assert.h is a header file in the standard library of the C programming language that defines the C preprocessor macro assert().[1] [2] In C++ it is also available through the <cassert> header file.


assert(a != 1);

This is a macro that implements a runtime assertion, which can be used to verify assumptions made by the program and print a diagnostic message if this assumption is false.

When executed, if the expression is false (that is, compares equal to 0), assert() will write information about the call that failed on stderr and then call abort(). The information it writes to stderr includes:

  • the source filename (the predefined macro __FILE__)
  • the source line number (the predefined macro __LINE__)
  • the source function (the predefined identifier __func__) (added in C99)
  • the text of expression that evaluated to 0 [1]

Example output of a program compiled on Linux:

program: program.c:5: main: Assertion `a != 1' failed.
Abort (core dumped)

Programmers can eliminate the assertions just by recompiling the program, without changing the source code: if the macro NDEBUG is defined before the inclusion of <assert.h>, the assert() macro may be defined simply as:

#define assert(ignore)((void) 0)

and therefore has no effect on the compilation unit, not even evaluating its argument. Therefore expressions passed to assert() must not contain side-effects since they will not happen when debugging is disabled. For instance:

assert(x = gets());

will not read a line and not assign to x when debugging is disabled.

Additional message

Although Microsoft has its own "assert with message" macro, there is no standarized variant of assert() that includes an error message. This can nevertheless be achieved using a comma operator, which discards all preceding values and only keep the last one:

assert(("Five fingers!", 2 + 2 == 5));
// or
#define assertmsg(x, msg) assert(((void) msg, x))
assertmsg(2 + 2 == 5, "Five fingers!");

Will yield something similar to:

program: program.c:5: main: Assertion `("Five fingers!", 2 + 2 == 5)' failed.

Static Assert

static_assert(sizeof(int)>20, "I need huge integers");

C++11 added a similar macro static_assert[3] that computes the value and prints a message at compile-time if it is false. It is possible to simulate this using a macro and templates, though most modern C++ compilers include built-in support (and they might not require the header file).

This feature was formally added in C11 as the keyword _Static_assert with an identical usage, and in <assert.h> a convenience macro static_assert is added.

It is possible to simulate a static assertion in older versions of C using a macro: #define static_assert(cond, str) char _temp[-!((void)str, (cond))], though the resulting error is cryptic. (It triggers an error because C allows zero-length arrays, but not negative-length ones.) Gnulib's version of the static assert uses sizeof and a structure to trigger a similar error.[4]


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

int test_assert(int x)
    assert(x <= 4);
    return x;

int main()
    int i;

    for (i=0; i<=9; i++)
        printf("i = %d\n", i);

    return 0;
i = 0
i = 1
i = 2
i = 3
i = 4
assert: assert.c:6: test_assert: Assertion `x <= 4' failed.


  1. International Standard for Programming Language C (C99), ISO/IEC 9899:1999, p. 169
  2. [Coding Programmer Page C / C++ Reference].
  4. "gnulib/lib/verify.h". coreutils. 24 November 2019.
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