# <assert.h>

<head><title>&lt;assert.h&gt;</title></head>

# `<assert.h>`

* * *

Include the standard header **`<assert.h>`**
to define the macro
[`assert`](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_assert.html#assert),
which is useful for diagnosing logic errors in
the program. You can eliminate the testing code produced by the macro
`assert` without removing the macro references from the program
by defining the macro
**`NDEBUG`**
in the program before you include
`<assert.h>`. Each time the program includes this header,
it redetermines the definition of the macro `assert`.

    #undef assert
    #if defined NDEBUG
    #define assert(test) (void)0
    #else
    #define assert(test) <void expression>
    #endifCopy to clipboard

## `assert`

    #undef assert
    #if defined NDEBUG
    #define assert(test) (void)0
    #else
    #define assert(test) <void expression>
    #endifCopy to clipboard

If the *int* expression `test` equals zero,
the macro writes to
[`stderr`](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_stdio.html#stderr)
a diagnostic message that includes:

- the text of `test`
- the source filename (the predefined macro
[`__FILE__`](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_preproc.html#__FILE__))
- the source line number (the predefined macro
[`__LINE__`](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_preproc.html#__LINE__))
- the function name (the predefined object
[`__func__`](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_preproc.html#__func__),
added with [C99](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_lib_over.html#C99))

It then calls
[`abort`](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_stdlib.html#abort).

You can write the macro `assert` in the program in any
[side-effects context](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_function.html#side-effects%20context).

* * *

See also the
**[Table of Contents](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_index.html#Table%20of%20Contents)** and the
**[Index](https://docs.qualcomm.com/doc/80-N3040-23/topic/_c_index.html)**.

*[Copyright](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_crit_pjp.html) © 1992-2006by P.J. Plauger and Jim Brodie. All rights reserved.*

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Last Published: Oct 06, 2026

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