i3/i3bar/src/child.c

/*
 * i3bar - an xcb-based status- and ws-bar for i3
 *
 * © 2010 Axel Wagner and contributors
 *
 * See file LICNSE for license information
 *
 * src/child.c: Getting Input for the statusline
 *
 */
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <signal.h>
#include <sys/wait.h>
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <ev.h>

#include "common.h"

/* Global variables for child_*() */
pid_t child_pid;

/* stdin- and sigchild-watchers */
ev_io    *stdin_io;
ev_child *child_sig;

/*
 * Stop and free() the stdin- and sigchild-watchers
 *
 */
void cleanup() {
    ev_io_stop(main_loop, stdin_io);
    ev_child_stop(main_loop, child_sig);
    FREE(stdin_io);
    FREE(child_sig);
    FREE(statusline);
}

/*
 * Callbalk for stdin. We read a line from stdin and store the result
 * in statusline
 *
 */
void stdin_io_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
    int fd = watcher->fd;
    int n = 0;
    int rec = 0;
    int buffer_len = STDIN_CHUNK_SIZE;
    char *buffer = malloc(buffer_len);
    memset(buffer, '\0', buffer_len);
    while(1) {
        n = read(fd, buffer + rec, buffer_len - rec);
        if (n == -1) {
            if (errno == EAGAIN) {
                /* remove trailing newline and finish up */
                buffer[rec-1] = '\0';
                break;
            }
            ELOG("read() failed!\n");
            exit(EXIT_FAILURE);
        }
        if (n == 0) {
            if (rec == buffer_len) {
                buffer_len += STDIN_CHUNK_SIZE;
                buffer = realloc(buffer, buffer_len);
            } else {
                /* remove trailing newline and finish up */
                buffer[rec-1] = '\0';
                break;
            }
        }
        rec += n;
    }
    if (strlen(buffer) == 0) {
        FREE(buffer);
        return;
    }
    FREE(statusline);
    statusline = buffer;
    DLOG("%s\n", buffer);
    draw_bars();
}

/*
 * We received a sigchild, meaning, that the child-process terminated.
 * We simply free the respective data-structures and don't care for input
 * anymore
 *
 */
void child_sig_cb(struct ev_loop *loop, ev_child *watcher, int revents) {
    DLOG("Child (pid: %d) unexpectedly exited with status %d\n",
           child_pid,
           watcher->rstatus);
    cleanup();
}

/*
 * 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.
 * 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], tmp[2];
        /* This pipe will be used to communicate between e.g. i3status and i3bar */
        pipe(fd);
        /* 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]);

                        dup2(fd[1], STDOUT_FILENO);

                        static const char *shell = NULL;

                        if ((shell = getenv("SHELL")) == NULL)
                            shell = "/bin/sh";

                        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);

    stdin_io = malloc(sizeof(ev_io));
    ev_io_init(stdin_io, &stdin_io_cb, STDIN_FILENO, EV_READ);
    ev_io_start(main_loop, stdin_io);

    /* We must cleanup, if the child unexpectedly terminates */
    child_sig = malloc(sizeof(ev_child));
    ev_child_init(child_sig, &child_sig_cb, child_pid, 0);
    ev_child_start(main_loop, child_sig);

}

/*
 * kill()s the child-process (if existent) and closes and
 * free()s the stdin- and sigchild-watchers
 *
 */
void kill_child() {
    if (child_pid != 0) {
        kill(child_pid, SIGQUIT);
    }
    cleanup();
}

/*
 * Sends a SIGSTOP to the child-process (if existent)
 *
 */
void stop_child() {
    if (child_pid != 0) {
        kill(child_pid, SIGSTOP);
    }
}

/*
 * Sends a SIGCONT to the child-process (if existent)
 *
 */
void cont_child() {
    if (child_pid != 0) {
        kill(child_pid, SIGCONT);
    }
}