text: strerror, strerror_r - get error message string
  Synopsis
  Description
  Return Value
  Errors
  Examples
  Application Usage
  Rationale
  Future Directions
  See Also
  #include <string.h>
  char *strerror(int errnum); IEEE Std 1003.1-2001 string pointed to shall not be modified by the application, but may be overwritten by a subsequent call to strerror() or perror().
  The contents of the error message strings returned by strerror() should be determined by the setting of the LC_MESSAGES category in the current locale.
  The implementation shall behave as if no function defined in this volume of IEEE Std 1003.1-2001 calls strerror().
  The strerror() function shall not change the setting of errno if successful.
  Since no return value is reserved to indicate an error, an application wishing to check for error situations should set errno to 0, then call strerror(), then check errno.
  The strerror() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.
  The strerror_r() function shall map the error number in errnum to a locale-dependent error message string and shall return the string in the buffer pointed to by strerrbuf, with length buflen.
  Upon successful completion, strerror() shall return a pointer to the generated message string. On error errno may be set, but no return value is reserved to indicate an error.
  Upon successful completion, strerror_r() shall return 0. Otherwise, an error number shall be returned to indicate the error.
  These functions may fail if:The following sections are informative.
  None.
  None.
  None.
  None.
  perror() , .