how to correctly use fork, exec, wait - c

Question

The shell i'm writing needs to execute a program given to it by the user. Here's the very shortened simplified version of my program
int main()
{
pid_t pid = getpid(); // this is the parents pid
char *user_input = NULL;
size_t line_sz = 0;
ssize_t line_ct = 0;
line_ct = getline(&user_input, &line_sz, stdin); //so get user input, store in user_input
for (;;)
{
pid_t child_pid = fork(); //fork a duplicate process
pid_t child_ppid = getppid(); //get the child's parent pid
if (child_ppid == pid) //if the current process is a child of the main process
{
exec(); //here I need to execute whatever program was given to user_input
exit(1); //making sure to avoid fork bomb
}
wait(); //so if it's the parent process we need to wait for the child process to finish, right?
}
}
Have I forked the new process & checked to see if it's a child process correctly
What exec could I use here for what I'm trying to do? What is the most simple way
What are my arguments to wait? the documentation I'm looking at isn't helping much
Assume the user might input something like ls, ps, pwd
Thanks.
Edit:
const char* hold = strdup(input_line);
char* argv[2];
argv[0] = input_line;
argv[1] = NULL;
char* envp[1];
envp[0] = NULL;
execve(hold, argv, envp);

Answer 1

Here's a simple, readable solution:
pid_t parent = getpid();
pid_t pid = fork();
if (pid == -1)
{
// error, failed to fork()
}
else if (pid > 0)
{
int status;
waitpid(pid, &status, 0);
}
else
{
// we are the child
execve(...);
_exit(EXIT_FAILURE); // exec never returns
}
The child can use the stored value parent if it needs to know the parent's PID (though I don't in this example). The parent simply waits for the child to finish. Effectively, the child runs "synchronously" inside the parent, and there is no parallelism. The parent can query status to see in what manner the child exited (successfully, unsuccessfully, or with a signal).