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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.
+