errno(3) — Linux manual pages

Name

errno — number of last error

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. Its value is significant only when the call returned an error (usually −1), and a function that does succeed is allowed to change errno.

Sometimes, when −1 is also a valid successful return value one has to zero errno before the call in order to detect possible errors.

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.

Valid error numbers are all non-zero; errno is never set to zero by any library function. All the error names specified by POSIX.1 must have distinct values, with the exception of EAGAIN and EWOULDBLOCK, which may be the same.

Below is a list of the symbolic error names that are defined on Linux. Some of these are marked POSIX.1, indicating that the name is defined by POSIX.1-2001, or C99, indicating that the name is defined by C99.

E2BIG

Argument list too long (POSIX.1)

EACCES

Permission denied (POSIX.1)

EADDRINUSE

Address already in use (POSIX.1)

EADDRNOTAVAIL

Address not available (POSIX.1)

EAFNOSUPPORT

Address family not supported (POSIX.1)

EAGAIN

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

EALREADY

Connection already in progress (POSIX.1)

EBADE

Invalid exchange

EBADF

Bad file descriptor (POSIX.1)

EBADFD

File descriptor in bad state

EBADMSG

Bad message (POSIX.1)

EBADR

Invalid request descriptor

EBADRQC

Invalid request code

EBADSLT

Invalid slot

EBUSY

Device or resource busy (POSIX.1)

ECANCELED

Operation canceled (POSIX.1)

ECHILD

No child processes (POSIX.1)

ECHRNG

Channel number out of range

ECOMM

Communication error on send

ECONNABORTED

Connection aborted (POSIX.1)

ECONNREFUSED

Connection refused (POSIX.1)

ECONNRESET

Connection reset (POSIX.1)

EDEADLK

Resource deadlock avoided (POSIX.1)

EDEADLOCK

Synonym for EDEADLK

EDESTADDRREQ

Destination address required (POSIX.1)

EDOM

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

EDQUOT

Disk quota exceeded (POSIX.1)

EEXIST

File exists (POSIX.1)

EFAULT

Bad address (POSIX.1)

EFBIG

File too large (POSIX.1)

EHOSTDOWN

Host is down

EHOSTUNREACH

Host is unreachable (POSIX.1)

EIDRM

Identifier removed (POSIX.1)

EILSEQ

Illegal byte sequence (POSIX.1, C99)

EINPROGRESS

Operation in progress (POSIX.1)

EINTR

Interrupted function call (POSIX.1)

EINVAL

Invalid argument (POSIX.1)

EIO

Input/output error (POSIX.1)

EISCONN

Socket is connected (POSIX.1)

EISDIR

Is a directory (POSIX.1)

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 halted

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

ELOOP

Too many levels of symbolic links (POSIX.1)

EMEDIUMTYPE

Wrong medium type

EMFILE

Too many open files (POSIX.1)

EMLINK

Too many links (POSIX.1)

EMSGSIZE

Message too long (POSIX.1)

EMULTIHOP

Multihop attempted (POSIX.1)

ENAMETOOLONG

Filename too long (POSIX.1)

ENETDOWN

Network is down (POSIX.1)

ENETRESET

Connection aborted by network (POSIX.1)

ENETUNREACH

Network unreachable (POSIX.1)

ENFILE

Too many open files in system (POSIX.1)

ENOBUFS

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

ENODATA

No message is available on the STREAM head read queue (POSIX.1)

ENODEV

No such device (POSIX.1)

ENOENT

No such file or directory (POSIX.1)

ENOEXEC

Exec format error (POSIX.1)

ENOKEY

Required key not available

ENOLCK

No locks available (POSIX.1)

ENOLINK

Link has been severed (POSIX.1)

ENOMEDIUM

No medium found

ENOMEM

Not enough space (POSIX.1)

ENOMSG

No message of the desired type (POSIX.1)

ENONET

Machine is not on the network

ENOPKG

Package not installed

ENOPROTOOPT

Protocol not available (POSIX.1)

ENOSPC

No space left on device (POSIX.1)

ENOSR

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

ENOSTR

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

ENOSYS

Function not implemented (POSIX.1)

ENOTBLK

Block device required

ENOTCONN

The socket is not connected (POSIX.1)

ENOTDIR

Not a directory (POSIX.1)

ENOTEMPTY

Directory not empty (POSIX.1)

ENOTSOCK

Not a socket (POSIX.1)

ENOTSUP

Operation not supported (POSIX.1)

ENOTTY

Inappropriate I/O control operation (POSIX.1)

ENOTUNIQ

Name not unique on network

ENXIO

No such device or address (POSIX.1)

EOPNOTSUPP

Operation not supported on socket (POSIX.1)

(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)

EPERM

Operation not permitted (POSIX.1)

EPFNOSUPPORT

Protocol family not supported

EPIPE

Broken pipe (POSIX.1)

EPROTO

Protocol error (POSIX.1)

EPROTONOSUPPORT

Protocol not supported (POSIX.1)

EPROTOTYPE

Protocol wrong type for socket (POSIX.1)

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

EROFS

Read-only file system (POSIX.1)

ESHUTDOWN

Cannot send after transport endpoint shutdown

ESPIPE

Invalid seek (POSIX.1)

ESOCKTNOSUPPORT

Socket type not supported

ESRCH

No such process (POSIX.1)

ESTALE

Stale file handle (POSIX.1)

This error can occur for NFS and for other file systems

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)

ETXTBSY

Text file busy (POSIX.1)

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)

EXDEV

Improper link (POSIX.1)

EXFULL

Exchange full

NOTES

A common mistake is to do

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

It was common in traditional C to declare errno manually (i.e., extern int errno) instead of including <errno.h>. Do not do this. It will not work with modern versions of the C library. However, on (very) old Unix systems, there may be no <errno.h> and the declaration is needed.

Name

Synopsis

DESCRIPTION

NOTES

SEE ALSO