strerror, strerror_s, strerrorlen_s

From cppreference.com

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, "ja_JP.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 Japanese: %s\n", errmsg);
#endif
    }
}

Possible output:

File opening error: No such file or directory
Now in German: Datei oder Verzeichnis nicht gefunden
Now in Japanese: そのようなファイル、又はディレクトリはありません

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

Compiler support
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Technical Specifications
Symbol index

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