Reaping zombies? [C] - c

Question

I'm having a few zombie processes that are running (defunct) in the background and I'm not quite sure how to kill them. I'm making a mini-shell, so it's basically like the terminal, but my own version of it.
Here is my code:
#include <stdio.h>
#include <string.h>
#include <stddef.h>
#include "main.h"
int main() {
/* Declared variables */
char buff[100];
char* args[20];
int arguments = 20;
/* Boolean value */
int done = 0;
while(done != 1) {
/* Print directory */
printf("%s>", getcwd(0,0));
/* Gets input */
fgets(buff, 100, stdin);
/* Checks to see if anything was entered */
if (buff[0] == '\n') {
printf("Error: Enter a command! (Example: ls -l)\n");
} else {
parseArgs(buff, args, 20, &arguments);
if (*args[0] == '\n') {
printf("Error: Enter a command! (Example: ls -l)\n");
} else if (strcmp(args[0], "exit") == 0) {
done = 1;
} else if (strcmp(args[0], "cd") == 0) {
/* Changes the directory */
int dir = chdir(args[1]);
if (dir != 0) {
printf("That directory isn't valid!\n");
}
} else {
int background = 0;
int count = 0;
/* Create a new process */
int process = fork();
if (process == -1) {
printf("Error: Unable to create a process!");
} else if (process == 0) {
/* Run user input */
int res = execvp(args[0], args);
if (res == -1) {
printf("\nError: Enter a command! (Example: ls -l)\n");
done = 1;
}
int reapingInfo;
waitpid(process, &reapingInfo, 0);
}
}
}
}
return (0);
}
Here is what I'm getting as an output when I run ls -l a few times and run the command: ps:
20978 pts /6 00:00:00 bash
21049 pts /6 00:00:00 main
21050 pts /6 00:00:00 ls <defunct>
21051 pts /6 00:00:00 ls <defunct>
21062 pts /6 00:00:00 ps
Any clues on how to reap these defunct processes?

Answer 1

Your logic following fork() doesn't look right. You have else if (process == 0), and in that branch (in the child process), you exec a new program and after that you attempt to call waitpid(). Since execvp never returns if the execution is successful, most of the time your waitpid never gets called.
I think you are missing an else somewhere. The waitpid should be done in the parent (the process in which fork() returned a strictly positive value). Then waitpid will reap the zombies; that's its job.
(By the way, if execvp fails, you probably want to call _exit(). Letting the child continue to run to completion is probably not right. For instance, any data in stdio buffers could be written twice.)

Answer 2

Here you can see an example for fork/exec/waitpid usage that should work in your code:
/* Create a new process */
int process = fork();
if (process == -1) {
printf("Error: Unable to create a process!");
} else if (process == 0) {
/* Run user input */
int res = execvp(args[0], args);
/* execvp only returns if it fails, not need to check if res == -1 */
printf("\nError: Enter a command! (Example: ls -l)\n");
exit(1); /* kill this process, if exec fails! */
} else {
int reapingInfo;
waitpid(process, &reapingInfo, 0);
}