Assert is a standard library function.  If the value you pass it is or evaluates to zero, assert will print which line in which file the assert failed and will exit the program.

Code:

#include <assert.h>

Usage:

// Is this pointer NULL?

assert(pointervar);

// Is the int initialized to zero?

assert(varname == 0);

// Is this number odd?

assert(varname%2);

Extensions:

None known.

Sources:

The C Programming Language 2nd Ed., Kernighan & Ritchie, pp. 253