assert() — Verify Condition

Standards/Extensions	C or C++	Dependencies
ISO C POSIX.1 XPG4 XPG4.2	both

Format

#include <assert.h>

void assert(int expression);

General Description

A macro that inserts diagnostics into a program. If the expression is false (zero), a diagnostic message of the form shown below is printed to stderr, and abort() is called to abnormally end the program. The assert() macro takes no action if the expression is true (nonzero).

The diagnostic message has the format:

   Assertion failed: expression, file filename, line line-number.

If you define NDEBUG to any value with a #define directive or with the DEFINE compiler option, the preprocessor expands all assert() invocations to void expressions.

Note: Using the #undef directive with the assert() macro results in undefined behavior. The assert() macro uses _FILE_ and _LINE_.

Returned Value

The assert() function has no returned value.

Example

CBC3BA08

/* CBC3BA08
   In this example, the assert() macro tests the string argument for a
   null string and an empty string, and verifies that the length argument
   is positive before proceeding.
 */
#include <stdio.h>
#include <assert.h>

void analyze(char *, int);
int main(void)
{
   char *string1 = "ABC";
   char *string2 = "";
   int length = 3;

   analyze(string1, length);
   printf("string1 %s is not null or empty, "
          "and has length %d \n", string1, length);
   analyze(string2, length);
   printf("string2 %s is not null or empty,"
          "and has length %d\n", string2, length);
}

void analyze(char *string, int length)
{
   assert(string != NULL);            /* cannot be NULL */
   assert(*string != '\0');    /* cannot be empty */
   assert(length > 0);          /* must be positive */
}

Output

String1 ABC is not null or empty, and has length 3
Assertion failed: *string != '\0', file: CBC3BA08 C       A1, line: 26