code{ICANON} bit by itself controls actual recognition of the KILL character and erasure of input, without which @code{ECHOK} is simply irrelevant. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t ECHOKE This bit is similar to @code{ECHOK}. It enables special display of the KILL character by erasing on the screen the entire line that has been killed. This is a BSD extension, and exists only in BSD systems and @gnulinuxhurdsystems{}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ECHONL If this bit is set and the @code{ICANON} bit is also set, then the newline (@code{'\n'}) character is echoed even if the @code{ECHO} bit is not set. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t ECHOCTL If this bit is set and the @code{ECHO} bit is also set, echo control characters with @samp{^} followed by the corresponding text character. Thus, control-A echoes as @samp{^A}. This is usually the preferred mode for interactive input, because echoing a control character back to the terminal could have some undesired effect on the terminal. This is a BSD extension, and exists only in BSD systems and @gnulinuxhurdsystems{}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t ISIG This bit controls whether the INTR, QUIT, and SUSP characters are recognized. The functions associated with these characters are performed if and only if this bit is set. Being in canonical or noncanonical input mode has no affect on the interpretation of these characters. You should use caution when disabling recognition of these characters. Programs that cannot be interrupted interactively are very user-unfriendly. If you clear this bit, your program should provide some alternate interface that allows the user to interactively send the signals associated with these characters, or to escape from the program. @cindex interactive signals, from terminal @xref{Signal Characters}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t IEXTEN POSIX.1 gives @code{IEXTEN} implementation-defined meaning, so you cannot rely on this interpretation on all systems. On BSD systems and @gnulinuxhurdsystems{}, it enables the LNEXT and DISCARD characters. @xref{Other Special}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t NOFLSH Normally, the INTR, QUIT, and SUSP characters cause input and output queues for the terminal to be cleared. If this bit is set, the queues are not cleared. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro tcflag_t TOSTOP If this bit is set and the system supports job control, then @code{SIGTTOU} signals are generated by background processes that attempt to write to the terminal. @xref{Access to the Terminal}. @end deftypevr The following bits are BSD extensions; they exist only on BSD systems and @gnuhurdsystems{}. @comment termios.h @comment BSD @deftypevr Macro tcflag_t ALTWERASE This bit determines how far the WERASE character should erase. The WERASE character erases back to the beginning of a word; the question is, where do words begin? If this bit is clear, then the beginning of a word is a nonwhitespace character following a whitespace character. If the bit is set, then the beginning of a word is an alphanumeric character or underscore following a character which is none of those. @xref{Editing Characters}, for more information about the WERASE character. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t FLUSHO This is the bit that toggles when the user types the DISCARD character. While this bit is set, all output is discarded. @xref{Other Special}. @end deftypevr @comment termios.h (optional) @comment BSD @deftypevr Macro tcflag_t NOKERNINFO Setting this bit disables handling of the STATUS character. @xref{Other Special}. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro tcflag_t PENDIN If this bit is set, it indicates that there is a line of input that needs to be reprinted. Typing the REPRINT character sets this bit; the bit remains set until reprinting is finished. @xref{Editing Characters}. @end deftypevr @c EXTPROC is too obscure to document now. --roland @node Line Speed @subsection Line Speed @cindex line speed @cindex baud rate @cindex terminal line speed @cindex terminal line speed The terminal line speed tells the computer how fast to read and write data on the terminal. If the terminal is connected to a real serial line, the terminal speed you specify actually controls the line---if it doesn't match the terminal's own idea of the speed, communication does not work. Real serial ports accept only certain standard speeds. Also, particular hardware may not support even all the standard speeds. Specifying a speed of zero hangs up a dialup connection and turns off modem control signals. If the terminal is not a real serial line (for example, if it is a network connection), then the line speed won't really affect data transmission speed, but some programs will use it to determine the amount of padding needed. It's best to specify a line speed value that matches the actual speed of the actual terminal, but you can safely experiment with different values to vary the amount of padding. There are actually two line speeds for each terminal, one for input and one for output. You can set them independently, but most often terminals use the same speed for both directions. The speed values are stored in the @code{struct termios} structure, but don't try to access them in the @code{struct termios} structure directly. Instead, you should use the following functions to read and store them: @comment termios.h @comment POSIX.1 @deftypefun speed_t cfgetospeed (const struct termios *@var{termios-p}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c Direct access to a single termios field, except on Linux, where @c multiple accesses may take place. No worries either way, callers @c must ensure mutual exclusion on such non-opaque types. This function returns the output line speed stored in the structure @code{*@var{termios-p}}. @end deftypefun @comment termios.h @comment POSIX.1 @deftypefun speed_t cfgetispeed (const struct termios *@var{termios-p}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} This function