errno(3)

NAME#

errno - number of last error

LIBRARY#

Standard C library (libc, -lc)

SYNOPSIS#

#include <errno.h>

DESCRIPTION#

The <errno.h> header file defines the integer variable errno, which is set by system calls and some library functions in the event of an error to indicate what went wrong.

errno#

The value in errno is significant only when the return value of the call indicated an error (i.e., -1 from most system calls; -1 or NULL from most library functions); a function that succeeds is allowed to change errno. The value of errno is never set to zero by any system call or library function.

For some system calls and library functions (e.g., getpriority(2)), -1 is a valid return on success. In such cases, a successful return can be distinguished from an error return by setting errno to zero before the call, and then, if the call returns a status that indicates that an error may have occurred, checking to see if errno has a nonzero value.

errno is defined by the ISO C standard to be a modifiable lvalue of type int, and must not be explicitly declared; errno may be a macro. errno is thread-local; setting it in one thread does not affect its value in any other thread.

Error numbers and names#

Valid error numbers are all positive numbers. The <errno.h> header file defines symbolic names for each of the possible error numbers that may appear in errno.

All the error names specified by POSIX.1 must have distinct values, with the exception of EAGAIN and EWOULDBLOCK, which may be the same. On Linux, these two have the same value on all architectures.

The error numbers that correspond to each symbolic name vary across UNIX systems, and even across different architectures on Linux. Therefore, numeric values are not included as part of the list of error names below. The perror(3) and strerror(3) functions can be used to convert these names to corresponding textual error messages.

On any particular Linux system, one can obtain a list of all symbolic error names and the corresponding error numbers using the errno(1) command (part of the moreutils package):

$ \nerrno -l\n
EPERM 1 Operation not permitted
ENOENT 2 No such file or directory
ESRCH 3 No such process
EINTR 4 Interrupted system call
EIO 5 Input/output error
...

The errno(1) command can also be used to look up individual error numbers and names, and to search for errors using strings from the error description, as in the following examples:

$ \nerrno 2\n
ENOENT 2 No such file or directory
$ \nerrno ESRCH\n
ESRCH 3 No such process
$ \nerrno -s permission\n
EACCES 13 Permission denied

List of error names#

In the list of the symbolic error names below, various names are marked as follows:

POSIX.1-2001

The name is defined by POSIX.1-2001, and is defined in later POSIX.1 versions, unless otherwise indicated.

POSIX.1-2008

The name is defined in POSIX.1-2008, but was not present in earlier POSIX.1 standards.

C99

The name is defined by C99.

Below is a list of the symbolic error names that are defined on Linux:

E2BIG

Argument list too long (POSIX.1-2001).

EACCES

Permission denied (POSIX.1-2001).

EADDRINUSE

Address already in use (POSIX.1-2001).

EADDRNOTAVAIL

Address not available (POSIX.1-2001).

EAFNOSUPPORT

Address family not supported (POSIX.1-2001).

EAGAIN

Resource temporarily unavailable (may be the same value as EWOULDBLOCK) (POSIX.1-2001).

EALREADY

Connection already in progress (POSIX.1-2001).

EBADE

Invalid exchange.

EBADF

Bad file descriptor (POSIX.1-2001).

EBADFD

File descriptor in bad state.

EBADMSG

Bad message (POSIX.1-2001).

EBADR

Invalid request descriptor.

EBADRQC

Invalid request code.

EBADSLT

Invalid slot.

EBUSY

Device or resource busy (POSIX.1-2001).

ECANCELED

Operation canceled (POSIX.1-2001).

ECHILD

No child processes (POSIX.1-2001).

ECHRNG

Channel number out of range.

ECOMM

Communication error on send.

ECONNABORTED

Connection aborted (POSIX.1-2001).

ECONNREFUSED

Connection refused (POSIX.1-2001).

ECONNRESET

Connection reset (POSIX.1-2001).

EDEADLK

Resource deadlock avoided (POSIX.1-2001).

