Given a child PID how can you get the parent PID

Question

I'm working on a project where I have a number of PIDs and I have to find out which of these are zombie processes and then kill their parent processes in order to kill the initial zombie process. I am not sure if there is any way to find out what the PPID of a given PID is. Any help would be appreciated.

Answer 1

At the source for the ps command, there is a function called get_proc_stats defined in proc/readproc.h that (among other things) returns the parent pid of a given pid. You need to do install libproc-dev to get this function. You can then do:

#include <proc/readproc.h>
void printppid(pid_t pid) {
    proc_t process_info;
    get_proc_stats(pid, &process_info);
    printf("Parent of pid=%d is pid=%d\n", pid, process_info.ppid);
}

This is taken from here. I never used this but according to author this may be helpful.

Answer 2

I've included Linux and macOS/BSD versions using only system libraries.

A pure C using only standard libraries for Linux:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXBUF      (BUFSIZ * 2)

int pgetppid(int pid) {
    int ppid;
    char buf[MAXBUF];
    char procname[32];  // Holds /proc/4294967296/status\0
    FILE *fp;

    snprintf(procname, sizeof(procname), "/proc/%u/status", pid);
    fp = fopen(procname, "r");
    if (fp != NULL) {
        size_t ret = fread(buf, sizeof(char), MAXBUF-1, fp);
        if (!ret) {
            return 0;
        } else {
            buf[ret++] = '\0';  // Terminate it.
        }
    }
    fclose(fp);
    char *ppid_loc = strstr(buf, "\nPPid:");
    if (ppid_loc) {
        ppid = sscanf(ppid_loc, "\nPPid:%d", &ppid);
        if (!ppid || ppid == EOF) {
            return 0;
        }
        return ppid;
    } else {
        return 0;
    }

}

int main () {
    int ppid, pid = 373;  // my current cron pid
    ppid = pgetppid(pid);
    printf("PPid = %d\n", ppid);
}

For macOS and FreeBSD use this:

#include <stdio.h>
#include <sys/sysctl.h>
#include <sys/param.h>
#include <sys/user.h>

int pgetppid(int pid) {

    struct kinfo_proc p;
    size_t len = sizeof(struct kinfo_proc);
    int mib[4] = { CTL_KERN, KERN_PROC, KERN_PROC_PID, pid };
    if (sysctl(mib, 4, &p, &len, NULL, 0) < 0)
        return 0;
    if (len == 0)
        return 0;
    int ret;
#if defined(__APPLE__)
        ret = p.kp_eproc.e_ppid; // macOS
    #elif defined(__FreeBSD__)
        ret = p.ki_ppid; // FreeBSD
    #else
        #error "Not supported, try adding an elif for this OS"
    #endif
    return ret;
}

int main () {
    int ppid, pid = 2420;  // my current cron pid
        ppid = pgetppid(pid);
    printf("PPid = %d\n", ppid);
}

Answer 3

I found an alternative solution on macOS, inspired by another answer. You can use the undocumented "libproc" methods.

To use libproc in Swift, add the following line to your bridging header:

#import <libproc.h>

Then you can get the ppid using this sample code:

let pid = pid_t(12345)
var shortinfo = proc_bsdshortinfo()
let status = proc_pidinfo(pid, PROC_PIDT_SHORTBSDINFO, 0, &shortinfo, Int32(MemoryLayout.size(ofValue: shortinfo)))
guard status > 0 else {
    let errno_stored = errno
    let errorDescription = String(cString:strerror(errno_stored)!)
    throw NSError(domain: "com.apple.libproc", code: Int(errno_stored), userInfo: [NSLocalizedDescriptionKey: errorDescription])
}
let ppid = pid_t(shortinfo.pbsi_ppid)

Answer 4

The function getppid() does that:

#include <unistd.h>

int main()
{
    pid_t ppid;

    ppid = getppid();

    return (0);
}