From Linux Man Pages
strerror, strerror_r - return string describing error number
SYNOPSIS
#include <string.h>
char *strerror(int errnum);
char *strerror_r(int errnum, char *buf, size_t buflen);
/* GNU-specific strerror_r() */
#define _XOPEN_SOURCE 600
#include <string.h>
int strerror_r(int errnum, char *buf, size_t buflen);
/* XSI-compliant strerror_r() */
DESCRIPTION
The strerror() function returns a string describing the error code passed in the argument errnum, possibly using
the LC_MESSAGES part of the current locale to select the appropriate language. This string must not be modified
by the application, but may be modified by a subsequent call to perror() or strerror(). No library function will
modify this string.
The strerror_r() function is similar to strerror(), but is thread safe. This function is available in two ver-
sions: an XSI-compliant version specified in POSIX.1-2001, and a GNU-specific version (available since glibc
2.0). If _XOPEN_SOURCE is defined with the value 600, then the XSI-compliant version is provided, otherwise the
GNU-specific version is provided.
The XSI-compliant strerror_r() is preferred for portable applications. It returns the error string in the user-
supplied buffer buf of length buflen.
The GNU-specific strerror_r() returns a pointer to a string containing the error message. This may be either a
pointer to a string that the function stores in buf, or a pointer to some (immutable) static string (in which
case buf is unused). If the function stores a string in buf, then at most buflen bytes are stored (the string
may be truncated if buflen is too small) and the string always includes a terminating null byte.
RETURN VALUE
The strerror() and strerror_r() functions return the appropriate error description string, or an "Unknown error
nnn" message if the error number is unknown.
The XSI-compliant strerror_r() function returns 0 on success; on error, -1 is returned and errno is set to indi-
cate the error.
ERRORS
EINVAL The value of errnum is not a valid error number.
ERANGE Insufficient storage was supplied to contain the error description string.
CONFORMING TO
POSIX.1-2001.
The GNU-specific strerror_r() function is a non-standard extension.
POSIX.1-2001 permits strerror() to set errno if the call encounters an error, but does not specify what value
should be returned as the function result in the event of an error. On some systems, strerror() returns NULL if
the error number is unknown. On other systems, strerror() returns a string something like "Error nnn occurred"
and sets errno to EINVAL if the error number is unknown.
RELATED
err(3), errno(3), error(3), perror(3), strsignal(3)
CATEGORY