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### 5.25 `strerror_r`—convert error number to string and copy to buffer

Synopsis

         #include <string.h>
         #ifdef _GNU_SOURCE
         char *strerror_r(int errnum, char *buffer, size_t n);
         #else
         int strerror_r(int errnum, char *buffer, size_t n);
         #endif

Description
`strerror_r` converts the error number errnum into a string and copies the result into the supplied buffer for a length up to n, including the NUL terminator. The value of errnum is usually a copy of `errno`. If `errnum` is not a known error number, the result is the empty string.

See `strerror` for how strings are mapped to `errnum`.

Returns
There are two variants: the GNU version always returns a NUL-terminated string, which is buffer if all went well, but which is another pointer if n was too small (leaving buffer untouched). If the return is not buffer, your application must not modify that string. The POSIX version returns 0 on success, EINVAL if `errnum` was not recognized, and ERANGE if n was too small. The variant chosen depends on macros that you define before inclusion of `string.h`.

Portability
`strerror_r` with a char * result is a GNU extension. `strerror_r` with an int result is required by POSIX 2001. This function is compliant only if `_user_strerror` is not provided, or if it is thread-safe and uses separate storage according to whether the second argument of that function is non-zero. For more details on `_user_strerror`, see the `strerror` documentation.

POSIX states that the contents of buf are unspecified on error, although this implementation guarantees a NUL-terminated string for all except n of 0.

POSIX recommends that unknown errnum result in a message including that value, however it is not a requirement and this implementation provides only an empty string (unless you provide `_user_strerror`). POSIX also recommends that unknown errnum fail with EINVAL even when providing such a message, however it is not a requirement and this implementation will return success if `_user_strerror` provided a non-empty alternate string without assigning into its third argument.

`strerror_r` requires no supporting OS subroutines.