returns the input line speed stored in the structure @code{*@var{termios-p}}. @end deftypefun @comment termios.h @comment POSIX.1 @deftypefun int cfsetospeed (struct termios *@var{termios-p}, speed_t @var{speed}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} This function stores @var{speed} in @code{*@var{termios-p}} as the output speed. The normal return value is @math{0}; a value of @math{-1} indicates an error. If @var{speed} is not a speed, @code{cfsetospeed} returns @math{-1}. @end deftypefun @comment termios.h @comment POSIX.1 @deftypefun int cfsetispeed (struct termios *@var{termios-p}, speed_t @var{speed}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} This function stores @var{speed} in @code{*@var{termios-p}} as the input speed. The normal return value is @math{0}; a value of @math{-1} indicates an error. If @var{speed} is not a speed, @code{cfsetospeed} returns @math{-1}. @end deftypefun @comment termios.h @comment BSD @deftypefun int cfsetspeed (struct termios *@var{termios-p}, speed_t @var{speed}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c There's no guarantee that the two calls are atomic, but since this is @c not an opaque type, callers ought to ensure mutual exclusion to the @c termios object. @c cfsetspeed ok @c cfsetispeed ok @c cfsetospeed ok This function stores @var{speed} in @code{*@var{termios-p}} as both the input and output speeds. The normal return value is @math{0}; a value of @math{-1} indicates an error. If @var{speed} is not a speed, @code{cfsetspeed} returns @math{-1}. This function is an extension in 4.4 BSD. @end deftypefun @comment termios.h @comment POSIX.1 @deftp {Data Type} speed_t The @code{speed_t} type is an unsigned integer data type used to represent line speeds. @end deftp The functions @code{cfsetospeed} and @code{cfsetispeed} report errors only for speed values that the system simply cannot handle. If you specify a speed value that is basically acceptable, then those functions will succeed. But they do not check that a particular hardware device can actually support the specified speeds---in fact, they don't know which device you plan to set the speed for. If you use @code{tcsetattr} to set the speed of a particular device to a value that it cannot handle, @code{tcsetattr} returns @math{-1}. @strong{Portability note:} In @theglibc{}, the functions above accept speeds measured in bits per second as input, and return speed values measured in bits per second. Other libraries require speeds to be indicated by special codes. For POSIX.1 portability, you must use one of the following symbols to represent the speed; their precise numeric values are system-dependent, but each name has a fixed meaning: @code{B110} stands for 110 bps, @code{B300} for 300 bps, and so on. There is no portable way to represent any speed but these, but these are the only speeds that typical serial lines can support. @comment termios.h @comment POSIX.1 @vindex B0 @comment termios.h @comment POSIX.1 @vindex B50 @comment termios.h @comment POSIX.1 @vindex B75 @comment termios.h @comment POSIX.1 @vindex B110 @comment termios.h @comment POSIX.1 @vindex B134 @comment termios.h @comment POSIX.1 @vindex B150 @comment termios.h @comment POSIX.1 @vindex B200 @comment termios.h @comment POSIX.1 @vindex B300 @comment termios.h @comment POSIX.1 @vindex B600 @comment termios.h @comment POSIX.1 @vindex B1200 @comment termios.h @comment POSIX.1 @vindex B1800 @comment termios.h @comment POSIX.1 @vindex B2400 @comment termios.h @comment POSIX.1 @vindex B4800 @comment termios.h @comment POSIX.1 @vindex B9600 @comment termios.h @comment POSIX.1 @vindex B19200 @comment termios.h @comment POSIX.1 @vindex B38400 @comment termios.h @comment GNU @vindex B57600 @comment termios.h @comment GNU @vindex B115200 @comment termios.h @comment GNU @vindex B230400 @comment termios.h @comment GNU @vindex B460800 @smallexample B0 B50 B75 B110 B134 B150 B200 B300 B600 B1200 B1800 B2400 B4800 B9600 B19200 B38400 B57600 B115200 B230400 B460800 @end smallexample @vindex EXTA @vindex EXTB BSD defines two additional speed symbols as aliases: @code{EXTA} is an alias for @code{B19200} and @code{EXTB} is an alias for @code{B38400}. These aliases are obsolete. @node Special Characters @subsection Special Characters In canonical input, the terminal driver recognizes a number of special characters which perform various control functions. These include the ERASE character (usually @key{DEL}) for editing input, and other editing characters. The INTR character (normally @kbd{C-c}) for sending a @code{SIGINT} signal, and other signal-raising characters, may be available in either canonical or noncanonical input mode. All these characters are described in this section. The particular characters used are specified in the @code{c_cc} member of the @code{struct termios} structure. This member is an array; each element specifies the character for a particular role. Each element has a symbolic constant that stands for the index of that element---for example, @code{VINTR} is the index of the element that specifies the INTR character, so storing @code{'='} in @code{@var{termios}.c_cc[VINTR]} specifies @samp{=} as the INTR character. @vindex _POSIX_VDISABLE On some systems, you can disable a particular special character function by specifying the value @code{_POSIX_VDISABLE} for that role. This value is unequal to any possible character code. @xref{Options for Files}, for more information about how to tell whether the operating system you are using supports @code{_POSIX_VDISABLE}. @menu * Editing Characters:: Special characters that terminate lines and delete text, and other editing functions. * Signal Characters:: Special characters that send or raise signals to or for certain classes of processes. * Start/Stop Characters:: Special characters that suspend or resume suspended output. * Other Special:: Other special characters for BSD systems: they can discard output, and print status. @end menu @node Editing Characters @subsubsection Characters for Input Editing These special characters are active only in canonical input mode. @xref{Canonical