Static assert in C

Question

What\'s the best way to achieve compile time static asserts in C (not C++), with particular emphasis on GCC?

Answer 1

Because:

1. _Static_assert() is now defined in gcc for all versions of C, and
2. static_assert() is defined in C++11 and later

The following simple macro for STATIC_ASSERT() therefore works in:

1. C++:
   1. C++11 (g++ -std=c++11) or later
2. C:
   1. gcc -std=c90
   2. gcc -std=c99
   3. gcc -std=c11
   4. gcc (no std specified)

Define STATIC_ASSERT as follows:

/* For C++: */
#ifdef __cplusplus
    #ifndef _Static_assert
        #define _Static_assert static_assert /* `static_assert` is part of C++11 or later */
    #endif
#endif
/* Now for gcc (C) (and C++, given the define above): */
#define STATIC_ASSERT(test_for_true) _Static_assert((test_for_true), "(" #test_for_true ") failed")

Now use it:

STATIC_ASSERT(1 > 2); // Output will look like: error: static assertion failed: "(1 > 2) failed" 

Examples:

Tested in Ubuntu using gcc 4.8.4:

Example 1: good gcc output (ie: the STATIC_ASSERT() codes works, but the condition was false, causing a compile-time assert):

$ gcc -Wall -o static_assert static_assert.c && ./static_assert
static_assert.c: In function ‘main’
static_assert.c:78:38: error: static assertion failed: "(1 > 2) failed"
#define STATIC_ASSERT(test_for_true) _Static_assert((test_for_true), "(" #test_for_true ") failed")
^
static_assert.c:88:5: note: in expansion of macro ‘STATIC_ASSERT’
STATIC_ASSERT(1 > 2);
^

Example 2: good g++ -std=c++11 output (ie: the STATIC_ASSERT() codes works, but the condition was false, causing a compile-time assert):

$ g++ -Wall -std=c++11 -o static_assert static_assert.c && ./static_assert
static_assert.c: In function ‘int main()’
static_assert.c:74:32: error: static assertion failed: (1 > 2) failed
#define _Static_assert static_assert /* static_assert is part of C++11 or later */
^
static_assert.c:78:38: note: in expansion of macro ‘_Static_assert’
#define STATIC_ASSERT(test_for_true) _Static_assert((test_for_true), "(" #test_for_true ") failed")
^
static_assert.c:88:5: note: in expansion of macro ‘STATIC_ASSERT’
STATIC_ASSERT(1 > 2);
^

Example 3: failed C++ output (ie: the assert code doesn't work properly at all, since this is using a version of C++ before C++11):

$ g++ -Wall -o static_assert static_assert.c && ./static_assert
static_assert.c:88:5: warning: identifier ‘static_assert’ is a keyword in C++11 [-Wc++0x-compat]
STATIC_ASSERT(1 > 2);
^
static_assert.c: In function ‘int main()’
static_assert.c:78:99: error: ‘static_assert’ was not declared in this scope
#define STATIC_ASSERT(test_for_true) _Static_assert((test_for_true), "(" #test_for_true ") failed")
^
static_assert.c:88:5: note: in expansion of macro ‘STATIC_ASSERT’
STATIC_ASSERT(1 > 2);
^

Full test results here:

/*
static_assert.c
- test static asserts in C and C++ using gcc compiler

Gabriel Staples
4 Mar. 2019 

To be posted in:
1. https://stackoverflow.com/questions/987684/does-gcc-have-a-built-in-compile-time-assert/987756#987756
2. https://stackoverflow.com/questions/3385515/static-assert-in-c/7287341#7287341

To compile & run:
  C:
    gcc -Wall -o static_assert static_assert.c && ./static_assert
    gcc -Wall -std=c90 -o static_assert static_assert.c && ./static_assert
    gcc -Wall -std=c99 -o static_assert static_assert.c && ./static_assert
    gcc -Wall -std=c11 -o static_assert static_assert.c && ./static_assert
  C++:
    g++ -Wall -o static_assert static_assert.c && ./static_assert
    g++ -Wall -std=c++98 -o static_assert static_assert.c && ./static_assert
    g++ -Wall -std=c++03 -o static_assert static_assert.c && ./static_assert
    g++ -Wall -std=c++11 -o static_assert static_assert.c && ./static_assert

-------------
TEST RESULTS:
-------------

1. `_Static_assert(false, "1. that was false");` works in:
  C:
    gcc -Wall -o static_assert static_assert.c && ./static_assert             YES
    gcc -Wall -std=c90 -o static_assert static_assert.c && ./static_assert    YES
    gcc -Wall -std=c99 -o static_assert static_assert.c && ./static_assert    YES
    gcc -Wall -std=c11 -o static_assert static_assert.c && ./static_assert    YES
  C++:
    g++ -Wall -o static_assert static_assert.c && ./static_assert             NO
    g++ -Wall -std=c++98 -o static_assert static_assert.c && ./static_assert  NO
    g++ -Wall -std=c++03 -o static_assert static_assert.c && ./static_assert  NO
    g++ -Wall -std=c++11 -o static_assert static_assert.c && ./static_assert  NO

2. `static_assert(false, "2. that was false");` works in:
  C:
    gcc -Wall -o static_assert static_assert.c && ./static_assert             NO
    gcc -Wall -std=c90 -o static_assert static_assert.c && ./static_assert    NO
    gcc -Wall -std=c99 -o static_assert static_assert.c && ./static_assert    NO
    gcc -Wall -std=c11 -o static_assert static_assert.c && ./static_assert    NO
  C++:
    g++ -Wall -o static_assert static_assert.c && ./static_assert             NO
    g++ -Wall -std=c++98 -o static_assert static_assert.c && ./static_assert  NO
    g++ -Wall -std=c++03 -o static_assert static_assert.c && ./static_assert  NO
    g++ -Wall -std=c++11 -o static_assert static_assert.c && ./static_assert  YES

3. `STATIC_ASSERT(1 > 2);` works in:
  C:
    gcc -Wall -o static_assert static_assert.c && ./static_assert             YES
    gcc -Wall -std=c90 -o static_assert static_assert.c && ./static_assert    YES
    gcc -Wall -std=c99 -o static_assert static_assert.c && ./static_assert    YES
    gcc -Wall -std=c11 -o static_assert static_assert.c && ./static_assert    YES
  C++:
    g++ -Wall -o static_assert static_assert.c && ./static_assert             NO
    g++ -Wall -std=c++98 -o static_assert static_assert.c && ./static_assert  NO
    g++ -Wall -std=c++03 -o static_assert static_assert.c && ./static_assert  NO
    g++ -Wall -std=c++11 -o static_assert static_assert.c && ./static_assert  YES

*/

#include 
#include 

/* For C++: */
#ifdef __cplusplus
    #ifndef _Static_assert
        #define _Static_assert static_assert /* `static_assert` is part of C++11 or later */
    #endif
#endif
/* Now for gcc (C) (and C++, given the define above): */
#define STATIC_ASSERT(test_for_true) _Static_assert((test_for_true), "(" #test_for_true ") failed")


int main(void)
{
    printf("Hello World\n");

    /*_Static_assert(false, "1. that was false");*/
    /*static_assert(false, "2. that was false");*/

    STATIC_ASSERT(1 > 2);

    return 0;
}

Related:

1. Use static_assert to check types passed to macro [my own answer]
   1. https://en.cppreference.com/w/cpp/types/is_same
   2. https://en.cppreference.com/w/cpp/language/decltype
2. Use static_assert to check types passed to macro
3. How to use static assert in C to check the types of parameters passed to a macro