static_assert(3)

NAME
static_assert, _Static_assert - fail compilation if assertion is false

LIBRARY
Standard C library (libc)

SYNOPSIS
#include <assert.h>

void static_assert(bool constant-expression, const char *msg);

/* Since C23: */
void static_assert(bool constant-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.

STANDARDS
C23.

HISTORY
C11.

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

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)                                        \
    (                                                         \
        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);
    }

SEE ALSO
assert(3)