NAME

       static_assert, _Static_assert - fail compilation if assertion is false

LIBRARY

       Standard C library (libc)

SYNOPSIS

       #include <assert.h>

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

       /* Since C23: */
       void static_assert(scalar 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.

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 <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);
           }

SEE ALSO

       assert(3)