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Assert.h considered harmful

Most C programmers become familiar with assertions from the the assert.h header. As such its a very good thing and has a nice simple implementation. However it is also inefficient and leads some people to the conclusion that assertion checking is an expensive luxury.

The implementation of assert.h as distributed with gcc looks like the following (after a bit of editing):

# ifndef NDEBUG
# define _assert(ex)	{if (!(ex)) \
                         {(void)fprintf(stderr, \
                           "Assertion failed: file \"%s\", line %d\n", \
                           __FILE__, __LINE__);exit(1);}}
# define assert(ex)	_assert(ex)
# else
# define _assert(ex)
# define assert(ex)
# endif

There are are two main problems with this:

  1. Code space overhead: each call to `assert' generates 2 function calls with 4 and 1 arguments plus strings for error messages. If assert.h had library code support we could make the implementation much more space efficient, e.g. by calling a single function on error detection.
  2. The default behaviour simply prints a message and dies, ideally you like to be able to use a debugger to determine why the assertion failed. Even if you run this under the debugger you can't observe the failures of variables are an assert failure because the process exits rather than aborting back to the debugger.

Of course everyone merely rewrites their own `assert' macro so these are not significant objections. The only problem is if the author uses the libraries without modification.

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