# set_constraint_handler_s, constraint_handler_t

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Defined in header `<stdlib.h>` |  |
constraint_handler_t set_constraint_handler_s( constraint_handler_t handler ); |  | (since C11)

Configures the handler to be called by all bounds-checked functions on a runtime constraint violation or restores the default handler (if `handler` is a null pointer)

The handler must be a pointer to function of type constraint_handler_t, which is defined as

Defined in header `<stdlib.h>` |  |
typedef void (*constraint_handler_t)( const char *restrict msg, void *restrict ptr, errno_t error); |  | (since C11)

On a runtime constraint violation, it is called with the following arguments:

1) pointer to character string that describes the error
2) pointer to an implementation-defined object or a null pointer. Examples of implementation-defined objects are objects that give the name of the function that detected the violation and the line number when the violation was detected
3) the error about to be returned by the calling function, if it happens to be one of the functions that return `errno_t`

If `set_constraint_handler_s` is never called, the default handler is implementation-defined: it may be abort_handler_s, ignore_handler_s, or some other implementation-defined handler.

  As with all bounds-checked functions, `set_constraint_handler_s` and `constraint_handler_t` 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 `<stdlib.h>`.

### Parameters

handler | - | pointer to function of type constraint_handler_t or a null pointer

### Return value

A pointer to the previously-installed runtime constraints handler. (note: this pointer is never a null pointer because calling set_constraint_handler_s( NULL ) sets up the system default handler)

### Example

Run this code

    #define __STDC_WANT_LIB_EXT1__ 1
    #include <string.h>
    #include <stdio.h>
    #include <stdlib.h>

    int main(void)
    {
    #ifdef __STDC_LIB_EXT1__
        char dst[2];
        set_constraint_handler_s(ignore_handler_s);
        int r = strcpy_s(dst, sizeof dst, "Too long!");
        printf("dst = \"%s\", r = %d\n", dst, r);
        set_constraint_handler_s(abort_handler_s);
        r = strcpy_s(dst, sizeof dst, "Too long!");
        printf("dst = \"%s\", r = %d\n", dst, r);
    #endif
    }

Possible output:

    dst = "", r = 22
    abort_handler_s was called in response to a runtime-constraint violation.

    The runtime-constraint violation was caused by the following expression in strcpy_s:
    (s1max <= (s2_len=strnlen_s(s2, s1max)) ) (in string_s.c:62)

    Note to end users: This program was terminated as a result
    of a bug present in the software. Please reach out to your
    software's vendor to get more help.
    Aborted

### References

- C11 standard (ISO/IEC 9899:2011):

  - K.3.6/2 constraint_handler_t (p: 604)

  - K.3.6.1.1 The set_constraint_handler_s function (p: 604-605)

### See also

abort_handler_s (C11) | abort callback for the bounds-checked functions (function)
ignore_handler_s (C11) | ignore callback for the bounds-checked functions (function)

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