How do I check that the sys call terminated/why it terminates?

Question

The output of the following code is just "hello!", which is confusing because when I do make qemu to start xv6 everything compiles well, but obviously something goes wrong with the function getiocounts.

I am new to sys calls, so there might be something obvios I'm missing. Please help! Thank you!

#include "types.h"
#include "stat.h"
#include "user.h"
#include "iostat.h"

int main(int argc, char** argv) {
    struct iostat* io;
    io =(struct iostat* ) malloc(sizeof(int)*2);
    int r=0;
    printf(1, "hello!");
    printf(1, "%d", getiocounts(io));
    r = io->readcount;
    printf(1, "hello!");
    printf(1, "%d", r);
    exit();
}

int sys_getiocounts( struct iostat* io){

    if(argptr(1, (void*)&io, sizeof(*io)) < 0)
        return -1;
    io->readcount = myproc()->r;
    io->writecount = myproc()->w;
    return 0;
}

score 0 · Accepted Answer · answered Feb 17 '22 at 13:50

Some mistakes in your kernel code:

Your system call is not correctly defined: its parameters prototype should be (void)
Your parameter reading is wrong: you must read the first parameter: argptr( 0, ...

int sys_getiocounts( void ){

    struct iostat* io;

    if(argptr(0, (void*) &io, sizeof *io) < 0)
        return -1;

    io->readcount = myproc()->r;
    io->writecount = myproc()->w;
    return 0;
}

And your user code could be cleaner on the allocation:

struct iostat* io = malloc(sizeof *io );
....
getiocounts(io);

or even without allocation:

struct iostat io;
...
getiocounts(&io);

How do I check that the sys call terminated/why it terminates?

1 Answers1