errno(3) - Linux man page

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() 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()). 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.

See Also

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

Referenced By

_syscall(2), cap_clear(3), cap_copy_ext(3), cap_get_file(3), cap_get_proc(3), cap_init(3), cap_to_text(3), intro(2), intro(3), ldap_get_dn(3), ldap_initialize(3), ldap_open(3), libmagic(3), lwres_net_ntop(3), nwerrors(3), zshparam(1), zzip_freopen(3), zzip_open_shared_io(3), zzip_read(3)