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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 ERR02-C. Avoid in-band error indicators.)
TR 24731-1 introduces introduced the new type errno_t that is defined to be type int in errno.h and elsewhere. Many of the functions defined in TR 24731-1 return values of this type [TR 24731-1]. The errno_t type should be used as the type of an object that may contain only contain values that might be found in errno. For example, a function that returns the value of errno should be declared as having the return type errno_t.
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Failing to test for error conditions can lead to vulnerabilities of varying severity. Declaring functions that return an errno with a return type of errno_t will not eliminate this problem but may reduce errors caused by programmers' misunderstanding the purpose of a return value.
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