EDEADLOCK

On most architectures, a synonym for EDEADLK. On some architectures (e.g., Linux MIPS, PowerPC, SPARC), it is a separate error code "File locking deadlock error".

EDESTADDRREQ

Destination address required (POSIX.1-2001).

EDOM

Mathematics argument out of domain of function (POSIX.1, C99).

EDQUOT

Disk quota exceeded (POSIX.1-2001).

EEXIST

File exists (POSIX.1-2001).

EFAULT

Bad address (POSIX.1-2001).

EFBIG

File too large (POSIX.1-2001).

EHOSTDOWN

Host is down.

EHOSTUNREACH

Host is unreachable (POSIX.1-2001).

EHWPOISON

Memory page has hardware error.

EIDRM

Identifier removed (POSIX.1-2001).

EILSEQ

Invalid or incomplete multibyte or wide character (POSIX.1, C99).

The text shown here is the glibc error description; in POSIX.1, this error is described as "Illegal byte sequence".

EINPROGRESS

Operation in progress (POSIX.1-2001).

EINTR

Interrupted function call (POSIX.1-2001); see signal(7).

EINVAL

Invalid argument (POSIX.1-2001).

EIO

Input/output error (POSIX.1-2001).

EISCONN

Socket is connected (POSIX.1-2001).

EISDIR

Is a directory (POSIX.1-2001).

EISNAM

Is a named type file.

EKEYEXPIRED

Key has expired.

EKEYREJECTED

Key was rejected by service.

EKEYREVOKED

Key has been revoked.

EL2HLT

Level 2 halted.

EL2NSYNC

Level 2 not synchronized.

EL3HLT

Level 3 halted.

EL3RST

Level 3 reset.

ELIBACC

Cannot access a needed shared library.

ELIBBAD

Accessing a corrupted shared library.

ELIBMAX

Attempting to link in too many shared libraries.

ELIBSCN

.lib section in a.out corrupted

ELIBEXEC

Cannot exec a shared library directly.

ELNRNG

Link number out of range.

ELOOP

Too many levels of symbolic links (POSIX.1-2001).

EMEDIUMTYPE

Wrong medium type.

EMFILE

Too many open files (POSIX.1-2001). Commonly caused by exceeding the RLIMIT_NOFILE resource limit described in getrlimit(2). Can also be caused by exceeding the limit specified in /proc/sys/fs/nr_open.

EMLINK

Too many links (POSIX.1-2001).

EMSGSIZE

Message too long (POSIX.1-2001).

EMULTIHOP

Multihop attempted (POSIX.1-2001).

ENAMETOOLONG

Filename too long (POSIX.1-2001).

ENETDOWN

Network is down (POSIX.1-2001).

ENETRESET

Connection aborted by network (POSIX.1-2001).

ENETUNREACH

Network unreachable (POSIX.1-2001).

ENFILE

Too many open files in system (POSIX.1-2001). On Linux, this is probably a result of encountering the /proc/sys/fs/file-max limit (see proc(5)).

ENOANO

No anode.

ENOBUFS

No buffer space available (POSIX.1 (XSI STREAMS option)).

ENODATA

The named attribute does not exist, or the process has no access to this attribute; see xattr(7).

In POSIX.1-2001 (XSI STREAMS option), this error was described as "No message is available on the STREAM head read queue".

ENODEV

No such device (POSIX.1-2001).

ENOENT

No such file or directory (POSIX.1-2001).

Typically, this error results when a specified pathname does not exist, or one of the components in the directory prefix of a pathname does not exist, or the specified pathname is a dangling symbolic link.

ENOEXEC

Exec format error (POSIX.1-2001).

ENOKEY

Required key not available.

ENOLCK

No locks available (POSIX.1-2001).

ENOLINK

Link has been severed (POSIX.1-2001).

ENOMEDIUM

No medium found.

ENOMEM

Not enough space/cannot allocate memory (POSIX.1-2001).

ENOMSG

No message of the desired type (POSIX.1-2001).

ENONET

Machine is not on the network.

ENOPKG

Package not installed.

ENOPROTOOPT

Protocol not available (POSIX.1-2001).

