Get pid at any depth with procfs

pppid.h

#include <sys/types.h>
#include <algorithm>
#include <cctype>
#include <fstream>
#include <string>

#define PROC_STR "/proc/%s/stat"

pid_t
_getpppid(const char *, int);

pid_t
getpppid(int level)
{
        if (level < 0) {
                return level;
        }
        if (level == 0) {
                return _getpppid("self", 0);
        }
        if (level == 1) {
                return _getpppid("self", 3);
        }
        return _getpppid(std::to_string(getpppid(level - 1)).c_str(), 3);
}

pid_t
_getpppid(const char *prefix, int skip_whitespaces)
{
        char buf[64];
        sprintf(buf, PROC_STR, prefix);
        std::ifstream f(buf);

        std::string buffer;
        while (std::getline(f, buffer)) {
                std::string pid;
                pid_t pid_;
                int space_ctr = 0;
                bool parens_open = false;
                for (int i = 0; i < buffer.length(); ++i) {
                        if (buffer.at(i) == '(') {
                                parens_open = true;
                        }
                        if (buffer.at(i) == ')') {
                                parens_open = false;
                        }
                        if (!parens_open) {
                                if (isspace(buffer.at(i))) {
                                        if (space_ctr == skip_whitespaces) {
                                                break;
                                        }
                                        pid.clear();
                                        space_ctr++;
                                        continue;
                                }
                        }
                        if (isdigit(buffer.at(i))) {
                                pid.push_back(buffer.at(i));
                        }
                }
                return std::stoi(pid);
        }
        return 0;
}