_doserrno, errno, _sys_errlist, and _sys_nerr

These global variables hold error codes used by the perror and strerror functions for printing error messages. Manifest constants for these variables are declared in STDLIB.H as follows:

extern int _doserrno;

extern int errno;

extern char *_sys_errlist[ ];

extern int _sys_nerr;

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. All errno values, defined as manifest constants in ERRNO.H, are UNIX-compatible. The values valid for 32-bit Windows applications are a subset of these UNIX values.

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 only, use _doserrno to access the operating-system error-code equivalents of errno codes. For other 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.

The following errno values are compatible with 32-bit Windows applications. Only ERANGE and EDOM are specified in the ANSI standard.