static_assert, _Static_assert - fail compilation if assertion is false

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

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>.

No value is returned.

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 <stddef.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)                                        \
(                                                         \
    0 * (int) sizeof(                                     \
        struct {                                          \
            static_assert(e);                             \
            int  ISO_C_forbids_a_struct_with_no_members;  \
        }                                                 \
    )                                                     \
)
#define is_same_type(a, b)  \
    __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]) \
                             + 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);
}

assert(3)