Assertions (assert())


The basic premise behind an  assert(bool_t value) is that the macro/function checks to make sure that  value is  true. The coder adds the checks where the expected result should be true. (i.e. don’t use them as an if statement).  If the value is not  true (i.e.  false), the software/firmware takes specific action. 

For a software application, this “specific action” may be printing out the filename and line number the  assert() was raised in, and then exiting the application. For a firmware application, this may be printing out the filename and line number the  assert() was raised in, and then restarting (it does not make sense to “quit” in a firmware environment).

Assertions refer to the use of both the standard C library  assert() macro, and the use of similar user-defined macros.

If you are looking for a full-blown assert() example, check out the compilable/testable assert() code snippet at

A Simple assert() Using A Macro

assert() is usually defined in C using a macro (rather than a function), due to the ability of automatically keep track of things such as the filename and line number that the  assert() was written on.

A basic assert macro could have the following definition:

Note that the ternary operator is used here instead of an if statement.

AssertRaised() is a function you have to write that will handle any raised assert() ‘s. It would have the following declaration:

A Note On __FILE__ and __LINE__

__FILE__ and  __LINE__ are special Standard Predefined Macros as defined by the C language specifications. For more info, see the File Names And Line Numbers section of the Preprocessor page.

__FILE__ and  __LINE__ can be removed from the macro in flash constrained embedded systems (storing the filename strings can take up a decent amount of flash).

Note that use of  __FILE__  may not take up as much flash memory as you think, as most compilers with any sort of optimisation turned on will collate all string literals which are exactly the same into one (i.e. all the the __FILE__ pointers in the same file will point to the same string literal).

Are there any other useful preprocessor keywords? How about __func__? This gets replaced with a const char * that contains the name of the current function the macro is in.

Printing The Failed assert() Condition

Even more useful than just having the filename and line number is also printing the expression which caused the failed  assert() . Luckily, this is easy to do, using the preprocessor stringification symbol#.

The function which handled the failed  assert() would then have the following declaration:

Note that this would consume much more flash memory than __FILE__ alone would, as rarely any of the const char * expression string literals would be the same .

Preventing Side-Effects

assert() checks should NOT have any side effects. That is, they should only do a boolean comparison between two variables/values. You should not embed assignments or function calls inside  assert() checks. This is to prevent the logic of your application changing when enabling/disabling  assert()’s. 

The  assert() macro can be re-written to stop developers from writing  assert()’s with side effects. This can be done by making clever use of the C comma operator (note that this is not the same as the comma used as a separator between variables in a function call).

Is using the GCC compiler, and you write an assert with side effects, you should get the following error:

Note that I was using gcc-5.1 when I was presented with this error.

A screenshot of the C compiler throwing an error when an assignment is attempted within the parameter passed to an assert().
A screenshot of the C compiler throwing an error when an assignment is attempted within the parameter passed to an assert().

Complete Code Example

I have written a working assert() example at It employs all of the functionality described above, including filename and line number reporting, expression printing, and syntax which disallows the use of assignments or function calls within the parameter passed to  assert().

Example assert() code on Ideone (
Example assert() code on Ideone (

Assert With printf() Style Messages

The below macro allows you to use assert with printf() style messages:

A working example of the above code can  be found at

External Links

Assert Yourself, by Niall Murphy, is a good read about using the assert() macro.

Posted: June 18th, 2013 at 9:34 am
Last Updated on: September 20th, 2017 at 6:38 am