K & R The C Programming Language 2ed.
The function atof must be declared and defined consistently. If atof itself and the call to it in main have inconsistent types in the same source file, the error will be detected by the compiler. But if (as is more likely) atof were compiled separately, the mismatch would not be detected (p.63)
Actually both gcc and clang can detect the mismatch across files. (Tested on gcc 4.8.2 and clang 3.4.)