static_assert(3)

💖 static_assert(3) 💖

okay bestie so this is a page from the Library Functions Manual — it's static_assert, and it's about to be your new fave compile-time bestie. no cap.

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✨ NAME ✨

static_assert, _Static_assert — fail compilation if assertion is false

like. it literally stops the build. on purpose. iconic behavior honestly.

📚 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 🦋

so this macro is basically assert(3)'s glow-up — same energy, but it works at compile time. if the input is false (i.e., it compares equal to zero), you get a compilation error, and the message is optional. 💅

and if the input is nonzero? literally nothing happens. no code is emitted. the compiler just vibes. ✨

ngl, msg has to be a string literal. like, that's the rules. but since C23, that second argument is optional, so you can just skip it. 🫶

also there's a keyword, _Static_assert(), that behaves identically, and the way that it works without including <assert.h>… chef's kiss. you don't even need the header. main character energy fr fr.

🍓 RETURN VALUE 🍓

No value is returned. like, at all. zero. it's not that kind of macro, bestie. 🙈

🌈 STANDARDS 🌈

C23.

⭐ HISTORY ⭐

C11.

back in C11, the second argument (msg) was mandatory — you had to give it one. but since C23, it can be omitted. glow-up confirmed. 👑

🌸 EXAMPLES 🌸

okay so quick tea: static_assert() can't be used in some places, like for example at global scope. 😭 so if you want it there, you can write a macro called must_be() in terms of static_assert(). the program below uses that macro to get the size of an array safely. obsess.

#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)

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and that's the whole page bestie!! 💖 written up in Linux man-pages 6.20, dated 2026-04-20 — that's the source and version and date, so now you know exactly which era of the docs you're reading from. slay, go assert something at compile time, ily!! 🧚💜