Ian Wienand <ianw@xxxxxxxxxxxxxxxxxx> writes: > Googling around, you will find many people saying that C99 disallows > undefined functions (e.g [1]), often citing C99 6.5.2.2. My reading > of this doesn't seem to suggest it disallows undefined functions; in > fact it talks about type promotion rules for undefined functions. I don't see that. I see type promotion rules for functions declared without a prototype, but that is not the same as functions which are not declared at all. > Can anyone point me to where C99 explicitly disallows a function > without a declaration, and if so why doesn't gcc in C99 mode give an > error rather than falling back to the C89 semantics? Or are the > numerous people suggesting C99 disallows undefined functions > incorrect? C99 disallows a function without a declaration in 6.5.2.2 paragraph 1: "The expression that denotes the called function shall have type pointer to function returning void or returning an object type other than an array type." This does not permit an undeclared symbol. As you note, gcc in c99 mode will always give a warning for an undeclared function; this is not true in c89 mode. In c99 mode, the compiler will give an error if you use the -pedantic-errors option. Or the -Werror-implicit-function-declaration option. Or the -Werror option. Ian
