ERRNO(3)

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.

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