ERRNO

Section: Linux Programmer's Manual (3)
Updated: 2014-09-06
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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 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.

Valid error numbers are all nonzero; 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.

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); see signal(7).
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 filesystem (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 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)
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.