path: root/manual/errno.texi

@node Error Reporting, Memory Allocation, Introduction, Top
@chapter Error Reporting
@cindex error reporting
@cindex reporting errors
@cindex error codes
@cindex status codes

Many functions in the GNU C library detect and report error conditions,
and sometimes your programs need to check for these error conditions.
For example, when you open an input file, you should verify that the
file was actually opened correctly, and print an error message or take
other appropriate action if the call to the library function failed.

This chapter describes how the error reporting facility works.  Your
program should include the header file @file{errno.h} to use this
facility.
@pindex errno.h

@menu
* Checking for Errors::         How errors are reported by library functions.
* Error Codes::                 Error code macros; all of these expand 
                                 into integer constant values.
* Error Messages::              Mapping error codes onto error messages.
@end menu

@node Checking for Errors, Error Codes,  , Error Reporting
@section Checking for Errors

Most library functions return a special value to indicate that they have
failed.  The special value is typically @code{-1}, a null pointer, or a
constant such as @code{EOF} that is defined for that purpose.  But this
return value tells you only that an error has occurred.  To find out
what kind of error it was, you need to look at the error code stored in the
variable @code{errno}.  This variable is declared in the header file
@file{errno.h}.
@pindex errno.h

@comment errno.h
@comment ANSI
@deftypevr {Variable} {volatile int} errno
The variable @code{errno} contains the system error number.  You can
change the value of @code{errno}.

Since @code{errno} is declared @code{volatile}, it might be changed
asynchronously by a signal handler; see @ref{Defining Handlers}.
However, a properly written signal handler saves and restores the value
of @code{errno}, so you generally do not need to worry about this
possibility except when writing signal handlers.

The initial value of @code{errno} at program startup is zero.  Many
library functions are guaranteed to set it to certain nonzero values
when they encounter certain kinds of errors.  These error conditions are
listed for each function.  These functions do not change @code{errno}
when they succeed; thus, the value of @code{errno} after a successful
call is not necessarily zero, and you should not use @code{errno} to
determine @emph{whether} a call failed.  The proper way to do that is
documented for each function.  @emph{If} the call the failed, you can
examine @code{errno}.

Many library functions can set @code{errno} to a nonzero value as a
result of calling other library functions which might fail.  You should
assume that any library function might alter @code{errno} when the
function returns an error.

@strong{Portability Note:} ANSI C specifies @code{errno} as a
``modifiable lvalue'' rather than as a variable, permitting it to be
implemented as a macro.  For example, its expansion might involve a
function call, like @code{*_errno ()}.  In fact, that is what it is
on the GNU system itself.  The GNU library, on non-GNU systems, does
whatever is right for the particular system.

There are a few library functions, like @code{sqrt} and @code{atan},
that return a perfectly legitimate value in case of an error, but also
set @code{errno}.  For these functions, if you want to check to see
whether an error occurred, the recommended method is to set @code{errno}
to zero before calling the function, and then check its value afterward.
@end deftypevr

@pindex errno.h
All the error codes have symbolic names; they are macros defined in
@file{errno.h}.  The names start with @samp{E} and an upper-case
letter or digit; you should consider names of this form to be
reserved names.  @xref{Reserved Names}.

The error code values are all positive integers and are all distinct,
with one exception: @code{EWOULDBLOCK} and @code{EAGAIN} are the same.
Since the values are distinct, you can use them as labels in a
@code{switch} statement; just don't use both @code{EWOULDBLOCK} and
@code{EAGAIN}.  Your program should not make any other assumptions about
the specific values of these symbolic constants.

The value of @code{errno} doesn't necessarily have to correspond to any
of these macros, since some library functions might return other error
codes of their own for other situations.  The only values that are
guaranteed to be meaningful for a particular library function are the
ones that this manual lists for that function.

On non-GNU systems, almost any system call can return @code{EFAULT} if
it is given an invalid pointer as an argument.  Since this could only
happen as a result of a bug in your program, and since it will not
happen on the GNU system, we have saved space by not mentioning
@code{EFAULT} in the descriptions of individual functions.

@node Error Codes, Error Messages, Checking for Errors, Error Reporting
@section Error Codes

@pindex errno.h
The error code macros are defined in the header file @file{errno.h}.
All of them expand into integer constant values.  Some of these error
codes can't occur on the GNU system, but they can occur using the GNU
library on other systems.

