Ptrace or waitpid gets stuck in C on macOS

Question

I'd like to write a mini-debugger with ptrace on OS X.

I want the parent process to make the child process run step by step.

This is what I tried, but the program gets stuck sometimes, it seems to be in an infinite loop or to be frozen.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <sys/user.h>
#include <sys/types.h>
#include <sys/ptrace.h>

int main(void)
{
    pid_t id = fork();

    if (id < 0)
        return 1;
    else if (id == 0)
    {
        ptrace(PT_TRACE_ME, 0, 0, 0);
        printf("Point 1\n");
        kill(getpid(), SIGSTOP);
        printf("Point 2\n");
        exit(1);
    }
    else
    {
        while (1)
        {
            int status = 0;
            pid_t retpid = waitpid(id, &status, WUNTRACED);

            if (retpid < 0)
            {
                printf("Waitpid error\n");
                exit(3);
            }

            if (WIFSTOPPED(status))
            {
                int ret = ptrace(PT_STEP, id, (caddr_t)1, 0);
                if (ret < 0)
                {
                    printf("Ptrace error\n");
                    exit(2);
                }
            }
            else
            {
                printf("Program has terminated\n");
                exit(0);
            }
        }
    }
}

Compile with cc bug.c, run with while true; do ./a.out; done, wait 30 seconds and it will freeze.

When it freezes, the last lines are:

Point 1
Point 2

I can't figure out what I'm doing wrong.

_{Running on macOS Sierra 10.12.6 with Apple LLVM version 8.1.0 (clang-802.0.42)}