Theassert()Macro ....................................................................................
The macro assert()can diagnose program bugs. It is defined in assert.h, and its proto-
type is
void assert(int expression);
The argument expressioncan be anything you want to test—a variable or any C expres-
sion. If expressionevaluates to TRUE,assert()does nothing. If expressionevaluates
toFALSE,assert()displays an error message on stderrand aborts program execution.
How do you use assert()? It is most frequently used to track down program bugs
(which are distinct from compilation errors). A bug doesn’t prevent a program from com-
piling, but it causes the program to give incorrect results or to run improperly (locking
up, for example). For instance, a financial-analysis program you’re writing might occa-
sionally give incorrect answers. You suspect that the problem is caused by the variable
interest_ratetaking on a negative value, which should never happen. To check this,
place the statement
assert(interest_rate >= 0);
at locations in the program where interest_rateis used. If the variable ever does
become negative, the assert()macro alerts you. You can then examine the relevant code
to locate the cause of the problem.
To see how assert()works, run Listing 19.3. If you enter a nonzero value, the program
displays the value and terminates normally. If you enter zero, the assert()macro forces
abnormal program termination. The exact error message you see will depend on your
compiler, but here’s a typical example:
Assertion failed: x, file list1903.c, line 13
Note that, in order for assert()to work, your program must be compiled in debug
mode. Refer to your compiler documentation for information on enabling debug mode
(as explained in a moment). When you later compile the final version in release mode,
theassert()macros are disabled.LISTING19.3 assert.c. Using the assert()macro
1: /* The assert() macro. */
2:
3: #include <stdio.h>
4: #include <assert.h>
5:
6: int main( void )
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