Question

Calls to missing functions in a C program are detected by
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Answer 1

Answer:

The compiler can see that there is no declaration in scope for the identifier designating the function. Most will emit a warning under those circumstances, and some will emit an error, or can be configured to do so.

But that's not the same thing as the compiler detecting that the function doesn't exist. It's the compiler detecting that the function name has not been declared. The compiler will exhibit the same behavior if you spell the function name correctly but do not include a prior declaration for it.

Furthermore, C90 and pre-standardization C permitted calls to functions without any prior declaration. Such calls do not conform to C99 or later, but most compilers still do accept them (usually with a warning) for compatibility purposes.