unshare user namespace, fork, map uid then execvp failing

Hello,
While trying to tinker with kernel namespaces and create a small container engine. I was trying to do the following sequence of actions:
- unshare the user namespace;
- Map the user in child process to root;
- execvp.

However, when running id, my code outputs the user as a nobody or fails without error.

Code:#include <sched.h>
#include <cstdio>
#include <cstring>
#include <cerrno>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/mount.h>
#include <system_error>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>

void unshare_user_namespace() {
if (0 != unshare(CLONE_NEWUSER)) {
fprintf(stderr, "%s\n", "USER unshare has failed");
exit(1);
}
}

void map_id() {
int pid = getpid();
char file[100];
if (0 > sprintf(file, "/proc/%d/uid_map", pid)) {
printf("Couldn't sprintf uid_map path.");
exit(1);
}

int fd;
fd = open(file, 1);
if (fd < 0) {
printf("Coudln't open file for writing.\n");
exit(1);
}

int uid = getuid();
char * buf;
if (0 > sprintf(buf, "0 %d 1", uid)) {
printf("Couldn't sprintf uid_map content.");
exit(1);
}

if (write(fd, buf, strlen(buf))) {
printf("Coudln't write mapping into file.\n");
exit(1);
}

free(buf);
close(fd);
}

void start(char * command, char ** args) {
unshare_user_namespace();
int fork_pid = fork();

if (-1 == fork_pid) {
fprintf(stderr, "%s\n", "couldn't fork");
exit(1);
}

if (0 == fork_pid) {
map_id();

if (-1 == execvp(command, args)) {
fprintf(stderr, "%s\n", "couldn't execvp");
exit(1);
}
}
}

int main(int argc, char ** argv) {
start(argv[1], & argv[1]);
int status;
wait( & status);
return 0;
}I tried reading the man pages for namespaces, unshare etc but couldn't figure out what's wrong with my code.

To run the code:
Code:$ g++ <file_containing_code> && ./a.out idAny help, suggestions are welcome.