Q:
Variable length argument lists
How does GCC treat variable length argument lists?
The function I want to call takes two arguments:
void function_name(char * name, char **args)
So this is what I call:
int main(int argc, char **argv)
{
int i = 0;
printf("%d
", function_name(argv[i++], &argv[i++]));
return 0;
}
According to my understanding GCC should first call the function with one argument. Then call it with two arguments etc. I thought the most extreme case is argc-2 args. But the execution results showed that the function was called with three arguments. What's the real situation? How is GCC able to handle this situation?
A:
Yes, you're correct. The arguments will be stored in the passed-in argument list.
When the called function starts execution, the first thing it will do is restore the base of the stack. The current stack pointer is stored in ESP and the saved stack pointer in EBP. So the stack now looks like this:
-----------------------------------------------------------------
| ESP | EBP | Old EBP | Old ESP | return address |... |
-----------------------------------------------------------------
After that, the function starts executing. The arguments are pushed onto the stack:
-----------------------------------------------------------------
| ESP | EBP | Old EBP | Old ESP | function parameters |... |
-----------------------------------------------------------------
After that, the first parameter gets evaluated and passed to the function. The stack now looks like this:
-----------------------------------------------------------------
| ESP | EBP | Old EBP | Old ESP | function parameters |... |
-----------------------------------------------------------------
After that, the second parameter gets evaluated and passed to the function. The stack now looks like this:
-----------------------------------------------------------------
| ESP | EBP | Old EBP | Old ESP | function parameters |... |
-----------------------------------------------------------------
After that, the third parameter gets evaluated and passed to the function. The stack now looks like this:
-----------------------------------------------------------------
| ESP | EBP | Old EBP | Old ESP | function parameters |... |
-----------------------------------------------------------------
The recursion continues. At the end of the first iteration, the stack now looks like this:
-----------------------------------------------------------------
| ESP | EBP | Old EBP | Old ESP | function parameters |... |
---------------------------------------------------------------- 01e38acffe
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