ENOSPC

No space left on device (POSIX.1-2001).

ENOSR

No STREAM resources (POSIX.1 (XSI STREAMS option)).

ENOSTR

Not a STREAM (POSIX.1 (XSI STREAMS option)).

ENOSYS

Function not implemented (POSIX.1-2001).

ENOTBLK

Block device required.

ENOTCONN

The socket is not connected (POSIX.1-2001).

ENOTDIR

Not a directory (POSIX.1-2001).

ENOTEMPTY

Directory not empty (POSIX.1-2001).

ENOTRECOVERABLE

State not recoverable (POSIX.1-2008).

ENOTSOCK

Not a socket (POSIX.1-2001).

ENOTSUP

Operation not supported (POSIX.1-2001).

ENOTTY

Inappropriate I/O control operation (POSIX.1-2001).

ENOTUNIQ

Name not unique on network.

ENXIO

No such device or address (POSIX.1-2001).

EOPNOTSUPP

Operation not supported on socket (POSIX.1-2001).

(ENOTSUP and EOPNOTSUPP have the same value on Linux, but according to POSIX.1 these error values should be distinct.)

EOVERFLOW

Value too large to be stored in data type (POSIX.1-2001).

EOWNERDEAD

Owner died (POSIX.1-2008).

EPERM

Operation not permitted (POSIX.1-2001).

EPFNOSUPPORT

Protocol family not supported.

EPIPE

Broken pipe (POSIX.1-2001).

EPROTO

Protocol error (POSIX.1-2001).

EPROTONOSUPPORT

Protocol not supported (POSIX.1-2001).

EPROTOTYPE

Protocol wrong type for socket (POSIX.1-2001).

ERANGE

Result too large (POSIX.1, C99).

EREMCHG

Remote address changed.

EREMOTE

Object is remote.

EREMOTEIO

Remote I/O error.

ERESTART

Interrupted system call should be restarted.

ERFKILL

Operation not possible due to RF-kill.

EROFS

Read-only filesystem (POSIX.1-2001).

ESHUTDOWN

Cannot send after transport endpoint shutdown.

ESPIPE

Invalid seek (POSIX.1-2001).

ESOCKTNOSUPPORT

Socket type not supported.

ESRCH

No such process (POSIX.1-2001).

ESTALE

Stale file handle (POSIX.1-2001).

This error can occur for NFS and for other filesystems.

ESTRPIPE

Streams pipe error.

ETIME

Timer expired (POSIX.1 (XSI STREAMS option)).

(POSIX.1 says "STREAM ioctl(2) timeout".)

ETIMEDOUT

Connection timed out (POSIX.1-2001).

ETOOMANYREFS

Too many references: cannot splice.

ETXTBSY

Text file busy (POSIX.1-2001).

EUCLEAN

Structure needs cleaning.

EUNATCH

Protocol driver not attached.

EUSERS

Too many users.

EWOULDBLOCK

Operation would block (may be same value as EAGAIN) (POSIX.1-2001).

EXDEV

Invalid cross-device link (POSIX.1-2001).

EXFULL

Exchange full.

NOTES#

A common mistake is to do

if (somecall() == -1) {
\n
    printf("somecall() failed\n");
\n
    if (errno == ...) { ... }
}

where errno no longer needs to have the value it had upon return from somecall() (i.e., it may have been changed by the printf(3)). If the value of errno should be preserved across a library call, it must be saved:

if (somecall() == -1) {
\n
    int errsv = errno;
\n
    printf("somecall() failed\n");
\n
    if (errsv == ...) { ... }
}

Note that the POSIX threads APIs do not set errno on error. Instead, on failure they return an error number as the function result. These error numbers have the same meanings as the error numbers returned in errno by other APIs.

On some ancient systems, <errno.h> was not present or did not declare errno, so that it was necessary to declare errno manually (i.e., extern int errno). Do not do this. It long ago ceased to be necessary, and it will cause problems with modern versions of the C library.

SEE ALSO#

errno(1), err(3), error(3), perror(3), strerror(3)