Program hangs after execvp(fork-and-exec simple console application) - fork

Question

I tried to write simple fork-and-exec application with the following code
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
int
spawn(char *program, char *arg_list[])
{
pid_t child_pid;
child_pid = fork();
if (child_pid != 0)
return child_pid;
else {
execvp(program, arg_list);
fprintf(stderr, "an error occured\n");
exit(EXIT_FAILURE);
}
}
int
main()
{
int child_stat;
char *arg_list[] = {
"ls",
"-l",
"/",
NULL
};
spawn("ls", arg_list);
printf("return to parent\n");
return 0;
}
Every goes fine, but child after becoming ls does not terminate.
Shell looks like
./main
return to parent
-- ls output here --
But command prompt does not appear, so I assume that one process hangs and the reason is not obvious for me. Can you, please, point to my mistake.
Thanks in advance.

Answer 1

I believe that it works correctly. However, the order of execution of the child and parent are not what you expect, so you see output you don't understand. On my Arch linux box (kernel 4.xx), it works fine, but the parent process returns first, leaving ls to overwrite and otherwise garble the command prompt. Do a wait() on the child process to get execution to happen in a specific order. Add a couple of lines of code after the spawn() call:
spawn("ls", arg_list);
printf("return to parent\n");
wait(&child_stat);
printf("Waited on child\n");
return 0;
After the fork() call, the parent process will call wait(), which won't return until a child process (if there is at least one) exits. Since there's only one child process, the output of ls occurs before the command prompt gets written out, and thus you will see what you believe you should see.