Assert.h

The assert macro implements runtime assertion. If the expression within it is false, the macro will print a message to stderr and call abort(), defined in stdlib.h. The message includes the source filename and the source line number from the macros __FILE__ and __LINE__, respectively.^[2] Since C99, the name of the function the assert statement is included as (__FUNC__) and the expression itself.^[3] In ANSI C, the expression in the assert macro is defined as signed integer, although any expression that can be implicitly cast to a signed integer may be used. In C99, the assert macro explicitly allows any scalar type.^[4] Two common uses of the assert macro are to assert that a pointer is not null and to ensure that an array index is in-bounds.^[5]

Below is a program using the assert macro. This program will always evaluate pointer as false, as pointer is a null pointer and does not point to a valid memory location:

#include <assert.h>
int main()
{
    void* pointer = 0;
    assert(pointer);
    return 0;
}

Upon compiling the program and running it, a message similar to the following will be output:

program: source.c:5: main: Assertion 'pointer' failed.
Aborted (core dumped)

The definition of the assert macro changes depending on the definition of another macro, NDEBUG. If NDEBUG is defined as a macro name, the assert macro is defined as #define assert(ignore) ((void)0),^[3] thus resulting in the macro not evaluating the expression. The use of NDEBUG may affect the overall behavior of a program if one or more assert statements contain side effects, as these statements are not evaluated.^[6]

The assert macro does not include an error message. However the comma operator can be used to add it to the printed expression, as in assert(("Not Orwellian", 2 + 2 == 5));.^[7]