OS161 execv Part 1

The function prototype

So the first thing we will do is decipher exactly what the prototype in unistd.h means.

int execv(const char *prog, char *const *args);

The syscall takes a constant character pointer to the program name. This could be a string literal “testbin/progname” or a string you have created. The second argument is more interesting although not complicated. It is simply an array of constant character pointers. A little more in depth this means that the variable args may be changed i.e.

args = new_args_array;

Each pointer in the array of constant character pointers that args points to however cannot be modified i.e.

args[0] = "Hello"; //"Hello" is a const char * but no go.

All of this however is kind of irrelevant because we will be passing these values through a syscall so gcc wont be generating any code and we do not have to respect these rules. The prototype may however give you a useful warning if you try something silly.

 Dropping into the Kernel

This part is all done for you. Here is a checklist of how to add a syscall just in case you forgot

1. Add the value to callno.h in kern/include/kern (already done)
2. Add the user syscall prototype to include/unistd.h (already done)
3. Add the prototype of the kernel version of the syscall to kern/include/syscall.h
4. Add a case to the switch statement in kern/arch/mips/mips/syscall.c
5. Add a call to the kernel version of the syscall to the case you just added. (you may not know all of the semantics you need just yet but just add a //TODO and if you have problems later do a “grep –r ‘TODO’ *” from your top level directoy.)

But what about our arguments?

The comment at the top of kern/arch/mips/mips/syscall.c is very informative on this matter.

When the user call into libc uses the syscall instruction to drop into the kernel.

That instruction jumps to the exception symbol in exception.S in kern/arch/mips/mips. This does some setup, sets up the trapframe and then jumps to mips_trap in kern/arch/mips/mips/trap.c and then it is going to hand off execution to mips_syscall.

During the setup of the trapframe before the call to mips_trap our arguments from userland were saved into the trap frame. The arguments are available at tf->a0, tf->a1, tf->a2, tf->a3, although we only need a0 and a1.

So now we have our arguments… but we haven’t done anything yet, have we? Not really, but maybe there will be a part 2?

- FlounderingZ