Question

Some old code that I just came across:

MLIST * new_mlist_link()
{
    MLIST *new_link = (MLIST * ) malloc(sizeof(MLIST));
    new_link->next  = NULL;
    new_link->mapi  = NULL;
    new_link->result = 0;
}

This was being called to build a linked list, however I noticed there is no statement:

return new_link;

Even without the return statement there, the list still got built properly. Why did this happen?

Edit: Platform: Mandriva 2009 64bit Linux 2.6.24.7-server GCC 4.2.3-6mnb1

Edit: Funny… this code also ran successfuly on about 5 different Linux installations, all different versions/flavors, as well as a Mac.

Answer 1

On 32-bit Windows, most of the time, the return value from a function is left in the EAX register. Similar setups are used in other OSes, though of course it’s compiler-specific. This particular function presumably stored the new_link variable in that same location, so when you returned without a return, the variable in that location was treated as the return value by the caller.

This is non-portable and very dangerous to actually do, but is also one of the little things that makes programming in C so much fun.