A simple ptrace-based debugger

Let's look at a code example that makes use of ptrace to create a debugger program:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <signal.h>
#include <elf.h>
#include <sys/types.h>
#include <sys/user.h>
#include <sys/stat.h>
#include <sys/ptrace.h>
#include <sys/mman.h>

typedef struct handle {
  Elf64_Ehdr *ehdr;
  Elf64_Phdr *phdr;
  Elf64_Shdr *shdr;
  uint8_t *mem;
  char *symname;
  Elf64_Addr symaddr;
  struct user_regs_struct pt_reg;
  char *exec;
} handle_t;

Elf64_Addr lookup_symbol(handle_t *, const char *);

int main(int argc, char **argv, char **envp)
{
  int fd;
  handle_t h;
  struct stat st;
  long trap, orig;
  int status, pid;
  char * args[2];
  if (argc < 3) {
    printf("Usage: %s <program> <function>\n", argv[0]);
    exit(0);
  }
  if ((h.exec = strdup...