@comment errno.h
@comment POSIX.1: Operation not permitted
@deftypevr Macro int EPERM
Operation not permitted; only the owner of the file (or other resource)
or processes with special privileges can perform the operation.
@end deftypevr

@comment errno.h
@comment POSIX.1: No such file or directory
@deftypevr Macro int ENOENT
No such file or directory.  This is a ``file doesn't exist'' error
for ordinary files that are referenced in contexts where they are
expected to already exist.
@end deftypevr

@comment errno.h
@comment POSIX.1: No such process
@deftypevr Macro int ESRCH
No process matches the specified process ID.
@end deftypevr

@comment errno.h
@comment POSIX.1: Interrupted system call
@deftypevr Macro int EINTR
Interrupted function call; an asynchronous signal occured and prevented
completion of the call.  When this happens, you should try the call
again.

You can choose to have functions resume after a signal that is handled,
rather than failing with @code{EINTR}; see @ref{Interrupted
Primitives}.
@end deftypevr

@comment errno.h
@comment POSIX.1: Input/output error
@deftypevr Macro int EIO
Input/output error; usually used for physical read or write errors.
@end deftypevr

@comment errno.h
@comment POSIX.1: Device not configured
@deftypevr Macro int ENXIO
No such device or address.  The system tried to use the device
represented by a file you specified, and it couldn't find the device.
This can mean that the device file was installed incorrectly, or that
the physical device is missing or not correctly attached to the
computer.
@end deftypevr

@comment errno.h
@comment POSIX.1: Argument list too long
@deftypevr Macro int E2BIG
Argument list too long; used when the arguments passed to a new program
being executed with one of the @code{exec} functions (@pxref{Executing a
File}) occupy too much memory space.  This condition never arises in the
GNU system.
@end deftypevr

@comment errno.h
@comment POSIX.1: Exec format error
@deftypevr Macro int ENOEXEC
Invalid executable file format.  This condition is detected by the
@code{exec} functions; see @ref{Executing a File}.
@end deftypevr

@comment errno.h
@comment POSIX.1: Bad file descriptor
@deftypevr Macro int EBADF
Bad file descriptor; for example, I/O on a descriptor that has been
closed or reading from a descriptor open only for writing (or vice
versa).
@end deftypevr

@comment errno.h
@comment POSIX.1: No child processes
@deftypevr Macro int ECHILD
There are no child processes.  This error happens on operations that are
supposed to manipulate child processes, when there aren't any processes
to manipulate.
@end deftypevr

@comment errno.h
@comment POSIX.1: Resource deadlock avoided
@deftypevr Macro int EDEADLK
Deadlock avoided; allocating a system resource would have resulted in a
deadlock situation.  The system does not guarantee that it will notice
all such situations.  This error means you got lucky and the system
noticed; it might just hang.  @xref{File Locks}, for an example.
@end deftypevr

@comment errno.h
@comment POSIX.1: Cannot allocate memory
@deftypevr Macro int ENOMEM
No memory available.  The system cannot allocate more virtual memory
because its capacity is full.
@end deftypevr

@comment errno.h
@comment POSIX.1: Permission denied
@deftypevr Macro int EACCES
Permission denied; the file permissions do not allow the attempted operation.
@end deftypevr

@comment errno.h
@comment POSIX.1: Bad address
@deftypevr Macro int EFAULT
Bad address; an invalid pointer was detected.
@end deftypevr

@comment errno.h
@comment BSD: Block device required
@deftypevr Macro int ENOTBLK
A file that isn't a block special file was given in a situation that
requires one.  For example, trying to mount an ordinary file as a file
system in Unix gives this error.
@end deftypevr

@comment errno.h
@comment POSIX.1: Device busy
@deftypevr Macro int EBUSY
Resource busy; a system resource that can't be shared is already in use.
For example, if you try to delete a file that is the root of a currently
mounted filesystem, you get this error.
@end deftypevr

@comment errno.h
@comment POSIX.1: File exists
@deftypevr Macro int EEXIST
File exists; an existing file was specified in a context where it only
makes sense to specify a new file.
@end deftypevr

@comment errno.h
@comment POSIX.1: Invalid cross-device link
@deftypevr Macro int EXDEV
An attempt to make an improper link across file systems was detected.
This happens not only when you use @code{link} (@pxref{Hard Links}) but
also when you rename a file with @code{rename} (@pxref{Renaming Files}).
@end deftypevr

@comment errno.h
@comment POSIX.1: Operation not supported by device
@deftypevr Macro int ENODEV
The wrong type of device was given to a function that expects a
particular sort of device.
@end deftypevr

