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.

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.

See Also

Reference

Global Variables

errno Constants

perror

strerror

_get_doserrno

_set_doserrno

_get_errno

_set_errno