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Evaluate assertion

If the argument expression of this macro with functional form compares equal to zero (i.e., the expression is false), a message is written to the standard error device and abort is called, terminating the program execution.

The specifics of the message shown depend on the particular library implementation, but it shall at least include: the expression whose assertion failed, the name of the source file, and the line number where it happened. A usual expression format is:

Assertion failed: expression, file filename, line line number

This macro is disabled if, at the moment of including <assert.h>, a macro with the name NDEBUG has already been defined. This allows for a coder to include as many assert calls as needed in a source code while debugging the program and then disable all of them for the production version by simply including a line like:

#define NDEBUG

at the beginning of the code, before the inclusion of <assert.h>.

Therefore, this macro is designed to capture programming errors, not user or run-time errors, since it is generally disabled after a program exits its debugging phase.


void assert (int expression);


Expression to be evaluated. If this expression evaluates to 0, this causes an assertion failure that terminates the program.

Return Value



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

void print_number(int *myInt) {
  assert (myInt!=NULL);
  printf ("Value: %d\n",*myInt);

int main (int argc, char *argv[])
  int a = 15;
  int *b = NULL;
  int *c = NULL;

  b = &a;

  printf("b: %p\n", b);
  print_number (b);
  print_number (c);

  return 0;
$ ./code 
b: 0xffa3cc54
Value: 15
code: code.c:5: print_number: Assertion `myInt!=NULL' failed.