@comment errno.h
@comment POSIX.1: Not a directory
@deftypevr Macro int ENOTDIR
A file that isn't a directory was specified when a directory is required.
@end deftypevr

@comment errno.h
@comment POSIX.1: Is a directory
@deftypevr Macro int EISDIR
File is a directory; attempting to open a directory for writing gives
this error.
@end deftypevr

@comment errno.h
@comment POSIX.1: Invalid argument
@deftypevr Macro int EINVAL
Invalid argument.  This is used to indicate various kinds of problems
with passing the wrong argument to a library function.
@end deftypevr

@comment errno.h
@comment POSIX.1: Too many open files
@deftypevr Macro int EMFILE
The current process has too many files open and can't open any more.
Duplicate descriptors do count toward this limit.
@c !!! In 4.4BSD and GNU, the number of open files is a resource limit
@c set with setrlimit.
@end deftypevr

@comment errno.h
@comment POSIX.1: Too many open files in system
@deftypevr Macro int ENFILE
There are too many distinct file openings in the entire system.  Note
that any number of linked channels count as just one file opening; see
@ref{Linked Channels}.  This error never occurs in the GNU system.
@end deftypevr

@comment errno.h
@comment POSIX.1: Inappropriate ioctl for device
@deftypevr Macro int ENOTTY
Inappropriate I/O control operation, such as trying to set terminal
modes on an ordinary file.
@end deftypevr

@comment errno.h
@comment BSD: Text file busy
@deftypevr Macro int ETXTBSY
An attempt to execute a file that is currently open for writing, or
write to a file that is currently being executed.  (The name stands
for ``text file busy''.)  This is not an error in the GNU system; the
text is copied as necessary.
@end deftypevr

@comment errno.h
@comment POSIX.1: File too large
@deftypevr Macro int EFBIG
File too big; the size of a file would be larger than allowed by the system.
@end deftypevr

@comment errno.h
@comment POSIX.1: No space left on device
@deftypevr Macro int ENOSPC
No space left on device; write operation on a file failed because the
disk is full.
@end deftypevr

@comment errno.h
@comment POSIX.1: Illegal seek
@deftypevr Macro int ESPIPE
Invalid seek operation (such as on a pipe).
@end deftypevr

@comment errno.h
@comment POSIX.1: Read-only file system
@deftypevr Macro int EROFS
An attempt was made to modify something on a read-only file system.
@end deftypevr

@comment errno.h
@comment POSIX.1: Too many links
@deftypevr Macro int EMLINK
Too many links; the link count of a single file is too large.
@code{rename} can cause this error if the file being renamed already has
as many links as it can take (@pxref{Renaming Files}).
@end deftypevr

@comment errno.h
@comment POSIX.1: Broken pipe
@deftypevr Macro int EPIPE
Broken pipe; there is no process reading from the other end of a pipe.
Every library function that returns this error code also generates a
@code{SIGPIPE} signal; this signal terminates the program if not handled
or blocked.  Thus, your program will never actually see @code{EPIPE}
unless it has handled or blocked @code{SIGPIPE}.
@end deftypevr

@comment errno.h
@comment ANSI: Numerical argument out of domain
@deftypevr Macro int EDOM
Domain error; used by mathematical functions when an argument value does
not fall into the domain over which the function is defined.
@end deftypevr

@comment errno.h
@comment ANSI: Numerical result out of range
@deftypevr Macro int ERANGE
Range error; used by mathematical functions when the result value is
not representable because of overflow or underflow.
@end deftypevr

@comment errno.h
@comment POSIX.1: Resource temporarily unavailable
@deftypevr Macro int EAGAIN
Resource temporarily unavailable; the call might work if you try again
later.  Only @code{fork} returns error code @code{EAGAIN} for such a
reason.
@c !!! sysv uses it somehow?  Don't say "only fork" when ==EWOULDBLOCK.
@end deftypevr

@comment errno.h
@comment BSD: Operation would block
@deftypevr Macro int EWOULDBLOCK
An operation that would block was attempted on an object that has
non-blocking mode selected.

