How to override assert macro in C?

Question

I want to create my own version of assert in which it does some log prints in case assert was called in NDEBUG mode.

I tried to do the LD_PRELOAD trick and redefine the assert macro but it seems to ignore the macro definition completely and overriding __assert_fail is irrelevant since it isn't called in case of NDEBUG.

How can I override the libc assert macro?

I do not want to create a different function since assert is already used heavily in the project.

Answer 1

Had the same problem using gcc on Cygwin/Windows and on Linux.

My solution is to overwrite the (weak) definition of the actual assertion failed handling function. Here is the code:

/*!
 * Overwrite the standard (weak) definition of the assert failed handling function.
 *
 * These functions are called by the assert() macro and are named differently and
 * have different signatures on different systems.
 * - On Cygwin/Windows its   __assert_func()
 * - On Linux its            __assert_fail()
 *
 * - Output format is changed to reflect the gcc error message style
 *
 * @param filename    - the filename where the error happened
 * @param line        - the line number where the error happened
 * @param assert_func - the function name where the error happened
 * @param expr        - the expression that triggered the failed assert
 */
#if defined( __CYGWIN__ )

void __assert_func( const char *filename, int line, const char *assert_func, const char *expr )

#elif defined( __linux__ )

void __assert_fail ( const char* expr, const char *filename, unsigned int line, const char *assert_func )

#else
# error "Unknown OS! Don't know how to overwrite the assert failed handling function. Follow assert() and adjust!"
#endif
{
    // gcc error message style output format:
    fprintf( stdout, "%s:%d:4: error: assertion \"%s\" failed in function %s\n",
             filename, line, expr, assert_func );

    abort();
}

Answer 2

The C99 rationale provides a sample on how to redefine the assert in a good way on page 113:

#undef assert
#ifdef NDEBUG
#define assert(ignore) ((void)0)
#else
extern void __gripe(char *_Expr, char *_File, int _Line, const char *_Func);
#define assert(expr) \
((expr) ? (void)0 :\
 __gripe(#expr, _ _FILE_ _,_ _LINE_ _,_ _func_ _))
#endif 

I'd include assert.h right before this code to make sure assert.h is used. Also notice that it calls a function that would do reporting logic, so that your code would be smaller.

Answer 3

It is a pretty simple thing to do, since assert is a macro. Given that you have this code:

#define NDEBUG
#include <assert.h>

int main( void )
{
  assert(0);

  return 0;
}

Then just do:

#ifdef NDEBUG
#undef assert
#define assert(x) if(!(x)){printf("hello world!");} // whatever code you want here
#endif

Note that this has to be done after #include <assert.h> though.

So if you want to stick your own definition into a common header file, and then use that header file to modify existing code, then your header file have to be included after assert.h.

my_assert.h

#include <assert.h>
#include <stdio.h>

#ifdef NDEBUG
#undef assert
#define assert(x) if(!(x)){printf("hello world!");}
#endif

main.c

#define NDEBUG    
#include <assert.h>
#include "my_assert.h"

int main( void )
{
  assert(0); // prints "hello world!"
  assert(1); // does nothing

  return 0;
}

Answer 4

An attempt to try to override the assert() macro in a large codebase can be difficult. For example, suppose you have code like:

#include <assert.h>
#include "my_assert.h"
#include "foo.h" // directly or indirectly includes <assert.h>

After this, any use of assert() will again use the system assert() macro and not the one that you have defined in "my_assert.h" (this is apparently part of the C design of the assert macro).

There are ways to avoid this, but you have to use nasty tricks like putting your own assert.h header in the include path before the system assert.h, which is somewhat error prone and non-portable.

I'd recommend using a different named macro than assert, and use regex tricks or a clang-rewriter to rename the various assert macros in your codebase to an assert macro that you can control. Example:

perl -p -i -e 's/\bassert\b *\( */my_assert( /;' `cat list_of_filenames`

(then adding "my_assert.h" or something like it to each of the files modified)

Answer 5

You can check if NDEBUG is defined and if it is then print whatever logs you want to print.