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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 ‘errno.h’ to use this facility.
| 1.1 Checking for Errors | How errors are reported by library functions. | |
| 1.2 Error Codes | Error code macros; all of these expand into integer constant values. | |
| 1.3 Error Messages | Mapping error codes onto error messages. |
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Most library functions return a special value to indicate that they have
failed. The special value is typically -1, a null pointer, or a
constant such as 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 errno. This variable is declared in the header file
‘errno.h’.
The variable errno contains the system error number. You can
change the value of errno.
Since errno is declared 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 errno, so you generally do not need to worry about this
possibility except when writing signal handlers.
The initial value of 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 errno
when they succeed; thus, the value of errno after a successful
call is not necessarily zero, and you should not use errno to
determine whether a call failed. The proper way to do that is
documented for each function. If the call the failed, you can
examine errno.
Many library functions can set 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 errno.
Portability Note: ANSI C specifies 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 *_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 sqrt and atan,
that return a perfectly legitimate value in case of an error, but also
set errno. For these functions, if you want to check to see
whether an error occurred, the recommended method is to set errno
to zero before calling the function, and then check its value afterward.
All the error codes have symbolic names; they are macros defined in ‘errno.h’. The names start with ‘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: EWOULDBLOCK and EAGAIN are the same.
Since the values are distinct, you can use them as labels in a
switch statement; just don’t use both EWOULDBLOCK and
EAGAIN. Your program should not make any other assumptions about
the specific values of these symbolic constants.
The value of 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 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
EFAULT in the descriptions of individual functions.
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The error code macros are defined in the header 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.
Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation.
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.
No process matches the specified process ID.
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 EINTR; see @ref{Interrupted
Primitives}.
Input/output error; usually used for physical read or write errors.
No such device or address. Typically, this means that a file representing a device has been installed incorrectly, and the system can’t find the right kind of device driver for it.
Argument list too long; used when the arguments passed to a new program
being executed with one of the exec functions (@pxref{Executing a
File}) occupy too much memory space. This condition never arises in the
GNU system.
Invalid executable file format. This condition is detected by the
exec functions; see @ref{Executing a File}.
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).
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.
Deadlock avoided; allocating a system resource would have resulted in a deadlock situation. @xref{File Locks}, for an example.
No memory available. The system cannot allocate more virtual memory because its capacity is full.
Permission denied; the file permissions do not allow the attempted operation.
Bad address; an invalid pointer was detected.
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.
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.
File exists; an existing file was specified in a context where it only makes sense to specify a new file.
An attempt to make an improper link across file systems was detected.
The wrong type of device was given to a function that expects a particular sort of device.
A file that isn’t a directory was specified when a directory is required.
File is a directory; attempting to open a directory for writing gives this error.
Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function.
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}.
The current process has too many files open and can’t open any more. Duplicate descriptors do count toward this limit.
Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.
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.
File too big; the size of a file would be larger than allowed by the system.
No space left on device; write operation on a file failed because the disk is full.
Invalid seek operation (such as on a pipe).
An attempt was made to modify a file on a read-only file system.
Too many links; the link count of a single file is too large.
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
SIGPIPE signal; this signal terminates the program if not handled
or blocked. Thus, your program will never actually see EPIPE
unless it has handled or blocked SIGPIPE.
Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined.
Range error; used by mathematical functions when the result value is not representable because of overflow or underflow.
Resource temporarily unavailable; the call might work if you try again
later. Only fork returns error code EAGAIN for such a
reason.
An operation that would block was attempted on an object that has non-blocking mode selected.
Portability Note: In 4.4BSD and GNU, EWOULDBLOCK and
EAGAIN are the same. Earlier versions of BSD (@pxref{Berkeley
Unix}) have two distinct codes, and use EWOULDBLOCK to indicate
an I/O operation that would block on an object with non-blocking mode
set, and EAGAIN for other kinds of errors.
An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected.
An operation is already in progress on an object that has non-blocking mode selected.
A file that isn’t a socket was specified when a socket is required.
No destination address was supplied on a socket operation.
The size of a message sent on a socket was larger than the supported maximum size.
The socket type does not support the requested communications protocol.
You specified a socket option that doesn’t make sense for the particular protocol being used by the socket. @xref{Socket Options}.
The socket domain does not support the requested communications protocol. @xref{Creating a Socket}.
The socket type is not supported.
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.
The socket communications protocol family you requested is not supported.
The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. @xref{Sockets}.
The requested socket address is already in use. @xref{Socket Addresses}.
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}.
A socket operation failed because the network was down.
A socket operation failed because the subnet containing the remost host was unreachable.
A network connection was reset because the remote host crashed.
A network connection was aborted locally.
A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting.
The kernel’s buffers for I/O operations are all in use.
You tried to connect a socket that is already connected. @xref{Connecting}.
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.
The socket has already been shut down.
A socket operation with a specified timeout received no response during the timeout period.
A remote host refused to allow the network connection (typically because it is not running the requested service).
Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links.
Filename too long (longer than PATH_MAX; @pxref{Limits for
Files}) or host name too long (in gethostname or
sethostname; @pxref{Host Identification}).
The remote host for a requested network connection is down.
The remote host for a requested network connection is not reachable.
Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory.
The file quota system is confused because there are too many users.
The user’s disk quota was exceeded.
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.
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.)
No locks available. This is used by the file locking facilities; see @ref{File Locks}.
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.
The experienced user will know what is wrong.
This error code has no purpose.
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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
strerror and perror give you the standard error message
for a given error code; the variable
program_invocation_short_name gives you convenient access to the
name of the program that encountered the error.
The strerror function maps the error code (see section Checking for Errors) specified by the errnum argument to a descriptive error
message string. The return value is a pointer to this string.
The value errnum normally comes from the variable errno.
You should not modify the string returned by strerror. Also, if
you make subsequent calls to strerror, the string might be
overwritten. (But it’s guaranteed that no library function ever calls
strerror behind your back.)
The function strerror is declared in ‘string.h’.
This function prints an error message to the stream stderr;
see @ref{Standard Streams}.
If you call perror with a message that is either a null
pointer or an empty string, perror just prints the error message
corresponding to errno, adding a trailing newline.
If you supply a non-null message argument, then perror
prefixes its output with this string. It adds a colon and a space
character to separate the message from the error string corresponding
to errno.
The function perror is declared in ‘stdio.h’.
strerror and 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.
Compatibility Note: The strerror function is a new
feature of ANSI C. Many older C systems do not support this function
yet.
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
program_invocation_short_name; the full file name is stored the
variable 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 argv[0].
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 program_invocation_name minus
everything up to the last slash, if any.)
Both program_invocation_name and
program_invocation_short_name are set up by the system before
main is called.
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 argv[0] in 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 main.
Here is an example showing how to handle failure to open a file
correctly. The function open_sesame tries to open the named file
for reading and returns a stream if successful. The fopen
library function returns a null pointer if it couldn’t open the file for
some reason. In that situation, open_sesame constructs an
appropriate error message using the strerror function, and
terminates the program. If we were going to make some other library
calls before passing the error code to strerror, we’d have to
save it in a local variable instead, because those other library
functions might overwrite errno in the meantime.
#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) {
fprintf (stderr, "%s: Couldn't open file %s; %s\n",
program_invocation_short_name, name, strerror (errno));
exit (EXIT_FAILURE);
} else
return stream;
}
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