strerror, strerror_s, strerrorlen_s

Defined in header `<string.h>`
char* strerror( int errnum );	(1)
errno_t strerror_s( char *buf, rsize_t bufsz, errno_t errnum );	(2)	(since C11)
size_t strerrorlen_s( errno_t errnum );	(3)	(since C11)

1) Returns a pointer to the textual description of the system error code errnum, identical to the description that would be printed by perror().

errnum is usually acquired from the errno variable, however the function accepts any value of type int. The contents of the string are locale-specific.

The returned string must not be modified by the program, but may be overwritten by a subsequent call to the strerror function. strerror is not required to be thread-safe. Implementations may be returning different pointers to static read-only string literals or may be returning the same pointer over and over, pointing at a static buffer in which strerror places the string.

2) Same as (1), except that the message is copied into user-provided storage buf. No more than bufsz-1 bytes are written, the buffer is always null-terminated. If the message had to be truncated to fit the buffer and bufsz is greater than 3, then only bufsz-4 bytes are written, and the characters "..." are appended before the null terminator. In addition, the following errors are detected at runtime and call the currently installed constraint handler function:

buf is a null pointer
bufsz is zero or greater than RSIZE_MAX

The behavior is undefined if writing to buf occurs past the end of the array, which can happen when the size of the buffer pointed to by buf is less than the number of characters in the error message which in turn is less than bufsz.

3) Computes the length of the untruncated locale-specific error message that strerror_s would write if it were called with errnum. The length does not include the null terminator.

As with all bounds-checked functions, strerror_s and strerrorlen_s are only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including string.h.

Parameters

errnum	-	integral value referring to an error code
buf	-	pointer to a user-provided buffer
bufsz	-	size of the user-provided buffer

Return value

1) Pointer to a null-terminated byte string corresponding to the errno error code errnum.

2) Zero if the entire message was successfully stored in buf, non-zero otherwise.

3) Length (not including the null terminator) of the message that strerror_s would return

Notes

POSIX allows subsequent calls to strerror to invalidate the pointer value returned by an earlier call. It also specifies that it is the LC_MESSAGES locale facet that controls the contents of these messages.

strerror_s is the only bounds-checked function that allows truncation, because providing as much information as possible about a failure was deemed to be more desirable. POSIX also defines strerror_r for similar purposes.

Example

Run this code

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <locale.h>
 
int main(void)
{
    FILE *fp = fopen(tmpnam((char[L_tmpnam]){0}), "r");
    if(fp==NULL) {
        printf("File opening error: %s\n", strerror(errno));
        setlocale(LC_MESSAGES, "de_DE.utf8");
        printf("Now in German: %s\n", strerror(errno));
#ifdef __STDC_LIB_EXT1__
        setlocale(LC_ALL, "zh_CN.utf8"); // printf needs CTYPE for multibyte output
        size_t errmsglen = strerrorlen_s(errno) + 1;
        char errmsg[errmsglen]; 
        strerror_s(errmsg, errmsglen, errno);
        printf("Now in Chinese: %s\n", errmsg);
#endif
    }
}

Possible output:

File opening error: No such file or directory
Now in German: Datei oder Verzeichnis nicht gefunden
Now in Chinese: 没有那个文件或目录

References

C11 standard (ISO/IEC 9899:2011):

7.24.6.2 The strerror function (p: 371)

K.3.7.4.2 The strerror_s function (p: 622)

K.3.7.4.3 The strerrorlen_s function (p: 623)

C99 standard (ISO/IEC 9899:1999):

7.21.6.2 The strerror function (p: 334)

C89/C90 standard (ISO/IEC 9899:1990):

4.11.6.2 The strerror function

Language
headers
Type support
Dynamic memory management
Error handling
Program utilities
Variadic function support
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Thread support (C11)
Atomic operations (C11)
Technical Specifications

perror	displays a character string corresponding of the current error to stderr (function)
errno	macro which expands to POSIX-compatible thread-local error number variable (macro variable)
C++ documentation for strerror