# set_constraint_handler_s, constraint_handler_t

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< c‎ | error

Defined in header ``<stdlib.h>`` |  |
constraint_handler_t set_constraint_handler_s( constraint_handler_t handler ); | (1) | (since C11)
typedef void (*constraint_handler_t)( const char* restrict msg, void* restrict ptr, errno_t error ); | (2) | (since C11)

1) 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).
2) The pointer to a handler that will be called on a runtime constraint violation.

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
msg | - | pointer to character string that describes the error
ptr | - | 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
error | - | the error about to be returned by the calling function, if it happens to be one of the functions that return `errno_t`

### 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

- C23 standard (ISO/IEC 9899:2024):

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

  - K.3.6.1.1 The set_constraint_handler_s function (p: TBD)

- C17 standard (ISO/IEC 9899:2018):

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

  - K.3.6.1.1 The set_constraint_handler_s function (p: TBD)

- 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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