assert Macro

If the argument is false, an assertion failure message is printed and the program is aborted.




void assert(scalar expression);


The expression to test.


The expression evaluates to zero or non-zero. If zero, the assertion fails, a message is printed and abort() is called, terminating program execution. In the case of MPLAB XC8, the message is printed to stdout; for other compilers, this is printed to stderr.

The message includes the source file name (__FILE__), the source line number (__LINE__), and the expression being evaluated.

If the macro NDEBUG (see NDEBUG Macro) is defined at the point where <assert.h> is included, the assert() macro will evaluate to a void expression, ((void)0), and not print any assertion message, nor abort program execution. Inclusion of <assert.h> can occur multiple times, even in the same source file, and the action of the assert() macro for each inclusion will be based on the state of NDEBUG at the point at which that inclusion takes place.


See the notes at the beginning of this chapter or section for information on using printf() or scanf() (and other functions reading and writing the stdin or stdout streams) in the example code.

int main(void)
  int a;

  a = 4;
#define NDEBUG        /* negate debugging - disable assert() functionality */
#include <assert.h>
  assert(a == 6);     /* no action performed, even though expression is false */

#undef NDEBUG         /* ensure assert() is active */
#include <assert.h>
  a = 7;
  assert(a == 7);     /* true - no action performed */
  assert(a == 8);     /* false - print message and abort */

Example Output

sampassert.c:14 a == 8 -- assertion failed