C calling conventions and passed arguments

Question

When making a function call in Linux (or OS X for that matter), can the callee modify the values of the arguments on the stack? I was under the assumption that since the ca

Answer 1

Although the caller (in some calling conventions) is the one that cleans up the arguments, all it's really doing is deallocating the space previously allocated on the stack to hold the argument values. The callee is free to modify the values during execution of the function, because the caller isn't going to look at their values later.

In the example you posted, GCC has emitted the popl %eax instruction to deallocate the space taken by the parameter on the stack. All it really needs to do is add 4 to %esp (the stack on x86 grows downwards in memory), and executing the popl %eax instruction is the shortest and fastest way to do this. If the compiler needed to deallocate 20 values, it would probably modify %esp directly instead of emitting 20 popl instructions.

You will probably find that the new value of %eax is not used in the following code.