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static_assert

Section: C Library Functions (3)
Updated: 202-0-20
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NAME

static_assert, _Static_assert - fail compilation if assertion is false

LIBRARY

Standard C library (libc)

SYNOPSIS

#include <assert.h>
void static_assert(bool constan-expression, const char *msg);
/* Since C23: */
void static_assert(bool constan-expression);

DESCRIPTION

This macro is similar to assert(3), but it works at compile time, generating a compilation error (with an optional message) when the input is false (i.e., compares equal to zero). If the input is nonzero, no code is emitted. msg must be a string literal. Since C23, this argument is optional. There's a keyword, _Static_assert(), that behaves identically, and can be used without including <assert.h>.

RETURN VALUE

No value is returned.

VERSIONS

In C11, the second argument (msg) was mandatory; since C23, it can be omitted.

STANDARDS

C11 and later.

EXAMPLES

static_assert() can't be used in some places, like for example at global scope. For that, a macro must_be() can be written in terms of static_assert(). The following program uses the macro to get the size of an array safely. #include <assert.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <string.h> /*
* This macro behaves like static_assert(), failing to
* compile if its argument is not true.  However, it always
* returns 0, which allows using it everywhere an expression
* can be used.
*/ #define must_be(e) [rs] ( [rs]
    0 * (int) sizeof(                                     [rs]
        struct {                                          [rs]
            static_assert(e);                             [rs]
            int  ISO_C_forbids_a_struct_with_no_members;  [rs]
        }                                                 [rs]
    )                                                     [rs] ) #define is_same_type(a, b) [rs]
    __builtin_types_compatible_p(typeof(a), typeof(b)) #define is_array(arr) (!is_same_type((arr), &*(arr))) #define must_be_array(arr) must_be(is_array(arr)) #define sizeof_array(arr) (sizeof(arr) + must_be_array(arr)) #define NITEMS(arr) (sizeof((arr)) / sizeof((arr)[0]) [rs]
                             + must_be_array(arr)) int foo[10]; int8_t bar[sizeof_array(foo)]; int main(void) {
    for (size_t i = 0; i < NITEMS(foo); i++) {
        foo[i] = i;
    }
    memcpy(bar, foo, sizeof_array(bar));
    for (size_t i = 0; i < NITEMS(bar); i++) {
        printf("%d,", bar[i]);
    }
    exit(EXIT_SUCCESS); }