The <assert.h> include file defines the assert macro. You must include assert.h when you use assert.
The definition of assert is in an #ifndef preprocessor block. If you have not defined the identifier NDEBUG through a #define directive or on the compilation command, the assert macro tests the assertion expression. If the assertion is false, the system prints a message to stderr, and raises an abort signal for the program. The system also does a Dump Job (DMPJOB) OUTPUT(*PRINT) when the assertion is false.
If NDEBUG is defined, assert is defined to do nothing. You can suppress program assertions by defining NDEBUG.