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Error Reporting

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.

Checking for Errors

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'.

Variable: volatile int errno

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 section Defining Signal 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. See section Reserved Names.

The error code values are all positive integers and are all distinct. (Since the values are distinct, you can use them as labels in a switch statement, for example.) 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.

Error Codes

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.

Macro: int EPERM

Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation.

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.

Macro: int ESRCH

No process matches the specified process ID.

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 EINTR; see section Primitives Interrupted by Signals.

Macro: int EIO

Input/output error; usually used for physical read or write errors.

Macro: int ENXIO

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.

Macro: int E2BIG

Argument list too long; used when the arguments passed to a new program being executed with one of the exec functions (see section Executing a File) occupy too much memory space. This condition never arises in the GNU system.

Macro: int ENOEXEC

Invalid executable file format. This condition is detected by the exec functions; see section Executing a File.

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).

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.

Macro: int EDEADLK

Deadlock avoided; allocating a system resource would have resulted in a deadlock situation. For an example, See section File Locks.

Macro: int ENOMEM

No memory available. The system cannot allocate more virtual memory because its capacity is full.

Macro: int EACCES

Permission denied; the file permissions do not allow the attempted operation.

Macro: int EFAULT

Bad address; an invalid pointer was detected.

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.

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.

Macro: int EEXIST

File exists; an existing file was specified in a context where it only makes sense to specify a new file.

Macro: int EXDEV

An attempt to make an improper link across file systems was detected.

Macro: int ENODEV

The wrong type of device was given to a function that expects a particular sort of device.

Macro: int ENOTDIR

A file that isn't a directory was specified when a directory is required.

Macro: int EISDIR

File is a directory; attempting to open a directory for writing gives this error.

Macro: int EINVAL

Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function.

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 section Linked Channels.

Macro: int EMFILE

The current process has too many files open and can't open any more. Duplicate descriptors do count toward this limit.

Macro: int ENOTTY

Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.

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.

Macro: int EFBIG

File too big; the size of a file would be larger than allowed by the system.

Macro: int ENOSPC

No space left on device; write operation on a file failed because the disk is full.

Macro: int ESPIPE

Invalid seek operation (such as on a pipe).

Macro: int EROFS

An attempt was made to modify a file on a read-only file system.

Macro: int EMLINK

Too many links; the link count of a single file is too large.

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 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.

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.

Macro: int ERANGE

Range error; used by mathematical functions when the result value is not representable because of overflow or underflow.

Macro: int EAGAIN

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 (see section 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.

Macro: int EALREADY

An operation is already in progress on an object that has non-blocking mode selected.

Macro: int ENOTSOCK

A file that isn't a socket was specified when a socket is required.


No destination address was supplied on a socket operation.

Macro: int EMSGSIZE

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. See section Socket Options.


The socket domain does not support the requested communications protocol. See section 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. See section Sockets.


The requested socket address is already in use. See section 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. See section Socket Addresses.

Macro: int ENETDOWN

A socket operation failed because the network was down.


A socket operation failed because the subnet containing the remost host was unreachable.

Macro: int ENETRESET

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.

Macro: int ENOBUFS

The kernel's buffers for I/O operations are all in use.

Macro: int EISCONN

You tried to connect a socket that is already connected. See section Making a Connection.

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.

Macro: int ESHUTDOWN

The socket has already been shut down.

Macro: int ETIMEDOUT

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).

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.


Filename too long (longer than PATH_MAX; see section Limits on File System Capacity) or host name too long (in gethostname or sethostname; see section Host Identification).

Macro: int EHOSTDOWN

The remote host for a requested network connection is down.


The remote host for a requested network connection is not reachable.

Macro: int ENOTEMPTY

Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory.

Macro: int EUSERS

The file quota system is confused because there are too many users.

Macro: int EDQUOT

The user's disk quota was exceeded.

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.

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.)

Macro: int ENOLCK

No locks available. This is used by the file locking facilities; see section File Locks.

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.

Macro: int ED

The experienced user will know what is wrong.


This error code has no purpose.

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 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.

Function: char * strerror (int errnum)

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'.

Function: void perror (const char *message)

This function prints an error message to the stream stderr; see section 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:

Variable: 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 argv[0].

Variable: 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 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>

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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