Getting pid and other process information from /proc/<pid>/status

Question

I need to get some information (pid is just an example, i know its much easier to get it in many other ways) from /proc/PID/status

I have tried to do it this way:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#include <fcntl.h>
#include <sys/procfs.h>
#include <sys/signal.h>
#include <sys/syscall.h>
#include <sys/param.h>

int main(){
        char buf[BUFSIZ], buffer[10];
        char pathbase[20], pathdir[20];
        FILE *fp;
        prstatus_t status;

        printf("Process ID: %d\n", getpid());
        printf("Parent process ID: %d\n", getppid());
        printf("Group ID: %d\n", getpgrp());
        printf("Session ID: %d\n", getsid(0));

        strcpy(pathbase,"/proc/");
        sprintf(buffer, "%d", getpid());
        strcat(pathbase, buffer);

        strcpy(pathdir, pathbase);
        strcat(pathdir,"/status");

        if((fp = fopen(pathdir, "r")) == NULL) perror("fopen");
        fread(&status, sizeof(prstatus_t), 1, fp);

        printf("Proces id: %d\n", status.pr_pid);
        printf("Proces ppid: %d\n", (int)status.pr_ppid);
        fclose(fp);
}

and it obviously wrong, cause the result i get is:

Process ID: 5474
Parent process ID: 3781
Group ID: 5474
Session ID: 3781
Proces id: 1735289198
Proces ppid: 1733560873

Answer 1

The thing is /proc/[pid]/status is a text file. So your fread is copying text into the struct status - so everything will look like gibberish.

You could read the status file line by line or you could use the /proc/[pid]/stat file which contains the same information on a single line ( status is for human consumption while stat is for program consumtion). To get the process id (or any other information) you would just have to tokenize that single line.