How to pass a value into a system call function in XV6?

Question

I am attempting to create a simple priority based scheduler in XV6. To do this, I also have to create a system call that will allow a process to set its priority. I have done everything required to create the system call as discussed here and elsewhere:

how do i add a system call / utility in xv6

The problem is, I cannot pass any variables when I call the function, or rather, it runs like nothing is wrong but the correct values do not show up inside the function.

Extern declaration (syscall.c):

...
extern int sys_setpty(void);

static int (*syscalls[])(void) = {
...
[SYS_setpty]  sys_setpty,
};

Sys-call Vector (syscall.h):

#define SYS_setpty 22

Implementation (sysproc.c):

void
sys_setpty(int pid, int pty)
{
  cprintf("function pid: %d \n", pid);
  cprintf("function pty: %d \n", pty);
}

(defs.h & user.h):

void setpty(int, int);

Macro (usys.S):

SYSCALL(setpty)

Function call:

setpty(3, 50);

Output:

function pid: 16843009
function pty: 16843009

The values are always the same exact number: 16843009. I have checked to see if the cprintf is working correctly by assigning values to pid and pty. I have spent about 6 hours trying every possible combination of everything I can think of and I'm starting to think that there is no built in mechanism for passing values via a system call in XV6. Am I missing something? Thank you in advance.

Answer 1

Passing arguments from user-level functions to kernel-level functions cannot be done in XV6. XV6 has its own built-in functions for passing arguments into a kernel function. For instance, to pass in an integer, the argint() function is called. In the implementation that I used for the set-priority function, that would look something like:

argint(0, &pid);

... to get the first argument which is the Process ID, and:

argint(1, &pty);

... to get the second argument which is the desired priority. The function call from the user process looks like this:

setpty(getpid(), priority);