errno - number of last error


#include <errno.h>


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 non-zero; 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 non-zero 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)
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))
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


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)


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