errno, _doserrno, _sys_errlist, and _sys_nerr

Global macros that hold error codes that are set during program execution, and string equivalents of the error codes for display.


#define errno   (*_errno())
#define _doserrno   (*__doserrno())
#define _sys_errlist (__sys_errlist())
#define _sys_nerr (*__sys_nerr())


Both errno and _doserrno are set to 0 by the runtime during program startup. errno is set on an error in a system-level call. Because errno holds the value for the last call that set it, this value may be changed by succeeding calls. Run-time library calls that set errno on an error do not clear errno on success. Always clear errno by calling _set_errno(0) immediately before a call that may set it, and check it immediately after the call.

On an error, errno is not necessarily set to the same value as the error code returned by a system call. For I/O operations, _doserrno stores the operating-system error-code equivalents of errno codes. For most non-I/O operations, the value of _doserrno is not set.

Each errno value is associated with an error message in _sys_errlist that can be printed by using one of the perror functions, or stored in a string by using one of the strerror or strerror_s functions. The perror and strerror functions use the _sys_errlist array and _sys_nerr—the number of elements in _sys_errlist—to process error information. Direct access to _sys_errlist and _sys_nerr is deprecated for code-security reasons. We recommend that you use the more secure, functional versions instead of the global macros, as shown here:

Global Macro Functional Equivalents
_doserrno _get_doserrno, _set_doserrno
errno _get_errno, _set_errno
_sys_errlist, _sys_nerr strerror_s, _strerror_s, _wcserror_s, __wcserror_s

Library math routines set errno by calling _matherr. To handle math errors differently, write your own routine according to the _matherr reference description and name it _matherr.

All errno values in the following table are predefined constants in <errno.h>, and are UNIX-compatible. Only ERANGE, EILSEQ, and EDOM are specified in the ISO C99 standard.

Constant System error message Value
EPERM Operation not permitted 1
ENOENT No such file or directory 2
ESRCH No such process 3
EINTR Interrupted function 4
EIO I/O error 5
ENXIO No such device or address 6
E2BIG Argument list too long 7
ENOEXEC Exec format error 8
EBADF Bad file number 9
ECHILD No spawned processes 10
EAGAIN No more processes or not enough memory or maximum nesting level reached 11
ENOMEM Not enough memory 12
EACCES Permission denied 13
EFAULT Bad address 14
EBUSY Device or resource busy 16
EEXIST File exists 17
EXDEV Cross-device link 18
ENODEV No such device 19
ENOTDIR Not a directory 20
EISDIR Is a directory 21
EINVAL Invalid argument 22
ENFILE Too many files open in system 23
EMFILE Too many open files 24
ENOTTY Inappropriate I/O control operation 25
EFBIG File too large 27
ENOSPC No space left on device 28
ESPIPE Invalid seek 29
EROFS Read-only file system 30
EMLINK Too many links 31
EPIPE Broken pipe 32
EDOM Math argument 33
ERANGE Result too large 34
EDEADLK Resource deadlock would occur 36
EDEADLOCK Same as EDEADLK for compatibility with older Microsoft C versions 36
ENAMETOOLONG Filename too long 38
ENOLCK No locks available 39
ENOSYS Function not supported 40
ENOTEMPTY Directory not empty 41
EILSEQ Illegal byte sequence 42
STRUNCATE String was truncated 80


Global macro Required header Optional header
errno <errno.h> or <stdlib.h>, <cerrno> or <cstdlib> (C++)
_doserrno, _sys_errlist, _sys_nerr <stdlib.h>, <cstdlib> (C++) <errno.h>, <cerrno> (C++)

The _doserrno, _sys_errlist, and _sys_nerr macros are Microsoft extensions. For more compatibility information, see Compatibility.

See also

Global Variables
errno Constants
perror, _wperror
strerror, _strerror, _wcserror, __wcserror
strerror_s, _strerror_s, _wcserror_s, __wcserror_s