Question

For Linux Operating System: Write a simple test program in C to find out PIDs of ALL ancestor (i.e., parent, grandparent ) processes of the current process or a given process specified by a PID. A parent process's PID of a given process pid can be found in /proc/$pid/status

Answer 1

#include <stdio.h>
#include <unistd.h>

int main(void)
{
FILE *fp;
int pid;
char buf[128];
int n=1;
// process id
printf("pid=%d\n", (int)getpid());
// parent process id
printf("ppid=%d\n", (int)getppid());

pid=getppid();

while(pid>1)
{
// Set the path of file to be opened in buf as mentioned in the question
sprintf(buf, "/proc/%d/stat", (int)pid);
// Open the file
fp=fopen(buf, "r");
if(fp == NULL) break;

// Ignore one number, then two strings, then read a number which the required PID
fscanf(fp, "%*d %*s %*s %d", &pid);
fclose(fp);
printf("grandparent%d id=%d\n", n++, (int)pid);
}
}