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Manual Pages  — ASSERT


assert – expression verification macro



#include <assert.h>



The assert() macro tests the given expression and if it is false, the calling process is terminated. A diagnostic message is written to stderr and the function abort(3) is called, effectively terminating the program.

If expression is true, the assert() macro does nothing.

The assert() macro may be removed at compile time by defining NDEBUG as a macro (e.g., by using the cc(1) option -D NDEBUG). Unlike most other include files, <assert.h> may be included multiple times. Each time whether or not NDEBUG is defined determines the behavior of assert from that point forward until the end of the unit or another include of <assert.h>.

The assert() macro should only be used for ensuring the developer's expectations hold true. It is not appropriate for regular run-time error detection.


The assertion:

    assert(1 == 0);

generates a diagnostic message similar to the following:

    Assertion failed: (1 == 0), function main, file main.c, line 100.

The following assert tries to assert there was no partial read:

    assert(read(fd, buf, nbytes) == nbytes);

However, there are two problems. First, it checks for normal conditions, rather than conditions that indicate a bug. Second, the code will disappear if NDEBUG is defined, changing the semantics of the program.


abort2(2), abort(3)


The assert() macro conforms to ISO/IEC 9899:1999 ("ISO C99").


An assert macro appeared in AT&T v7 .

ASSERT (3) May 31, 2018

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