Populating a va_list

Question

Is there a way to create a va_list from scratch? I'm trying to call a function that takes a va_list as a parameter:

func(void **entry, int num_args, va_list args, char *key);  

...from a function that doesn't take a variable number of arguments. The only way I can think of is to create an intermediary function that takes varargs and then passing along its va_list, which is pretty stupid:

void stupid_func(void **entry, char *key, int num_args, ...) {     va_list args;     va_start(args, num_args);      func(entry, num_args, args, key);      va_end(args); } 

Is there a better way? I can't change func's signature.

Answer 1

This is a bad idea because the va_list abstraction is there to hide some grim compiler/architecture specific details regarding stack-pointers and what not. And it is pretty much bound to the function's scope once initialized. If you wind the stack and reference a previous frames va_args out of scope, things can go bad. You can pass them around but ...

expect bugs

See: http://lists.freebsd.org/pipermail/freebsd-amd64/2004-August/001946.html

Also checkout man(3) va_copy and friends for safer handling of va_args and passing them around.

IMHO the va_args stuff is not very neat. In the past I have dealt with this by initializing structures/opaque pointers on the heap then using pointer arithmetic to work the data. But this is a hack and depends on circumstances.

Answer 2

I understand and agree with Aiden's warnings - va_list and friends are dangerous since they hide low-level calling conventions. But ... in this situation I think you have no other option. Put the static ... function into the .c file so nobody else can see it, sort of a proxy to the function you need to call, test the hell out of it, and be done. Just make sure you don't expose the variadic arguments up the call chain.