Header File 

errno.h 

Syntax 

extern int errno; 

Description 

errno is used by perror to print error messages when certain library routines fail to accomplish their appointed tasks. 

When an error in a math or system call occurs, errno is set to indicate the type of error. Sometimes errno and _doserrno are equivalent. At other times, errno does not contain the actual operating system error code, which is contained in _doserrno. Still other errors might occur that set only errno, not _doserrno.