/*****************************************************************************
- fork daemon:
- (https://github.com/pasce/daemon-skeleton-linux-c/)
- (https://nullraum.net/how-to-create-a-daemon-in-c/)
- forking a daemon requires the following steps:
- o - fork off the parent process, so now the child runs in background.
- o - setsid to create a new session. the process is now the session leader and the process group leader of the new process group.
- o - catch signals
- o - fork again. let the parent terminate so the child gets rid of session leader.
- o - umask
- o - chdir
- o - close all open fds.
*****************************************************************************/
/*****************************************************************************
* NAME: UpdateSignalHandling
*
* DESCRIPTION:
* this function sets up the signal handling and masking for the daemon
* process. the following is done here:
*
* o - change some default signal disposition with sigaction().
* o - block all signals. we will use sigwait() in a dedicated thread
* to process the signals we are interested in.
*
* the failures in this function are non-fatal. so we keep going after
* taking a note of the error code.
*
* INPUTS:
* NONE.
*
* RETURN:
* int.
*/
static
int
UpdateSignalHandling(
void
)
{
sigset_t sigset;
struct sigaction sa = { .sa_handler = SIG_IGN };
int status = 0;
if (sigfillset(&sa.sa_mask) == 0)
{
static int IGNORE_SIG[] = { SIGINT, SIGTERM, SIGCHLD, SIGHUP, SIGPIPE, SIGIO };
unsigned int i = 0;
for (i = 0; i < sizeof(IGNORE_SIG)/sizeof(IGNORE_SIG[0]); i++)
{
if (sigaction(IGNORE_SIG[i], &sa, NULL) != 0)
{
status = -errno;
}
}
}
else
{
status = -errno;
}
if (sigfillset(&sigset) == 0)
{
static int UNMASK_SIG[] = { SIGBUS, SIGFPE, SIGILL, SIGSEGV };
unsigned int i = 0;
int r = 0;
for (i = 0; i < sizeof(UNMASK_SIG)/sizeof(UNMASK_SIG[0]); i++)
{
(void)sigdelset(&sigset, UNMASK_SIG[i]);
}
/*
* per man page, the use of sigprocmask() is unspecified in MT process.
* use pthread_sigmask() instead.
*/
r = pthread_sigmask(SIG_BLOCK, &sigset, NULL);
if (r != 0)
{
status = -r;
}
}
else
{
status = -errno;
}
return status;
}
/*****************************************************************************
* NAME: MakeDir
*
* DESCRIPTION:
* this function makes the directory recursively.
*
* INPUTS:
* DirPath - [in] the dir path.
* Mode - [in] the mode/permission of the dirs.
*
* RETURN:
* int. errno is also set just for convenience of ForkDaemon().
*/
static
int
MakeDir(
char const* DirPath,
mode_t Mode
)
{
int status = 0;
if (mkdir(DirPath, Mode) != 0)
{
status = -errno;
if (status == -ENOENT)
{
size_t len = strlen(DirPath);
char* end = strchr(DirPath, '/');
if (len > 0 && end != NULL && DirPath + len > end + 1)
{
char* path = strdup(DirPath);
if (path != NULL)
{
char* p = path;
status = 0;
for ( ; ; )
{
if ((p = strchr(p + 1, '/')) != NULL)
{
*p = 0;
}
if (mkdir(path, Mode) != 0 && errno != EEXIST)
{
status = -errno;
break;
}
if (p == NULL)
{
break;
}
*p = '/';
}
free(path);
}
else
{
status = -ENOMEM;
}
}
}
else if (status == -EEXIST)
{
status = 0;
}
}
errno = -status;
return status;
}
/*****************************************************************************
* NAME: ForkDaemon
*
* DESCRIPTION:
* this function forks the process into a daemon and exits the parent.
* by doing this the orphaned child will be monitored by init.
*
* INPUTS:
* WorkingDir - [in] the working directory of the daemon.
*
* RETURN:
* STATUS. if the return value is STATUS_PARENT, the caller is
* forked as the parent and must clean up if needed and exit ASAP.
*/
#define STATUS_PARENT 1
static
int
ForkDaemon(
char const* WorkingDir
)
{
int status = 0;
pid_t pid = fork();
if (pid == 0)
{
if (setsid() != -1)
{
int r = UpdateSignalHandling();
if (r != 0)
{
/* signal handler update failures are non-fatal. */
printf("failed to update signal handlers (%d). still ok to proceed.", r);
}
pid = fork();
if (pid == 0)
{
(void)umask(S_IWGRP | S_IWOTH);
if (chdir(WorkingDir) == 0 ||
(errno == ENOENT &&
MakeDir(WorkingDir, S_IRWXU) == LW_STATUS_OK &&
chdir(WorkingDir) == 0))
{
/* now close all FDs. in case trace/log uses FDs, we uninit then reinit it. */
int fd;
(void)fprintf(stderr, "CommServer running as service...\n");
/*all fds opened are being closed here*/
for (fd = sysconf(_SC_OPEN_MAX); fd >= 0; fd--)
{
(void)close(fd);
}
}
else
{
status = -errno;
(void)fprintf(stderr, "failed to enter/create working dir: %s (%d)\n",
WorkingDir, status);
}
}
else if (pid != -1)
{
status = STATUS_PARENT;
}
else
{
status = -errno;
}
}
else
{
status = -errno;
}
}
else if (pid != -1)
{
status = STATUS_PARENT;
}
else
{
status = -errno;
}
return status;
}
-
第一次fork的作用是让shell 认为本条命令 已经终止，不用挂在终端输入上。还有一个作用是为后面setsid服务。setsid的调用者不能是进程组组长(group leader). 此时父进程是进程组组长。
-
setsid() 是本函数最重要的一个调用。它完成了daemon函数想要做的大部分事情。调用完整个函数。子进程是会话组长(sid==pid)，也是进程组组长(pgid == pid)，并且脱离了原来控制终端。到了这一步，基本上不管控制终端如何怎么样。新的进程都不会收到那些信号。
-
经过前面2个步骤，基本想要做的都做了。第2次fork不是必须的。也看到很多开源服务没有fork第二次。fork第二次主要目的是。防止进程再次打开一个控制终端。因为打开一个控制终端的前提条件是该进程必须是会话组长。再fork一次，子进程ID != sid（sid是进程父进程的sid）。所以也无法打开新的控制终端。
daemon目的就是防止终端产生的一些信号让进程退出。上面函数并没有直接调用signal函数去处理它。而是间接通过fork和setsid函数使用更少代码优雅处理。而被有些人误以为是僵死进程的原因需要这样处理。