libc.info: assert

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`assert'--Macro for Debugging Diagnostics

*Synopsis*
     #include <assert.h>
     void assert(int EXPRESSION);
   *Description*
Use this macro to embed debuggging diagnostic statements in your
programs.  The argument EXPRESSION should be an expression which
evaluates to true (nonzero) when your program is working as you
intended.
   When EXPRESSION evaluates to false (zero), `assert' calls `abort',
after first printing a message showing what failed and where:
      Assertion failed: EXPRESSION, file FILENAME, line LINENO
   The macro is defined to permit you to turn off all uses of `assert'
at compile time by defining `NDEBUG' as a preprocessor variable.   If
you do this, the `assert' macro expands to

(void(0))

*Returns*
`assert' does not return a value.
*Portability*
The `assert' macro is required by ANSI, as is the behavior when
`NDEBUG' is defined.
   Supporting OS subroutines required (only if enabled): `close',
`fstat', `getpid', `isatty', `kill', `lseek', `read', `sbrk', `write'.