errno, _doserrno, _sys_errlist, and _sys_nerr

These global variables hold error codes used by the perror and strerror functions for printing error messages. The more secure functional versions listed in this table should be used in place of their global variable.

Global Variable	Functional Equivalents
_doserrno	_get_doserrno, _set_doserrno
_errno	_get_errno, _set_errno

These variables are declared in STDLIB.H as

extern int _doserrno; 
extern int errno; 
extern char *_sys_errlist[ ];
extern int _sys_nerr;

Remarks

errno and _errno are connected to the same variable. errno is defined with the code #define errno (*_errno())

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. Always check errno immediately before and after a call that may set it.

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, use _doserrno to access the operating-system error-code equivalents of errno codes. For most non-I/O operations the value of _doserrno is undefined.

Each errno value is associated with an error message that can be printed using perror or stored in a string using strerror. perror and strerror use the _sys_errlist array and _sys_nerr, the number of elements in _sys_errlist, to process error information.

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, defined as predefined constants in ERRNO.H, are UNIX-compatible and are listed below. Only ERANGE, EILSEQ and EDOM are specified in the ANSI 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

See Also

Reference

Global Variables

errno Constants

perror

strerror

_get_doserrno

_set_doserrno

_get_errno

_set_errno