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Something worth mentioning would have been those libc calls where the only way to tell if a return value of 0 is an error is to check errno. And of course, as the article says, errno is only set in error, you need to set it to 0 before making that libc call.

I think strtol was one such function, but there were others.

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getservbyname Is also one of those functions, it returns NULL on error or for signaling it couldn’t find what you looked for.

strtol() isn't actually such a case - it sets errno on range errors but returns 0 for valid input "0"; the classic examples are actually getpriority() and sched_yield() which can legitimately return -1 on success.

It is such a case, it's just not about zero. If the input is too large to be represented in a signed long, strtol(3) returns LONG_MAX and sets errno to ERANGE. However, if the input was the string form of LONG_MAX, it returns LONG_MAX and doesn't set errno to anything. In fact my strtol(3) manpage explicitly states "This function does not modify errno on success".

Thus, to distinguish between an overflow and a legitimate maximum value, you need to set errno to 0 before calling it, because something else you called previously may have already set it to ERANGE.

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