第 37 章 守护进程 (Daemons)

# 查看 daemon 进程（Linux）
$ ps -eo 'pid ppid pgid sid tty cmd' | grep -E 'd\s' | head -5

// 摘自《The Linux Programming Interface》第 37 章 — Listing 37-2
// 摘自 daemons/become_daemon.c
int becomeDaemon(int flags) {
    int maxfd, fd;
    /* Step 1: fork + parent exit */
    switch (fork()) {
    case -1: return -1;
    case 0: break;                  /* Child falls through */
    default: _exit(EXIT_SUCCESS);   /* Parent exits */
    }
    /* Step 2: setsid */
    if (setsid() == -1) return -1;
    /* Step 3: second fork */
    switch (fork()) {
    case -1: return -1;
    case 0: break;
    default: _exit(EXIT_SUCCESS);
    }
    /* Step 4-7 based on flags */
    if (!(flags & BD_NO_UMASK0)) umask(0);
    if (!(flags & BD_NO_CHDIR)) chdir("/");
    if (!(flags & BD_NO_CLOSE_FILES)) {
        maxfd = sysconf(_SC_OPEN_MAX);
        if (maxfd == -1) maxfd = BD_MAX_CLOSE;  /* 8192 fallback */
        for (fd = 0; fd < maxfd; fd++) close(fd);
    }
    if (!(flags & BD_NO_REOPEN_STD_FDS)) {
        close(STDIN_FILENO);
        fd = open("/dev/null", O_RDWR);    /* 应该 == 0 */
        if (fd != STDIN_FILENO) return -1;
        dup2(STDIN_FILENO, STDOUT_FILENO);
        dup2(STDIN_FILENO, STDERR_FILENO);
    }
    return 0;
}

// 摘自《The Linux Programming Interface》第 37 章
static volatile sig_atomic_t terminate = 0;
static void termHandler(int sig) { terminate = 1; }
int main() {
    signal(SIGTERM, termHandler);
    /* daemon 业务逻辑 */
    while (!terminate) {
        /* 工作单元 */
    }
    /* 清理：flush log / close fd / unlink PID file */
    cleanup();
    return 0;
}

// 摘自《The Linux Programming Interface》第 37 章 — Listing 37-3
// 摘自 daemons/daemon_SIGHUP.c
static volatile sig_atomic_t hupReceived = 0;
static void sighupHandler(int sig) {
    hupReceived = 1;   /* 简洁 — handler 内只设标志 */
}
int main(int argc, char *argv[]) {
    const int SLEEP_TIME = 15;
    int count = 0, unslept;
    struct sigaction sa;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    sa.sa_handler = sighupHandler;
    sigaction(SIGHUP, &sa, NULL);
    becomeDaemon(0);
    logOpen(LOG_FILE);
    readConfigFile(CONFIG_FILE);
    unslept = SLEEP_TIME;
    for (;;) {
        unslept = sleep(unslept);    /* Returns > 0 if interrupted */
        if (hupReceived) {
            logClose();
            logOpen(LOG_FILE);
            readConfigFile(CONFIG_FILE);
            hupReceived = 0;
        }
        if (unslept == 0) {
            count++;
            logMessage("Main: %d", count);
            unslept = SLEEP_TIME;
        }
    }
}

// 摘自《The Linux Programming Interface》第 37 章
#include <syslog.h>
void openlog(const char *ident, int log_options, int facility);
void syslog(int priority, const char *format, ...);
void closelog(void);
int setlogmask(int mask_priority);

openlog(argv[0], LOG_PID | LOG_CONS, LOG_LOCAL0);
syslog(LOG_ERROR, "Bad argument: %s", argv[1]);   /* facility 默认 LOG_LOCAL0 */
syslog(LOG_USER | LOG_INFO, "Exiting");            /* 显式 facility */

*.err               /dev/tty10
auth.notice         root
*.debug;mail.none;news.none   -/var/log/messages

// 摘自《The Linux Programming Interface》第 37 章
openlog("mydaemon", LOG_PID | LOG_CONS, LOG_DAEMON);
syslog(LOG_INFO, "Daemon started (PID=%ld)", (long) getpid());
syslog(LOG_ERR, "Failed to open %s: %m", filename);  /* %m 替换 errno */
setlogmask(LOG_UPTO(LOG_WARNING));   /* 只记录 WARNING 及以上 */
closelog();