Double-fork() to avoid zombies

i3bar/src/child.c

@@ -12,6 +12,7 @@
 #include <unistd.h>
 #include <sys/types.h>
 #include <signal.h>
+#include <sys/wait.h>
 #include <stdio.h>
 #include <fcntl.h>
 #include <string.h>
@@ -100,19 +101,29 @@ void child_sig_cb(struct ev_loop *loop, ev_child *watcher, int revents) {
 /*
  * Start a child-process with the specified command and reroute stdin.
  * We actually start a $SHELL to execute the command so we don't have to care
- * about arguments and such
+ * about arguments and such.
+ * We also double-fork() to avoid zombies and pass the pid of the child through a
+ * temporary pipe back to i3bar
  *
  */
 void start_child(char *command) {
     child_pid = 0;
     if (command != NULL) {
-        int fd[2];
+        int fd[2], tmp[2];
+        /* This pipe will be used to communicate between e.g. i3status and i3bar */
         pipe(fd);
-        child_pid = fork();
-        switch (child_pid) {
+        /* We also need this temporary pipe to get back the pid of i3status */
+        pipe(tmp);
+        switch (fork()) {
             case -1:
                 ELOG("Couldn't fork()\n");
                 exit(EXIT_FAILURE);
+            case 0:
+                /* Double-fork(), so the child gets reparented to init */
+                switch(child_pid = fork()) {
+                    case -1:
+                        ELOG("Couldn't fork() twice\n");
+                        exit(EXIT_FAILURE);
                     case 0:
                         /* Child-process. Reroute stdout and start shell */
                         close(fd[0]);
@@ -126,15 +137,31 @@ void start_child(char *command) {
 
                         execl(shell, shell, "-c", command, (char*) NULL);
                         return;
+                    default:
+                        /* Temporary parent. We tell i3bar about the pid of i3status and exit */
+                        write(tmp[1], &child_pid, sizeof(int));
+                        close(tmp[0]);
+                        close(tmp[1]);
+                        exit(EXIT_SUCCESS);
+                    }
             default:
                 /* Parent-process. Rerout stdin */
                 close(fd[1]);
 
                 dup2(fd[0], STDIN_FILENO);
 
+                /* We also need to get the pid of i3status from the temporary pipe */
+                size_t rec = 0;
+                while (rec < sizeof(int)) {
+                    rec += read(tmp[0], &child_pid, sizeof(int) - rec);
+                }
+                /* The temporary pipe is no longer needed */
+                close(tmp[0]);
+                close(tmp[1]);
                 break;
         }
     }
+    wait(0);
 
     /* We set O_NONBLOCK because blocking is evil in event-driven software */
     fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK);