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daemon.c
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daemon.c
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#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <paths.h>
#include <sched.h>
#include <pwd.h>
#include <assert.h>
#include "daemon.h"
bool daemon_daemonize(const char *working_directory, const char *user) {
/* the return value */
bool result = false;
/* the process owner */
struct passwd *user_info = NULL;
assert(NULL != working_directory);
/* get the process owner ID */
if (NULL != user) {
user_info = getpwnam(user);
if (NULL == user_info) {
goto end;
}
}
/* change the process working directory */
if (-1 == chdir(working_directory)) {
goto end;
}
/* create a child process */
switch (fork()) {
case (-1):
goto end;
case 0:
break;
default:
exit(EXIT_SUCCESS);
}
/* make the child process a session leader */
if (((pid_t) -1) == setsid()) {
goto end;
}
/* create a child process, again */
switch (fork()) {
case (-1):
goto end;
case 0:
break;
default:
exit(EXIT_SUCCESS);
}
/* make the daemon mount namespace private, so it's impossible to replace
* files it tries to access by mounting a file system */
if (-1 == unshare(CLONE_NEWNS)) {
goto end;
}
/* drop the process permissions, by setting its owner */
if (NULL != user_info) {
if (-1 == setuid(user_info->pw_uid)) {
goto end;
}
if (-1 == seteuid(user_info->pw_uid)) {
goto end;
}
}
/* replace the standard input pipe with /dev/null */
if (-1 == close(STDIN_FILENO)) {
goto end;
}
if (-1 == open(_PATH_DEVNULL, O_RDONLY)) {
goto end;
}
/* do the same with the standard output and error output pipes */
if (-1 == close(STDOUT_FILENO)) {
goto end;
}
if (-1 == open(_PATH_DEVNULL, O_WRONLY)) {
goto end;
}
if (-1 == close(STDERR_FILENO)) {
goto end;
}
if (STDERR_FILENO != dup(STDIN_FILENO)) {
goto end;
}
/* set the file permissions mask */
(void) umask(0);
/* report success */
result = true;
end:
return result;
}
bool daemon_init(daemon_t *daemon,
const char *working_directory,
const char *user) {
/* a signal action */
struct sigaction signal_action = {{0}};
/* the socket flags */
int flags = 0;
/* the return value */
bool result = false;
assert(NULL != daemon);
assert(NULL != working_directory);
/* get the file descriptor flags */
flags = fcntl(daemon->fd, F_GETFL);
if (-1 == flags) {
goto end;
}
/* daemonize */
if (false == daemon_daemonize(working_directory, user)) {
goto end;
}
/* pick the minimum real-time signal */
daemon->io_signal = SIGRTMIN;
/* block io_signal, SIGCHLD and SIGTERM signals */
if (-1 == sigemptyset(&daemon->signal_mask)) {
goto end;
}
if (-1 == sigaddset(&daemon->signal_mask, daemon->io_signal)) {
goto end;
}
if (-1 == sigaddset(&daemon->signal_mask, SIGTERM)) {
goto end;
}
if (-1 == sigprocmask(SIG_SETMASK, &daemon->signal_mask, NULL)) {
goto end;
}
/* prevent child processes from becoming zombie processes */
signal_action.sa_flags = SA_NOCLDWAIT | SA_NOCLDSTOP;
signal_action.sa_sigaction = NULL;
if (-1 == sigemptyset(&signal_action.sa_mask)) {
goto end;
}
if (-1 == sigaction(SIGCHLD, &signal_action, NULL)) {
goto end;
}
/* set the file descriptor I/O signal */
if (-1 == fcntl(daemon->fd, F_SETSIG, daemon->io_signal)) {
goto end;
}
/* enable non-blocking, asynchronous I/O */
if (-1 == fcntl(daemon->fd, F_SETFL, flags | O_NONBLOCK | O_ASYNC)) {
goto end;
}
/* change the file descriptor ownership */
if (-1 == fcntl(daemon->fd, F_SETOWN, getpid())) {
goto end;
}
/* report success */
result = true;
end:
return result;
}
bool daemon_wait(const daemon_t *daemon, int *received_signal) {
assert(NULL != daemon);
assert(NULL != received_signal);
if (0 != sigwait(&daemon->signal_mask, received_signal)) {
return false;
}
if ((daemon->io_signal != *received_signal) &&
(SIGTERM != *received_signal)) {
return false;
}
return true;
}
pid_t daemon_fork() {
/* a signal mask */
sigset_t signal_mask = {{0}};
/* the return value */
pid_t pid = (-1);
/* empty the signal mask */
if (-1 == sigemptyset(&signal_mask)) {
return (-1);
}
/* spawn a child process */
pid = fork();
if (0 == pid) {
/* reset the child process signal mask */
if (-1 == sigprocmask(SIG_SETMASK, &signal_mask, NULL)) {
exit(EXIT_FAILURE);
}
}
return pid;
}