@strong{Portability Note:} In 4.4BSD and GNU, @code{EWOULDBLOCK} and
@code{EAGAIN} are the same.  Earlier versions of BSD (@pxref{Berkeley
Unix}) have two distinct codes, and use @code{EWOULDBLOCK} to indicate
an I/O operation that would block on an object with non-blocking mode
set, and @code{EAGAIN} for other kinds of errors.@refill
@end deftypevr

@comment errno.h
@comment BSD: Operation now in progress
@deftypevr Macro int EINPROGRESS
An operation that cannot complete immediately was initiated on an object
that has non-blocking mode selected.  Some functions that must always
block (such as @code{connect}; @pxref{Connecting}) never return
@code{EWOULDBLOCK}.
@c !!! EAGAIN?
Instead, they return @code{EINPROGRESS} to indicate that the operation
has begun and will take some time.  Attempts to manipulate the object
before the call completes return @code{EALREADY}.
@end deftypevr

@comment errno.h
@comment BSD: Operation already in progress
@deftypevr Macro int EALREADY
An operation is already in progress on an object that has non-blocking
mode selected.
@end deftypevr

@comment errno.h
@comment BSD: Socket operation on non-socket
@deftypevr Macro int ENOTSOCK
A file that isn't a socket was specified when a socket is required.
@end deftypevr

@comment errno.h
@comment BSD: Destination address required
@deftypevr Macro int EDESTADDRREQ
No destination address was supplied on a socket operation that needed one.
@end deftypevr

@comment errno.h
@comment BSD: Message too long
@deftypevr Macro int EMSGSIZE
The size of a message sent on a socket was larger than the supported
maximum size.  
@end deftypevr

@comment errno.h
@comment BSD: Protocol wrong type for socket
@deftypevr Macro int EPROTOTYPE
The socket type does not support the requested communications protocol.
@end deftypevr

@comment errno.h
@comment BSD: Protocol not available
@deftypevr Macro int ENOPROTOOPT
You specified a socket option that doesn't make sense for the
particular protocol being used by the socket.  @xref{Socket Options}.
@end deftypevr

@comment errno.h
@comment BSD: Protocol not supported
@deftypevr Macro int EPROTONOSUPPORT
The socket domain does not support the requested communications protocol
(perhaps because the requested protocol is completely invalid.)
@xref{Creating a Socket}.
@end deftypevr

@comment errno.h
@comment BSD: Socket type not supported
@deftypevr Macro int ESOCKTNOSUPPORT
The socket type is not supported.
@end deftypevr

@comment errno.h
@comment BSD: Operation not supported
@deftypevr Macro int EOPNOTSUPP
The operation you requested is not supported.  Some socket functions
don't make sense for all types of sockets, and others may not be implemented
for all communications protocols.
@end deftypevr

@comment errno.h
@comment BSD: Protocol family not supported
@deftypevr Macro int EPFNOSUPPORT
The socket communications protocol family you requested is not supported.
@end deftypevr

@comment errno.h
@comment BSD: Address family not supported by protocol family
@deftypevr Macro int EAFNOSUPPORT
The address family specified for a socket is not supported; it is
inconsistent with the protocol being used on the socket.  @xref{Sockets}.
@end deftypevr

@comment errno.h
@comment BSD: Address already in use
@deftypevr Macro int EADDRINUSE
The requested socket address is already in use.  @xref{Socket Addresses}.
@end deftypevr

@comment errno.h
@comment BSD: Can't assign requested address
@deftypevr Macro int EADDRNOTAVAIL
The requested socket address is not available; for example, you tried
to give a socket a name that doesn't match the local host name.
@xref{Socket Addresses}.
@end deftypevr

@comment errno.h
@comment BSD: Network is down
@deftypevr Macro int ENETDOWN
A socket operation failed because the network was down.
@end deftypevr

@comment errno.h
@comment BSD: Network is unreachable
@deftypevr Macro int ENETUNREACH
A socket operation failed because the subnet containing the remost host
was unreachable.
@end deftypevr

@comment errno.h
@comment BSD: Network dropped connection on reset
@deftypevr Macro int ENETRESET
A network connection was reset because the remote host crashed.
@end deftypevr

@comment errno.h
@comment BSD: Software caused connection abort
@deftypevr Macro int ECONNABORTED
A network connection was aborted locally.
@end deftypevr

@comment errno.h
@comment BSD: Connection reset by peer
@deftypevr Macro int ECONNRESET
A network connection was closed for reasons outside the control of the
local host, such as by the remote machine rebooting or an unrecoverable
protocol violation.
@end deftypevr

@comment errno.h
@comment BSD: No buffer space available
@deftypevr Macro int ENOBUFS
The kernel's buffers for I/O operations are all in use.
@c !!! this will probably never happen in GNU (I'm presuming the
@c eventual implementation of the network won't want to use it); you get
@c ENOMEM instead.
@c ??? I think the network code should convert ENOMEM into ENOBUFS
@c ??? just to be compatible--rms.
@end deftypevr