or Not}. @comment termios.h @comment POSIX.1 @deftypevr Macro int VEOF @cindex EOF character This is the subscript for the EOF character in the special control character array. @code{@var{termios}.c_cc[VEOF]} holds the character itself. The EOF character is recognized only in canonical input mode. It acts as a line terminator in the same way as a newline character, but if the EOF character is typed at the beginning of a line it causes @code{read} to return a byte count of zero, indicating end-of-file. The EOF character itself is discarded. Usually, the EOF character is @kbd{C-d}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro int VEOL @cindex EOL character This is the subscript for the EOL character in the special control character array. @code{@var{termios}.c_cc[VEOL]} holds the character itself. The EOL character is recognized only in canonical input mode. It acts as a line terminator, just like a newline character. The EOL character is not discarded; it is read as the last character in the input line. @c !!! example: this is set to ESC by 4.3 csh with "set filec" so it can @c complete partial lines without using cbreak or raw mode. You don't need to use the EOL character to make @key{RET} end a line. Just set the ICRNL flag. In fact, this is the default state of affairs. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro int VEOL2 @cindex EOL2 character This is the subscript for the EOL2 character in the special control character array. @code{@var{termios}.c_cc[VEOL2]} holds the character itself. The EOL2 character works just like the EOL character (see above), but it can be a different character. Thus, you can specify two characters to terminate an input line, by setting EOL to one of them and EOL2 to the other. The EOL2 character is a BSD extension; it exists only on BSD systems and @gnulinuxhurdsystems{}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro int VERASE @cindex ERASE character This is the subscript for the ERASE character in the special control character array. @code{@var{termios}.c_cc[VERASE]} holds the character itself. The ERASE character is recognized only in canonical input mode. When the user types the erase character, the previous character typed is discarded. (If the terminal generates multibyte character sequences, this may cause more than one byte of input to be discarded.) This cannot be used to erase past the beginning of the current line of text. The ERASE character itself is discarded. @c !!! mention ECHOE here Usually, the ERASE character is @key{DEL}. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro int VWERASE @cindex WERASE character This is the subscript for the WERASE character in the special control character array. @code{@var{termios}.c_cc[VWERASE]} holds the character itself. The WERASE character is recognized only in canonical mode. It erases an entire word of prior input, and any whitespace after it; whitespace characters before the word are not erased. The definition of a ``word'' depends on the setting of the @code{ALTWERASE} mode; @pxref{Local Modes}. If the @code{ALTWERASE} mode is not set, a word is defined as a sequence of any characters except space or tab. If the @code{ALTWERASE} mode is set, a word is defined as a sequence of characters containing only letters, numbers, and underscores, optionally followed by one character that is not a letter, number, or underscore. The WERASE character is usually @kbd{C-w}. This is a BSD extension. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro int VKILL @cindex KILL character This is the subscript for the KILL character in the special control character array. @code{@var{termios}.c_cc[VKILL]} holds the character itself. The KILL character is recognized only in canonical input mode. When the user types the kill character, the entire contents of the current line of input are discarded. The kill character itself is discarded too. The KILL character is usually @kbd{C-u}. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro int VREPRINT @cindex REPRINT character This is the subscript for the REPRINT character in the special control character array. @code{@var{termios}.c_cc[VREPRINT]} holds the character itself. The REPRINT character is recognized only in canonical mode. It reprints the current input line. If some asynchronous output has come while you are typing, this lets you see the line you are typing clearly again. The REPRINT character is usually @kbd{C-r}. This is a BSD extension. @end deftypevr @node Signal Characters @subsubsection Characters that Cause Signals These special characters may be active in either canonical or noncanonical input mode, but only when the @code{ISIG} flag is set (@pxref{Local Modes}). @comment termios.h @comment POSIX.1 @deftypevr Macro int VINTR @cindex INTR character @cindex interrupt character This is the subscript for the INTR character in the special control character array. @code{@var{termios}.c_cc[VINTR]} holds the character itself. The INTR (interrupt) character raises a @code{SIGINT} signal for all processes in the foreground job associated with the terminal. The INTR character itself is then discarded. @xref{Signal Handling}, for more information about signals. Typically, the INTR character is @kbd{C-c}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro int VQUIT @cindex QUIT character This is the subscript for the QUIT character in the special control character array. @code{@var{termios}.c_cc[VQUIT]} holds the character itself. The QUIT character raises a @code{SIGQUIT} signal for all processes in the foreground job associated with the terminal. The QUIT character itself is then discarded. @xref{Signal Handling}, for more information about signals. Typically, the QUIT character is @kbd{C-\}. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro int VSUSP @cindex SUSP character @cindex suspend character This is the subscript for the SUSP character in the special control character array. @code{@var{termios}.c_cc[VSUSP]} holds the character itself. The SUSP (suspend) character is recognized only if the implementation supports job control (@pxref{Job Control}). It causes a @code{SIGTSTP} signal to be sent to all processes in the foreground job associated with the terminal. The SUSP character itself is then discarded. @xref{Signal Handling}, for more information about signals. Typically, the SUSP character is @kbd{C-z}. @end deftypevr Few applications disable the normal interpretation of the SUSP character. If your program does this, it should provide some other mechanism for the user to stop the job. When the user invokes this mechanism, the program should send a @code{SIGTSTP} signal to the process group of the process, not just to the process itself. @xref{Signaling Another Process}. @comment termios.h @comment BSD @deftypevr Macro int VDSUSP @cindex DSUSP character @cindex delayed suspend character This is the subscript for the DSUSP character in the special control character array. @code{@var{termios}.c_cc[VDSUSP]} holds the character itself. The DSUSP (suspend) character is recognized only if the implementation supports job control (@pxref{Job Control}). It sends a @code{SIGTSTP} signal, like the SUSP character, but not right away---only when the program tries to read it as input. Not all systems with job control support DSUSP; only BSD-compatible systems (including @gnuhurdsystems{}). @xref{Signal Handling}, for more information about signals. Typically, the DSUSP character is @kbd{C-y}. @end deftypevr @node Start/Stop Characters @subsubsection Special Characters for Flow Control These special characters may be active in either canonical or noncanonical input mode, but their use is controlled by the flags @code{IXON} and @code{IXOFF} (@pxref{Input Modes}). @comment termios.h @comment POSIX.1 @deftypevr Macro int VSTART @cindex START character This is the subscript for the START character in the special control character array. @code{@var{termios}.c_cc[VSTART]} holds the character itself. The START character is used to support the @code{IXON} and @code{IXOFF} input modes. If @code{IXON} is set, receiving a START character resumes suspended output; the START character itself is discarded. If @code{IXANY} is set, receiving any character at all resumes suspended output; the resuming character is not discarded unless it is the START character. @code{IXOFF} is set, the system may also transmit START characters to the terminal. The usual value for the START character is @kbd{C-q}. You may not be able to change this value---the hardware may insist on using @kbd{C-q} regardless of what you specify. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro int VSTOP @cindex STOP character This is the subscript for the STOP character in the special control character array. @code{@var{termios}.c_cc[VSTOP]} holds the character itself. The STOP character is used to support the @code{IXON} and @code{IXOFF} input modes. If @code{IXON} is set, receiving a STOP character causes output to be suspended; the STOP character itself is discarded. If @code{IXOFF} is set, the system may also transmit STOP characters to the terminal, to prevent the input queue from overflowing. The usual value for the STOP character is @kbd{C-s}. You may not be able to change this value---the hardware may insist on using @kbd{C-s} regardless of what you specify. @end deftypevr @node Other Special @subsubsection Other Special Characters @comment termios.h @comment BSD @deftypevr Macro int VLNEXT @cindex LNEXT character This is the subscript for the LNEXT character in the special control character array. @code{@var{termios}.c_cc[VLNEXT]} holds the character itself. The LNEXT character is recognized only when @code{IEXTEN} is set, but in both canonical and noncanonical mode. It disables any special significance of the next character the user types. Even if the character would normally perform some editing function or generate a signal, it is read as a plain character. This is the analogue of the @kbd{C-q} command in Emacs. ``LNEXT'' stands for ``literal next.'' The LNEXT character is usually @kbd{C-v}. This character is available on BSD systems and @gnulinuxhurdsystems{}. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro int VDISCARD @cindex DISCARD character This is the subscript for the DISCARD character in the special control character array. @code{@var{termios}.c_cc[VDISCARD]} holds the character itself. The DISCARD character is recognized only when @code{IEXTEN} is set, but in both canonical and noncanonical mode. Its effect is to toggle the discard-output flag. When this flag is set, all program output is discarded. Setting the flag also discards all output currently in the output buffer. Typing any other character resets the flag. This character is available on BSD systems and @gnulinuxhurdsystems{}. @end deftypevr @comment termios.h @comment BSD @deftypevr Macro int VSTATUS @cindex STATUS character This is the subscript for the STATUS character in the special control character array. @code{@var{termios}.c_cc[VSTATUS]} holds the character itself. The STATUS character's effect is to print out a status message about how the current process is running. The STATUS character is recognized only in canonical mode, and only if @code{NOKERNINFO} is not set. This character is available only on BSD systems and @gnuhurdsystems{}. @end deftypevr @node Noncanonical Input @subsection Noncanonical Input In noncanonical input mode, the special editing characters such as ERASE and KILL are ignored. The system facilities for the user to edit input are disabled in noncanonical mode, so that all input characters (unless they are special for signal or flow-control purposes) are passed to the application program exactly as typed. It is up to the application program to give the user ways to edit the input, if appropriate. Noncanonical mode offers special parameters called MIN and TIME for controlling whether and how long to wait for input to be available. You can even use them to avoid ever waiting---to return immediately with whatever input is available, or with no input. The MIN and TIME are stored in elements of the @code{c_cc} array, which is a member of the @w{@code{struct termios}} structure. Each element