Question 16.5

This program runs perfectly on one machine, but I get weird results on another. Stranger still, adding or removing debugging printouts changes the symptoms...

Lots of things could be going wrong; here are a few of the more common things to check:

uninitialized local variables (see also question 7.1)
integer overflow, especially on 16-bit machines, especially of an intermediate result when doing things like a * b / c (see also question 3.14)
undefined evaluation order (see questions 3.1 through 3.4)
omitted declaration of external functions, especially those which return something other than int (see questions 1.25 and 14.2)
dereferenced null pointers (see section 5)
improper malloc/free use: assuming malloced memory contains 0, assuming freed storage persists, freeing something twice (see also questions 7.20 and 7.19)
pointer problems in general (see also question 16.8)
mismatch between printf format and arguments, especially trying to print long ints using %d (see question 12.9)
trying to malloc(256 * 256 * sizeof(double)), especially on machines with limited memory (see also questions 7.16 and 19.23)
array bounds problems, especially of small, temporary buffers, perhaps used for constructing strings with sprintf (see also questions 7.1 and 12.21)
invalid assumptions about the mapping of typedefs, especially size_t
floating point problems (see questions 14.1 and 14.4)
anything you thought was a clever exploitation of the way you believe code is generated for your specific system

Proper use of function prototypes can catch several of these problems; lint would catch several more. See also questions 16.3, 16.4, and 18.4.

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