@comment errno.h
@comment BSD: Socket is already connected
@deftypevr Macro int EISCONN
You tried to connect a socket that is already connected.
@xref{Connecting}.
@end deftypevr

@comment errno.h
@comment BSD: Socket is not connected
@deftypevr Macro int ENOTCONN
The socket is not connected to anything.  You get this error when you
try to transmit data over a socket, without first specifying a destination
for the data.
@end deftypevr

@comment errno.h
@comment BSD: Can't send after socket shutdown
@deftypevr Macro int ESHUTDOWN
The socket has already been shut down.
@end deftypevr

@comment errno.h
@comment BSD: Connection timed out
@deftypevr Macro int ETIMEDOUT
A socket operation with a specified timeout received no response during
the timeout period.
@end deftypevr

@comment errno.h
@comment BSD: Connection refused
@deftypevr Macro int ECONNREFUSED
A remote host refused to allow the network connection (typically because
it is not running the requested service).
@end deftypevr

@comment errno.h
@comment BSD: Too many levels of symbolic links
@deftypevr Macro int ELOOP
Too many levels of symbolic links were encountered in looking up a file name.
This often indicates a cycle of symbolic links.
@end deftypevr

@comment errno.h
@comment POSIX.1: File name too long
@deftypevr Macro int ENAMETOOLONG
Filename too long (longer than @code{PATH_MAX}; @pxref{Limits for
Files}) or host name too long (in @code{gethostname} or
@code{sethostname}; @pxref{Host Identification}).
@end deftypevr

@comment errno.h
@comment BSD: Host is down
@deftypevr Macro int EHOSTDOWN
The remote host for a requested network connection is down.
@end deftypevr

@comment errno.h
@comment BSD: No route to host
@deftypevr Macro int EHOSTUNREACH
The remote host for a requested network connection is not reachable.
@end deftypevr

@comment errno.h
@comment POSIX.1: Directory not empty
@deftypevr Macro int ENOTEMPTY
Directory not empty, where an empty directory was expected.  Typically,
this error occurs when you are trying to delete a directory.
@end deftypevr

@comment errno.h
@comment BSD: Too many users
@deftypevr Macro int EUSERS
The file quota system is confused because there are too many users.
@c This can probably happen in a GNU system when using NFS.
@end deftypevr

@comment errno.h
@comment BSD: Disc quota exceeded
@deftypevr Macro int EDQUOT
The user's disk quota was exceeded.
@end deftypevr

@comment errno.h
@comment BSD: Stale NFS file handle
@deftypevr Macro int ESTALE
Stale NFS file handle.  This indicates an internal confusion in the NFS
system which is due to file system rearrangements on the server host.
Repairing this condition usually requires unmounting and remounting
the NFS file system on the local host.
@end deftypevr

@comment errno.h
@comment BSD: Too many levels of remote in path
@deftypevr Macro int EREMOTE
An attempt was made to NFS-mount a remote file system with a file name that
already specifies an NFS-mounted file.
(This is an error on some operating systems, but we expect it to work
properly on the GNU system, making this error code impossible.)
@end deftypevr

@comment errno.h
@comment POSIX.1: No locks available
@deftypevr Macro int ENOLCK
No locks available.  This is used by the file locking facilities;
see @ref{File Locks}.
This error never occurs in the GNU system.
@end deftypevr

@comment errno.h
@comment POSIX.1: Function not implemented
@deftypevr Macro int ENOSYS
Function not implemented.  Some functions have commands or options defined
that might not be supported in all implementations, and this is the kind
of error you get if you request them and they are not supported.
@end deftypevr

@comment errno.h
@comment GNU: Inappropriate operation for background process
@deftypevr Macro int EBACKGROUND
In the GNU system, servers supporting the @code{term} protocol return
this error for certain operations when the caller is not in the
foreground process group of the terminal.  Users do not usually see this
error because functions such as @code{read} and @code{write} translate
it into a @code{SIGTTIN} or @code{SIGTTOU} signal.  @xref{Job Control},
for information on process groups and these signals.
@end deftypevr

@comment errno.h
@comment GNU: Translator died
@deftypevr Macro int EDIED
In the GNU system, opening a file returns this error when the file is
translated by a program and the translator program dies while starting
up, before it has connected to the file.
@end deftypevr

@comment errno.h
@comment GNU: ?
@deftypevr Macro int ED
The experienced user will know what is wrong.
@end deftypevr

@comment errno.h
@comment GNU: You really blew it this time
@deftypevr Macro int EGREGIOUS
You did @strong{what}?
@end deftypevr