of this array has a particular role, and each element has a symbolic constant that stands for the index of that element. @code{VMIN} and @code{VMAX} are the names for the indices in the array of the MIN and TIME slots. @comment termios.h @comment POSIX.1 @deftypevr Macro int VMIN @cindex MIN termios slot This is the subscript for the MIN slot in the @code{c_cc} array. Thus, @code{@var{termios}.c_cc[VMIN]} is the value itself. The MIN slot is only meaningful in noncanonical input mode; it specifies the minimum number of bytes that must be available in the input queue in order for @code{read} to return. @end deftypevr @comment termios.h @comment POSIX.1 @deftypevr Macro int VTIME @cindex TIME termios slot This is the subscript for the TIME slot in the @code{c_cc} array. Thus, @code{@var{termios}.c_cc[VTIME]} is the value itself. The TIME slot is only meaningful in noncanonical input mode; it specifies how long to wait for input before returning, in units of 0.1 seconds. @end deftypevr The MIN and TIME values interact to determine the criterion for when @code{read} should return; their precise meanings depend on which of them are nonzero. There are four possible cases: @itemize @bullet @item Both TIME and MIN are nonzero. In this case, TIME specifies how long to wait after each input character to see if more input arrives. After the first character received, @code{read} keeps waiting until either MIN bytes have arrived in all, or TIME elapses with no further input. @code{read} always blocks until the first character arrives, even if TIME elapses first. @code{read} can return more than MIN characters if more than MIN happen to be in the queue. @item Both MIN and TIME are zero. In this case, @code{read} always returns immediately with as many characters as are available in the queue, up to the number requested. If no input is immediately available, @code{read} returns a value of zero. @item MIN is zero but TIME has a nonzero value. In this case, @code{read} waits for time TIME for input to become available; the availability of a single byte is enough to satisfy the read request and cause @code{read} to return. When it returns, it returns as many characters as are available, up to the number requested. If no input is available before the timer expires, @code{read} returns a value of zero. @item TIME is zero but MIN has a nonzero value. In this case, @code{read} waits until at least MIN bytes are available in the queue. At that time, @code{read} returns as many characters as are available, up to the number requested. @code{read} can return more than MIN characters if more than MIN happen to be in the queue. @end itemize What happens if MIN is 50 and you ask to read just 10 bytes? Normally, @code{read} waits until there are 50 bytes in the buffer (or, more generally, the wait condition described above is satisfied), and then reads 10 of them, leaving the other 40 buffered in the operating system for a subsequent call to @code{read}. @strong{Portability note:} On some systems, the MIN and TIME slots are actually the same as the EOF and EOL slots. This causes no serious problem because the MIN and TIME slots are used only in noncanonical input and the EOF and EOL slots are used only in canonical input, but it isn't very clean. @Theglibc{} allocates separate slots for these uses. @comment termios.h @comment BSD @deftypefun void cfmakeraw (struct termios *@var{termios-p}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c There's no guarantee the changes are atomic, but since this is not an @c opaque type, callers ought to ensure mutual exclusion to the termios @c object. This function provides an easy way to set up @code{*@var{termios-p}} for what has traditionally been called ``raw mode'' in BSD. This uses noncanonical input, and turns off most processing to give an unmodified channel to the terminal. It does exactly this: @smallexample @var{termios-p}->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP |INLCR|IGNCR|ICRNL|IXON); @var{termios-p}->c_oflag &= ~OPOST; @var{termios-p}->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN); @var{termios-p}->c_cflag &= ~(CSIZE|PARENB); @var{termios-p}->c_cflag |= CS8; @end smallexample @end deftypefun @node BSD Terminal Modes @section BSD Terminal Modes @cindex terminal modes, BSD The usual way to get and set terminal modes is with the functions described in @ref{Terminal Modes}. However, on some systems you can use the BSD-derived functions in this section to do some of the same thing. On many systems, these functions do not exist. Even with @theglibc{}, the functions simply fail with @code{errno} = @code{ENOSYS} with many kernels, including Linux. The symbols used in this section are declared in @file{sgtty.h}. @comment termios.h @comment BSD @deftp {Data Type} {struct sgttyb} This structure is an input or output parameter list for @code{gtty} and @code{stty}. @table @code @item char sg_ispeed Line speed for input @item char sg_ospeed Line speed for output @item char sg_erase Erase character @item char sg_kill Kill character @item int sg_flags Various flags @end table @end deftp @comment sgtty.h @comment BSD @deftypefun int gtty (int @var{filedes}, struct sgttyb *@var{attributes}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c Direct ioctl, BSD only. This function gets the attributes of a terminal. @code{gtty} sets *@var{attributes} to describe the terminal attributes of the terminal which is open with file descriptor @var{filedes}. @end deftypefun @comment sgtty.h @comment BSD @deftypefun int stty (int @var{filedes}, const struct sgttyb *@var{attributes}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c Direct ioctl, BSD only. This function sets the attributes of a terminal. @code{stty} sets the terminal attributes of the terminal which is open with file descriptor @var{filedes} to those described by *@var{filedes}. @end deftypefun @node Line Control @section Line Control Functions @cindex terminal line control functions These functions perform miscellaneous control actions on terminal devices. As regards terminal access, they are treated like doing output: if any of these functions is used by a background process on its controlling terminal, normally all processes in the process group are sent a @code{SIGTTOU} signal. The exception is if the calling process itself is ignoring or blocking @code{SIGTTOU} signals, in which case the operation is performed and no signal is sent. @xref{Job Control}. @cindex break condition, generating @comment termios.h @comment POSIX.1 @deftypefun int tcsendbreak (int @var{filedes}, int @var{duration}) @safety{@prelim{}@mtunsafe{@mtasurace{:tcattr(filedes)/bsd}}@asunsafe{}@acunsafe{@acucorrupt{/bsd}}} @c On Linux, this calls just one out of two ioctls; on BSD, it's two @c ioctls with a select (for the delay only) in between, the first @c setting and the latter clearing the break status. The BSD @c implementation may leave the break enabled if cancelled, and threads @c and signals may cause the break to be interrupted before requested. This function generates a break condition by transmitting a stream of zero bits on the terminal associated with the file descriptor @var{filedes}. The duration of the break is controlled by the @var{duration} argument. If zero, the duration is between 0.25 and 0.5 seconds. The meaning of a nonzero value depends on the operating system. This function does nothing if the terminal is not an asynchronous serial data port. The return value is normally zero. In the event of an error, a value of @math{-1} is returned. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{filedes} is not a valid file descriptor. @item ENOTTY The @var{filedes} is not associated with a terminal device. @end table @end deftypefun @cindex flushing terminal output queue @cindex terminal output queue, flushing @comment termios.h @comment POSIX.1 @deftypefun int tcdrain (int @var{filedes}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c Direct ioctl. The @code{tcdrain} function waits until all queued output to the terminal @var{filedes} has been transmitted. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time @code{tcdrain} is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to @code{tcdrain} should be protected using cancellation handlers. @c ref pthread_cleanup_push / pthread_cleanup_pop The return value is normally zero. In the event of an error, a value of @math{-1} is returned. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{filedes} is not a valid file descriptor. @item ENOTTY The @var{filedes} is not associated with a terminal device. @item EINTR The operation was interrupted by delivery of a signal. @xref{Interrupted Primitives}. @end table @end deftypefun @cindex clearing terminal input queue @cindex terminal input queue, clearing @comment termios.h @comment POSIX.1 @deftypefun int tcflush (int @var{filedes}, int @var{queue}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c Direct ioctl. The @code{tcflush} function is used to clear the input and/or output queues associated with the terminal file @var{filedes}. The @var{queue} argument specifies which queue(s) to clear, and can be one of the following values: @c Extra blank lines here make it look better. @table @code @vindex TCIFLUSH @item TCIFLUSH Clear any input data received, but not yet read. @vindex TCOFLUSH @item TCOFLUSH Clear any output data written, but not yet transmitted. @vindex TCIOFLUSH @item TCIOFLUSH Clear both queued input and output. @end table The return value is normally zero. In the event of an error, a value of @math{-1} is returned. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{filedes} is not a valid file descriptor. @item ENOTTY The @var{filedes} is not associated with a terminal device. @item EINVAL A bad value was supplied as the @var{queue} argument. @end table It is unfortunate that this function is named @code{tcflush}, because the term ``flush'' is normally used for quite another operation---waiting until all output is transmitted---and using it for discarding input or output would be confusing. Unfortunately, the name @code{tcflush} comes from POSIX and we cannot change it. @end deftypefun @cindex flow control, terminal @cindex terminal flow control @comment termios.h @comment POSIX.1 @deftypefun int tcflow (int @var{filedes}, int @var{action}) @safety{@prelim{}@mtunsafe{@mtasurace{:tcattr(filedes)/bsd}}@asunsafe{}@acsafe{}} @c Direct ioctl on Linux. On BSD, the TCO* actions are a single ioctl, @c whereas the TCI actions first call tcgetattr and then write to the fd @c the c_cc character corresponding to the action; there's a window for @c another thread to change the xon/xoff characters. The @code{tcflow} function is used to perform operations relating to XON/XOFF flow control on the terminal file specified by @var{filedes}. The @var{action} argument specifies what operation to perform, and can be one of the following values: @table @code @vindex TCOOFF @item TCOOFF Suspend transmission of output. @vindex TCOON @item TCOON Restart transmission of output. @vindex TCIOFF @item TCIOFF Transmit a STOP character. @vindex TCION @item TCION Transmit a START character. @end table For more information about the STOP and START characters, see @ref{Special Characters}. The return value is normally zero. In the event of an error, a value of @math{-1} is returned. The following @code{errno} error conditions are defined for this function: @table @code @vindex EBADF @item EBADF The @var{filedes} is not a valid file descriptor. @vindex ENOTTY @item ENOTTY The @var{filedes} is not associated with a terminal device. @vindex EINVAL @item EINVAL A bad value was supplied as the @var{action} argument. @end table @end deftypefun @node Noncanon Example @section Noncanonical Mode Example Here is an example program that shows how you can set up a terminal device to read single characters in noncanonical input mode, without echo. @smallexample @include termios.c.texi @end smallexample This program is careful to restore the original terminal modes before exiting or terminating with a signal. It uses the @code{atexit} function (@pxref{Cleanups on Exit}) to make sure this is done by @code{exit}. @ignore @c !!!! the example doesn't handle any