Explicitly Checking Internal Consistency
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When you're writing a program, it's often a good idea to put in checks at strategic places for ``impossible'' errors or violations of basic assumptions.  These kinds of checks are helpful in debugging problems with the interfaces between different parts of the program, for example.

The assert macro, defined in the header file assert.h, provides a convenient way to abort the program while printing some debugging information about where in the program the error was detected.

Once you think your program is debugged, you can disable the error checks performed by the assert macro by recompiling with the macro NDEBUG defined.  This means you don't actually have to change the program source code to disable these checks.

But disabling these consistency checks is undesirable unless they make the program significantly slower.  All else being equal, more error checking is good no matter who is running the program.  A wise user would rather have a program crash, visibly, than have it return nonsense without indicating anything might be wrong.

 -- Macro: void assert (int EXPRESSION)
     Verify the programmer's belief that EXPRESSION should be nonzero at a certain point in the program.

     If NDEBUG is not defined, assert tests the value of EXPRESSION.  If it is false (zero), assert aborts the program (see Aborting a Program) after printing a message of the form:

          FILE:LINENUM: FUNCTION: Assertion `EXPRESSION' failed.

     on the standard error stream stderr (see Standard Streams). The filename and line number are taken from the C preprocessor macros __FILE__ and __LINE__ and specify where the call to assert was written.  When using the GNU C compiler, the name of the function which calls assert is taken from the built-in variable __PRETTY_FUNCTION__; with older compilers, the function name and following colon are omitted.

     If the preprocessor macro NDEBUG is defined before assert.h is included, the assert macro is defined to do absolutely nothing.  Even the argument expression EXPRESSION is not evaluated, so you should avoid calling assert with arguments that involve side effects.

     For example, assert (++i > 0); is a bad idea, because i will not be incremented if NDEBUG is defined.

Sometimes the ``impossible'' condition you want to check for is an error return from an operating system function.  Then it is useful to display not only where the program crashes, but also what error was returned. The assert_perror macro makes this easy.

 -- Macro: void assert_perror (int ERRNUM)
     Similar to assert, but verifies that ERRNUM is zero.

     If NDEBUG is defined, assert_perror tests the value of ERRNUM.  If it is nonzero, assert_perror aborts the program after a printing a message of the form:

          FILE:LINENUM: FUNCTION: ERROR TEXT

     on the standard error stream.  The file name, line number, and function name are as for assert.  The error text is the result of strerror (ERRNUM).  See Error Messages.

     Like assert, if NDEBUG is defined before assert.h is included, the assert_perror macro does absolutely nothing.  It does not evaluate the argument, so ERRNUM should not have any side effects.  It is best for ERRNUM to be a just simple variable reference; often it will be errno.

     This macro is a GNU extension.

Usage note: The assert facility is designed for detecting internal inconsistency; it is not suitable for reporting invalid input or improper usage.

The information in the diagnostic messages provided by the assert macro is intended to to help you, the programmer, track down the cause of a bug, but is not really useful in telling a user of your program why his or her input was invalid or why a command could not be carried out. So you can't use assert to print the error messages for these eventualities.

What's more, your program should not abort when given invalid input, as assert would do---it should exit with nonzero status after printing its error messages, or perhaps read another command or move on to the next input file.

See Error Messages, for information on printing error messages for problems that do not represent bugs in the program.