@comment errno.h
@comment GNU: Computer bought the farm
@deftypevr Macro int EIEIO
Go home and have a glass of warm, dairy-fresh milk.
@end deftypevr

@comment errno.h
@comment GNU: Gratuitous error
@deftypevr Macro int EGRATUITOUS
This error code has no purpose.
@end deftypevr


@node Error Messages,  , Error Codes, Error Reporting
@section Error Messages

The library has functions and variables designed to make it easy for
your program to report informative error messages in the customary
format about the failure of a library call.  The functions
@code{strerror} and @code{perror} give you the standard error message
for a given error code; the variable
@code{program_invocation_short_name} gives you convenient access to the
name of the program that encountered the error.

@comment string.h
@comment ANSI
@deftypefun {char *} strerror (int @var{errnum})
The @code{strerror} function maps the error code (@pxref{Checking for
Errors}) specified by the @var{errnum} argument to a descriptive error
message string.  The return value is a pointer to this string.

The value @var{errnum} normally comes from the variable @code{errno}.

You should not modify the string returned by @code{strerror}.  Also, if
you make subsequent calls to @code{strerror}, the string might be
overwritten.  (But it's guaranteed that no library function ever calls
@code{strerror} behind your back.)

The function @code{strerror} is declared in @file{string.h}.
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypefun void perror (const char *@var{message})
This function prints an error message to the stream @code{stderr};
see @ref{Standard Streams}.

If you call @code{perror} with a @var{message} that is either a null
pointer or an empty string, @code{perror} just prints the error message 
corresponding to @code{errno}, adding a trailing newline.

If you supply a non-null @var{message} argument, then @code{perror}
prefixes its output with this string.  It adds a colon and a space 
character to separate the @var{message} from the error string corresponding
to @code{errno}.

The function @code{perror} is declared in @file{stdio.h}.
@end deftypefun

@code{strerror} and @code{perror} produce the exact same message for any
given error code; the precise text varies from system to system.  On the
GNU system, the messages are fairly short; there are no multi-line
messages or embedded newlines.  Each error message begins with a capital
letter and does not include any terminating punctuation.

@strong{Compatibility Note:}  The @code{strerror} function is a new
feature of ANSI C.  Many older C systems do not support this function
yet.

@cindex program name
@cindex name of running program
Many programs that don't read input from the terminal are designed to
exit if any system call fails.  By convention, the error message from
such a program should start with the program's name, sans directories.
You can find that name in the variable
@code{program_invocation_short_name}; the full file name is stored the
variable @code{program_invocation_name}:

@comment errno.h
@comment GNU
@deftypevar {char *} program_invocation_name 
This variable's value is the name that was used to invoke the program
running in the current process.  It is the same as @code{argv[0]}.  Note
that this is not necessarily a useful file name; often it contains no
directory names.  @xref{Program Arguments}.
@end deftypevar

@comment errno.h
@comment GNU
@deftypevar {char *} program_invocation_short_name 
This variable's value is the name that was used to invoke the program
running in the current process, with directory names removed.  (That is
to say, it is the same as @code{program_invocation_name} minus
everything up to the last slash, if any.)
@end deftypevar

The library initialization code sets up both of these variables before
calling @code{main}.

@strong{Portability Note:} These two variables are GNU extensions.  If
you want your program to work with non-GNU libraries, you must save the
value of @code{argv[0]} in @code{main}, and then strip off the directory
names yourself.  We added these extensions to make it possible to write
self-contained error-reporting subroutines that require no explicit
cooperation from @code{main}.

Here is an example showing how to handle failure to open a file
correctly.  The function @code{open_sesame} tries to open the named file
for reading and returns a stream if successful.  The @code{fopen}
library function returns a null pointer if it couldn't open the file for
some reason.  In that situation, @code{open_sesame} constructs an
appropriate error message using the @code{strerror} function, and
terminates the program.  If we were going to make some other library
calls before passing the error code to @code{strerror}, we'd have to
save it in a local variable instead, because those other library
functions might overwrite @code{errno} in the meantime.

@smallexample
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

FILE *
open_sesame (char *name)
@{ 
  FILE *stream;

  errno = 0;                     
  stream = fopen (name, "r");
  if (stream == NULL)
    @{
      fprintf (stderr, "%s: Couldn't open file %s; %s\n",
               program_invocation_short_name, name, strerror (errno));
      exit (EXIT_FAILURE);
    @}
  else
    return stream;
@}
@end smallexample