errno, _doserrno, _sys_errlist, and _sys_nerr

Article
04/28/2015

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())

Remarks

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

Requirements

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.

errno, _doserrno, _sys_errlist, and _sys_nerr

Remarks

Requirements

See Also

Reference

Additional resources