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 a message
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.
If NDEBUG is not defined, assert tests the value of
expression. If it is false (zero), assert aborts the
program (see section Aborting a Program) after printing a message of the
form:
`file':linenum: function: Assertion `expression' failed.
on the standard error stream stderr (see section 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 made. 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.
Warning: Even the argument expression expression is not
evaluated if NDEBUG is in effect. So never use 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.
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 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 section 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 just a 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 by the user of the
program.
The information in the diagnostic messages printed by the assert
and assert_perror macro is intended to help you, the programmer,
track down the cause of a bug, but is not really useful for telling a user
of your program why his or her input was invalid or why a command could not
be carried out. What's more, your program should not abort when given
invalid input, as assert would do--it should exit with nonzero
status (see section Exit Status) after printing its error messages, or perhaps
read another command or move on to the next input file.
See section Error Messages, for information on printing error messages for problems that do not represent bugs in the program.
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