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routines for build-time assertions


Rusty Russell <>


This code provides routines which will cause compilation to fail should some assertion be untrue: such failures are preferable to run-time assertions, but much more limited since they can only depends on compile-time constants.

These assertions are most useful when two parts of the code must be kept in sync: it is better to avoid such cases if possible, but seconds best is to detect invalid changes at build time.

For example, a tricky piece of code might rely on a certain element being at the start of the structure. To ensure that future changes don't break it, you would catch such changes in your code like so:


#include <stddef.h>
#include <ccan/build_assert/build_assert.h>

struct foo {
        char string[5];
        int x;

static char *foo_string(struct foo *foo)
        // This trick requires that the string be first in the structure
        BUILD_ASSERT(offsetof(struct foo, string) == 0);
        return (char *)foo;


CC0 (Public domain)