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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
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.
err(3), error(3), perror(3), strerror(3)
2006-02-09 ERRNO(3)