Following @Lee Duhem's hint, I made the following function that returns the nth ancestor of the current process (the 2nd ancestor is the grandparent).
/* Get the process ID of the calling process's nth ancestor. */
pid_t getapid(int n) {
pid_t pid = getpid();
while(n>0 && pid){ // process with pid 0 has no parent
// strlen("/proc/") == 6
// max [pid] for 64 bits is 4194304 then strlen("[pid]") < 7
// strlen("/stat") == 5
// then strlen("/proc/[pid]/stat") < 6 + 7 + 5
char proc_stat_path[6+7+5+1];
sprintf(proc_stat_path, "/proc/%d/stat", pid);
// open "/proc/<pid>/stat"
FILE *fh = fopen(proc_stat_path, "r");
if (fh == NULL) {
fprintf(stderr, "Failed opening %s: ", proc_stat_path);
perror("");
exit(1);
}
// seek to the last ')'
int c;
long pos = 0;
while ((c = fgetc(fh)) != EOF) {
if (c == ')')
pos = ftell(fh);
}
fseek(fh, pos, SEEK_SET);
// get parent
fscanf(fh, " %*c %d", &pid);
// close "/proc/<pid>/stat"
fclose(fh);
// decrement n
n--;
}
if(n>0)
return -1;
else
return pid;
}