signals! The signals handled in the example are the ones that typically occur due to actions of the user. It might be desirable to handle other signals such as SIGSEGV that can result from bugs in the program. @end ignore The shell is supposed to take care of resetting the terminal modes when a process is stopped or continued; see @ref{Job Control}. But some existing shells do not actually do this, so you may wish to establish handlers for job control signals that reset terminal modes. The above example does so. @node Pseudo-Terminals @section Pseudo-Terminals @cindex pseudo-terminals A @dfn{pseudo-terminal} is a special interprocess communication channel that acts like a terminal. One end of the channel is called the @dfn{master} side or @dfn{master pseudo-terminal device}, the other side is called the @dfn{slave} side. Data written to the master side is received by the slave side as if it was the result of a user typing at an ordinary terminal, and data written to the slave side is sent to the master side as if it was written on an ordinary terminal. Pseudo terminals are the way programs like @code{xterm} and @code{emacs} implement their terminal emulation functionality. @menu * Allocation:: Allocating a pseudo terminal. * Pseudo-Terminal Pairs:: How to open both sides of a pseudo-terminal in a single operation. @end menu @node Allocation @subsection Allocating Pseudo-Terminals @cindex allocating pseudo-terminals @pindex stdlib.h This subsection describes functions for allocating a pseudo-terminal, and for making this pseudo-terminal available for actual use. These functions are declared in the header file @file{stdlib.h}. @comment stdlib.h @comment GNU @deftypefun int getpt (void) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{@acsfd{}}} @c On BSD, tries to open multiple potential pty names, returning on the @c first success. On Linux, try posix_openpt first, then fallback to @c the BSD implementation. The posix implementation opens the ptmx @c device, checks with statfs that /dev/pts is a devpts or that /dev is @c a devfs, and returns the fd; static variables devpts_mounted and @c have_no_dev_ptmx are safely initialized so as to avoid repeated @c tests. The @code{getpt} function returns a new file descriptor for the next available master pseudo-terminal. The normal return value from @code{getpt} is a non-negative integer file descriptor. In the case of an error, a value of @math{-1} is returned instead. The following @code{errno} conditions are defined for this function: @table @code @item ENOENT There are no free master pseudo-terminals available. @end table This function is a GNU extension. @end deftypefun @comment stdlib.h @comment SVID, XPG4.2 @deftypefun int grantpt (int @var{filedes}) @safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascudlopen{} @ascuplugin{} @ascuheap{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsfd{} @acsmem{}}} @c grantpt @mtslocale @ascudlopen @ascuplugin @ascuheap @asulock @acucorrupt @aculock @acsfd @acsmem @c unix/grantpt:pts_name @acsuheap @acsmem @c ptsname_internal dup ok (but this is Linux-only!) @c memchr dup ok @c realloc dup @acsuheap @acsmem @c malloc dup @acsuheap @acsmem @c free dup @acsuheap @acsmem @c fcntl dup ok @c getuid dup ok @c chown dup ok @c sysconf(_SC_GETGR_R_SIZE_MAX) ok @c getgrnam_r @mtslocale @ascudlopen @ascuplugin @ascuheap @asulock @acucorrupt @aculock @acsfd @acsmem @c getgid dup ok @c chmod dup ok @c fork dup @aculock @c [child] @c setrlimit @c dup2 @c CLOSE_ALL_FDS @c execle @c _exit @c waitpid dup ok @c WIFEXITED dup ok @c WEXITSTATUS dup ok @c free dup @ascuheap @acsmem The @code{grantpt} function changes the ownership and access permission of the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor @var{filedes}. The owner is set from the real user ID of the calling process (@pxref{Process Persona}), and the group is set to a special group (typically @dfn{tty}) or from the real group ID of the calling process. The access permission is set such that the file is both readable and writable by the owner and only writable by the group. On some systems this function is implemented by invoking a special @code{setuid} root program (@pxref{How Change Persona}). As a consequence, installing a signal handler for the @code{SIGCHLD} signal (@pxref{Job Control Signals}) may interfere with a call to @code{grantpt}. The normal return value from @code{grantpt} is @math{0}; a value of @math{-1} is returned in case of failure. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{filedes} argument is not a valid file descriptor. @item EINVAL The @var{filedes} argument is not associated with a master pseudo-terminal device. @item EACCES The slave pseudo-terminal device corresponding to the master associated with @var{filedes} could not be accessed. @end table @end deftypefun @comment stdlib.h @comment SVID, XPG4.2 @deftypefun int unlockpt (int @var{filedes}) @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{/bsd}}@acunsafe{@acsmem{} @acsfd{}}} @c unlockpt @ascuheap/bsd @acsmem @acsfd @c /bsd @c ptsname_r dup @ascuheap @acsmem @acsfd @c revoke ok (syscall) @c /linux @c ioctl dup ok The @code{unlockpt} function unlocks the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor @var{filedes}. On many systems, the slave can only be opened after unlocking, so portable applications should always call @code{unlockpt} before trying to open the slave. The normal return value from @code{unlockpt} is @math{0}; a value of @math{-1} is returned in case of failure. