Many existing functions that return errno are declared as returning a value of type int. It is semantically unclear by looking at the function declaration or prototype if these functions return an error status or a value or worse, some combination of the two. (See guideline recommendation ERR02-C. Avoid in-band error indicators.)
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This noncompliant code example, however, does comply with guideline rule ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only after the function returns a value indicating failure.
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