va_list 和 va_arg

Question

I using va_list like this:

void foo(const char* firstArg, ...) {
    va_list args;
    va_start (args, firstArg);
    for (const char* arg = firstArg; arg != NULL; arg = va_arg(arg, const char*)) {
         // do something with arg
    }

    va_end(args);
}

foo("123", "234", "345")

the first three arguments was passed to foo correctly, but where "345" is done,

 arg = va_arg(arg, const char*) 

set some other freak value to arg .

so What's the problem. I using llvm3.0 as my compiler.

Answer 1

C does not automatically put a NULL at the end of a ... argument list. If you want to use NULL to detect the end of the arguments, you must pass it explicitly. Some functions (such as printf ) use earlier parameters to decide when they have reached the end of the arguments.

( Edit : And actually if you want to put a NULL at the end, you need to cast it to the appropriate type so that it gets passed as the correct type of null pointer.)

Answer 2

I think the loop should be as follows:

for (const char* arg = firstArg; arg != NULL; arg = va_arg(args, const char*))

The change is va_arg(args, const char*) instead of va_arg(arg/*<<==*/, const char*) .