Static Assertion

Static assertions are used to check if a condition is true when the code is compiled. If it isn’t, the compiler is required to issue an error message and stop the compiling process.

A static assertion is one that is checked at compile time, not run time. The condition must be a constant expression, and if false will result in a compiler error. The first argument, the condition that is checked, must be a constant expression, and the second a string literal.

Unlike assert, _Static_assert is a keyword. A convenience macro static_assert is defined in <assert.h>.

#include <assert.h>

enum {N = 5};
_Static_assert(N == 5, "N does not equal 5");
static_assert(N > 10, "N is not greater than 10");  /* compiler error */

Prior to C11, there was no direct support for static assertions. However, in C99, static assertions could be emulated with macros that would trigger a compilation failure if the compile time condition was false. Unlike _Static_assert, the second parameter needs to be a proper token name so that a variable name can be created with it. If the assertion fails, the variable name is seen in the compiler error, since that variable was used in a syntactically incorrect array declaration.

#define STATIC_MSG(msg, l) STATIC_MSG2(msg, l)
#define STATIC_MSG2(msg,l) on_line_##l##__##msg
#define STATIC_ASSERT(x, msg) extern char STATIC_MSG(msg, __LINE__) [(x)?1:-1]
 
enum { N = 5 };
STATIC_ASSERT(N == 5, N_must_equal_5);
STATIC_ASSERT(N > 5, N_must_be_greater_than_5); /* compile error */

Before C99, you could not declare variables at arbitrary locations in a block, so you would have to be extremely cautious about using this macro, ensuring that it only appears where a variable declaration would be valid.