comp.lang.c FAQ list · Question 16.5

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

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

• uninitialized local variables [footnote] (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, or have ``narrow'' or variable arguments (see questions 1.25, 11.3, 14.2, and 15.1)
• dereferenced null pointers (see section 5)
• improper malloc/free use: assuming malloc'ed memory contains 0, assuming freed storage persists, freeing something twice, corrupting the malloc arena (see also questions 7.19 and 7.20)
• pointer problems in general (see also questions 16.7 and 16.8)
• mismatch between printf format and arguments, especially trying to print long ints using %d (see questions 12.7 and 12.9)
• trying to allocate more memory than an unsigned int can count, 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 [footnote] (see also questions 7.1, 12.21, and 19.28)
• invalid assumptions about the mapping of typedefs, especially size_t (see question 7.15)
• floating point problems (see questions 14.1 and 14.4a)
• 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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