assert() — Verify condition

Standards

Standards / Extensions	C or C++	Dependencies
ISO C POSIX.1 XPG4 XPG4.2 C99 Single UNIX Specification, Version 3	both

Format

#include <assert.h>

void assert(int expression);

General description

The assert() macro inserts diagnostics into a program. If the expression (which will have a scalar type) is false (that is, compares equal to 0), 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).

Without a compiler that is designed to support C99, or when compiling C++ code, the diagnostic message has the following format:

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

With a compiler that is designed to support C99, or when compiling C++ code, the diagnostic message has the following format:

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

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

Note: The assert() function is a macro. Using the #undef directive with the assert() macro results in undefined behavior. The assert() macro uses __FILE__, __LINE__ and, with a compiler that is designed to support C99, __func__...

Returned value

assert() returns no values.

Example

CELEBA08

⁄* CELEBA08

   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 without a compiler that is designed to support C99

String1 ABC is not NULL or empty, and has length 3
Assertion failed: *string != '\0', file: CELEBA08 C       A1, line: 26

Output with a compiler that is designed to support C99