Question

So, this seemed simple at first, but after crawling Google and here, the answer doesn’t seem as simple as I first thought.

Basically, I’m editing a MINIX kernel as part of a practical for my Operating Systems course, and I have to add a little function that spits out the number of running processes when you hit a function key in the Information Server. I’ve figured out how to integrate the functionality so all the other stuff works, but for the life of me, I can not figure out how to get the current number of processes running in the system into my C code and into a variable to print out.

First I thought there’d be a nifty Syscall like SYS_NUMPROCS or something that’d return the value, but no luck.
Then, I tried piping output from a system(“ps -ax | wc -l”) to a file and the file wouldn’t create. I tried using popen() and no luck there either – even with a simple “ls” read into a buffer, it just bombs the code and “hangs” the run of the code, so there’s no output.

So now I’m truly stumped, and any help would be super awesome, because at this point I’ve exhausted all the obvious options.

The only two things I can think of now would be a loop counting all the processes, but first you have to get to the system’s process list, and I’ve heard vague things said about /proc/ as a directory, but I haven’t a clue how to access/run through that or how it’d link up to getting the number of processes in the first place.

Thanks a stack (lol pun), guys 🙂

Also, I haven’t included code explicitly because nothing I’ve written aside from basic printf’ing for cosmetic output, because none of the things I’ve tried gave me any joy :/

Edit notes: Guys, this is a kernel edit – I’m writing the function to printf the information in a system C file, then recompiling the kernel and rebooting the system to test. It’s a UNIX (MINIX) kernel, not a Linux kernel, and it’s not a user mode program.

My code for popen(), as some of you requested, is as follows:

public void cos_dmp(){
    char buffer[512];
    FILE * f;

    f = popen("ps -ax | wc -l","r");

    fgets(buffer, sizeof(buffer),f);

  //buffer should now contain result of popen()

     printf(buffer);
}

That’s a bit of a hacked together version from what I remember and keeping it ultra simple and showing you guys that’s what I was trying to do. Again though, there must be a better way to do this aside from essentially calling the output of a system() call.

Edit again: the above code woks perfectly from a user program but won’t work from the kernel function. Anybody have an idea why?:/

Answer 1

struct kinfo kinfo;
int nr_tasks, nr_procs;
getsysinfo(PM_PROC_NR, SI_KINFO, &kinfo);
nr_procs = kinfo.nr_pro;

This will get you the number of processes running