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{filedes} argument is not a valid file descriptor. @item EINVAL The @var{filedes} argument is not associated with a master pseudo-terminal device. @end table @end deftypefun @comment stdlib.h @comment SVID, XPG4.2 @deftypefun {char *} ptsname (int @var{filedes}) @safety{@prelim{}@mtunsafe{@mtasurace{:ptsname}}@asunsafe{@ascuheap{/bsd}}@acunsafe{@acsmem{} @acsfd{}}} @c ptsname @mtasurace:ptsname @ascuheap/bsd @acsmem @acsfd @c ptsname_r dup @ascuheap/bsd @acsmem @acsfd If the file descriptor @var{filedes} is associated with a master pseudo-terminal device, the @code{ptsname} function returns a pointer to a statically-allocated, null-terminated string containing the file name of the associated slave pseudo-terminal file. This string might be overwritten by subsequent calls to @code{ptsname}. @end deftypefun @comment stdlib.h @comment GNU @deftypefun int ptsname_r (int @var{filedes}, char *@var{buf}, size_t @var{len}) @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{/bsd}}@acunsafe{@acsmem{} @acsfd{}}} @c ptsname_r @ascuheap/bsd @acsmem @acsfd @c /hurd @c term_get_peername ok @c strlen dup ok @c memcpy dup ok @c /bsd @c isatty dup ok @c strlen dup ok @c ttyname_r dup @ascuheap @acsmem @acsfd @c stat dup ok @c /linux @c ptsname_internal ok @c isatty dup ok @c ioctl dup ok @c strlen dup ok @c itoa_word dup ok @c stpcpy dup ok @c memcpy dup ok @c fxstat64 dup ok @c MASTER_P ok @c major ok @c gnu_dev_major ok @c minor ok @c gnu_dev_minor ok @c minor dup ok @c xstat64 dup ok @c S_ISCHR dup ok @c SLAVE_P ok @c major dup ok @c minor dup ok The @code{ptsname_r} function is similar to the @code{ptsname} function except that it places its result into the user-specified buffer starting at @var{buf} with length @var{len}. This function is a GNU extension. @end deftypefun @strong{Portability Note:} On @w{System V} derived systems, the file returned by the @code{ptsname} and @code{ptsname_r} functions may be STREAMS-based, and therefore require additional processing after opening before it actually behaves as a pseudo terminal. @c FIXME: xref STREAMS Typical usage of these functions is illustrated by the following example: @smallexample int open_pty_pair (int *amaster, int *aslave) @{ int master, slave; char *name; master = getpt (); if (master < 0) return 0; if (grantpt (master) < 0 || unlockpt (master) < 0) goto close_master; name = ptsname (master); if (name == NULL) goto close_master; slave = open (name, O_RDWR); if (slave == -1) goto close_master; if (isastream (slave)) @{ if (ioctl (slave, I_PUSH, "ptem") < 0 || ioctl (slave, I_PUSH, "ldterm") < 0) goto close_slave; @} *amaster = master; *aslave = slave; return 1; close_slave: close (slave); close_master: close (master); return 0; @} @end smallexample @node Pseudo-Terminal Pairs @subsection Opening a Pseudo-Terminal Pair @cindex opening a pseudo-terminal pair These functions, derived from BSD, are available in the separate @file{libutil} library, and declared in @file{pty.h}. @comment pty.h @comment BSD @deftypefun int openpty (int *@var{amaster}, int *@var{aslave}, char *@var{name}, const struct termios *@var{termp}, const struct winsize *@var{winp}) @safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascudlopen{} @ascuplugin{} @ascuheap{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsfd{} @acsmem{}}} @c openpty @mtslocale @ascudlopen @ascuplugin @ascuheap @asulock @acucorrupt @aculock @acsfd @acsmem @c getpt @acsfd @c grantpt @mtslocale @ascudlopen @ascuplugin @ascuheap @asulock @acucorrupt @aculock @acsfd @acsmem @c unlockpt dup @ascuheap/bsd @acsmem @acsfd @c openpty:pts_name @acsuheap @acsmem @acsfd @c ptsname_r dup @ascuheap/bsd @acsmem @acsfd @c realloc dup @acsuheap @acsmem @c malloc dup @acsuheap @acsmem @c free dup @acsuheap @acsmem @c open dup @acsfd @c free dup @acsuheap @acsmem @c tcsetattr dup ok @c ioctl dup ok @c strcpy dup ok @c close dup @acsfd This function allocates and opens a pseudo-terminal pair, returning the file descriptor for the master in @var{*amaster}, and the file descriptor for the slave in @var{*aslave}. If the argument @var{name} is not a null pointer, the file name of the slave pseudo-terminal device is stored in @code{*name}. If @var{termp} is not a null pointer, the terminal attributes of the slave are set to the ones specified in the structure that @var{termp} points to (@pxref{Terminal Modes}). Likewise, if the @var{winp} is not a null pointer, the screen size of the slave is set to the values specified in the structure that @var{winp} points to. The normal return value from @code{openpty} is @math{0}; a value of @math{-1} is returned in case of failure. The following @code{errno} conditions are defined for this function: @table @code @item ENOENT There are no free pseudo-terminal pairs available. @end table @strong{Warning:} Using the @code{openpty} function with @var{name} not set to @code{NULL} is @strong{very dangerous} because it provides no protection against overflowing the string @var{name}. You should use the @code{ttyname} function on the file descriptor returned in @var{*slave} to find out the file name of the slave pseudo-terminal device instead. @end deftypefun @comment pty.h @comment BSD @deftypefun int forkpty (int *@var{amaster}, char *@var{name}, const struct termios *@var{termp}, const struct winsize *@var{winp}) @safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascudlopen{} @ascuplugin{} @ascuheap{} @asulock{}}@acunsafe{@acucorrupt{} @aculock{} @acsfd{} @acsmem{}}} @c forkpty @mtslocale @ascudlopen @ascuplugin @ascuheap @asulock @acucorrupt @aculock @acsfd @acsmem @c openpty dup @mtslocale @ascudlopen @ascuplugin @ascuheap @asulock @acucorrupt @aculock @acsfd @acsmem @c fork dup @aculock @c close dup @acsfd @c /child @c close dup @acsfd @c login_tty dup @mtasurace:ttyname @ascuheap @asulock @aculock @acsmem @acsfd @c _exit dup ok @c close dup @acsfd This function is similar to the @code{openpty} function, but in addition, forks a new process (@pxref{Creating a Process}) and makes the newly opened slave pseudo-terminal device the controlling terminal (@pxref{Controlling Terminal}) for the child process. If the operation is successful, there are then both parent and child processes and both see @code{forkpty} return, but with different values: it returns a value of @math{0} in the child process and returns the child's process ID in the parent process. If the allocation of a pseudo-terminal pair or the process creation failed, @code{forkpty} returns a value of @math{-1} in the parent process. @strong{Warning:} The @code{forkpty} function has the same problems with respect to the @var{name} argument as @code{openpty}. @end deftypefun