9.2. Diagnostics

While you are debugging programs, it is often useful to check that the value of an expression is the one that you expected. The assert function provides such a diagnostic aid.

In order to use assert you must first include the header file <assert.h>. The function is defined as

#include <assert.h>

void assert(int expression)

If the expression evaluates to zero (i.e. false) then assert will write a message about the failing expression, including the name of the source file, the line at which the assertion was made and the expression itself. After this, the abort function is called, which will halt the program.

assert(1 == 2);

/* Might result in */

Assertion failed: 1 == 2, file silly.c, line 15

Assert is actually defined as a macro, not as a real function. In order to disable assertions when a program is found to work satisfactorily, defining the name NDEBUG before including <assert.h> will disable assertions totally. You should beware of side effects that the expression may have: when assertions are turned off with NDEBUG, the expression is not evaluated. Thus the following example will behave unexpectedly when debugging is turned off with the #define NDEBUG.

#define NDEBUG
#include <assert.h>

void
func(void)
{
        int c;
        assert((c = getchar()) != EOF);
        putchar(c);
}

Note that assert returns no value.