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-@node Consistency Checking, Mathematics, Low-Level Terminal Interface, Top
-@chapter Explicitly Checking Internal Consistency
-@cindex consistency checking
-@cindex impossible events
-@cindex assertions
-
-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.
-
-@pindex assert.h
-The @code{assert} macro, defined in the header file @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.
-
-@vindex NDEBUG
-Once you think your program is debugged, you can disable the error
-checks performed by the @code{assert} macro by recompiling with the
-macro @code{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.
-
-@comment assert.h
-@comment ANSI
-@deftypefn Macro void assert (int @var{expression})
-Verify the programmer's belief that @var{expression} should be nonzero
-at a certain point in the program.
-
-If @code{NDEBUG} is not defined, @code{assert} tests the value of
-@var{expression}. If it is false (zero), @code{assert} aborts the
-program (@pxref{Aborting a Program}) after printing a message of the
-form:
-
-@smallexample
-@file{@var{file}}:@var{linenum}: @var{function}: Assertion `@var{expression}' failed.
-@end smallexample
-
-@noindent
-on the standard error stream @code{stderr} (@pxref{Standard Streams}).
-The filename and line number are taken from the C preprocessor macros
-@code{__FILE__} and @code{__LINE__} and specify where the call to
-@code{assert} was written. When using the GNU C compiler, the name of
-the function which calls @code{assert} is taken from the built-in
-variable @code{__PRETTY_FUNCTION__}; with older compilers, the function
-name and following colon are omitted.
-
-If the preprocessor macro @code{NDEBUG} is defined before
-@file{assert.h} is included, the @code{assert} macro is defined to do
-absolutely nothing. Even the argument expression @var{expression} is
-not evaluated, so you should avoid calling @code{assert} with arguments
-that involve side effects.
-
-For example, @code{assert (++i > 0);} is a bad idea, because @code{i}
-will not be incremented if @code{NDEBUG} is defined.
-@end deftypefn
-
-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 @code{assert_perror} macro makes this easy.
-
-@comment assert.h
-@comment GNU
-@deftypefn Macro void assert_perror (int @var{errnum})
-Similar to @code{assert}, but verifies that @var{errnum} is zero.
-
-If @code{NDEBUG} is defined, @code{assert_perror} tests the value of
-@var{errnum}. If it is nonzero, @code{assert_perror} aborts the program
-after a printing a message of the form:
-
-@smallexample
-@file{@var{file}}:@var{linenum}: @var{function}: @var{error text}
-@end smallexample
-
-@noindent
-on the standard error stream. The file name, line number, and function
-name are as for @code{assert}. The error text is the result of
-@w{@code{strerror (@var{errnum})}}. @xref{Error Messages}.
-
-Like @code{assert}, if @code{NDEBUG} is defined before @file{assert.h}
-is included, the @code{assert_perror} macro does absolutely nothing. It
-does not evaluate the argument, so @var{errnum} should not have any side
-effects. It is best for @var{errnum} to be a just simple variable
-reference; often it will be @code{errno}.
-
-This macro is a GNU extension.
-@end deftypefn
-
-@strong{Usage note:} The @code{assert} facility is designed for
-detecting @emph{internal inconsistency}; it is not suitable for
-reporting invalid input or improper usage.
-
-The information in the diagnostic messages provided by the @code{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 @code{assert} to print the error messages for these
-eventualities.
-
-What's more, your program should not abort when given invalid input, as
-@code{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.
-
-@xref{Error Messages}, for information on printing error messages for
-problems that @emph{do not} represent bugs in the program.
-