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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 checks are helpful in debugging problems due to misunderstandings between different parts of the program.
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.
Macro: void assert (int expression)
Verify the programmer's belief that expression should be nonzero at this 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 section Aborting a Program) after printing a message of the
form:
`file':linenum: 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 written.
If the preprocessor macro NDEBUG is defined at the point where
`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.
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
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. 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 (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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