daemonize of a process? - c

Question

I'm learning apue and I try to daemonize a process according to the code sample in apue. The code is as follows:
#include <syslog.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/resource.h>
int damonize(const char *cmd)
{
int i, fd0, fd1, fd2;
pid_t pid;
struct rlimit rl;
struct sigaction sa;
umask(0);
if(getrlimit(RLIMIT_NOFILE, &rl) < 0)
{
return 1;
}
if((pid = fork()) < 0)
{
return 2;
}
else if(pid != 0)
{
exit(0);
}
setsid();
sa.sa_handler = SIG_IGN;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
if(sigaction(SIGHUP, &sa, NULL) < 0)
{
return 3;
}
if((pid = fork()) < 0)
{
return 4;
}
else if(pid > 0)
{
exit(0);
}
if(chdir("/") < 0)
{
return 5;
}
if(rl.rlim_max == RLIM_INFINITY)
{
rl.rlim_max = 1024;
}
for(i = 0; i < rl.rlim_max; i++)
{
close(i);
}
fd0 = open("/dev/null", O_RDWR);
fd1 = dup(0);
fd2 = dup(0);
openlog(cmd, LOG_CONS, LOG_DAEMON);
if(fd0 != 0 || fd1 != 1 || fd2 != 2)
{
syslog(LOG_ERR, "unexpected file descriptors %d %d %d\n", fd0, fd1, fd2);
return 6;
}
}
int main(void)
{
FILE *fp;
int id;
fp = fopen("test.txt", "w+");
id = damonize("ls");
fprintf(fp, "%d", id);
fclose(fp);
exit(0);
}
I run the above program and use ps -axj, but there's no daemon process created by the program, and threre's no output in the file test.txt. My question is
What's wrong in my code? What causes the above two problems?

Answer 1

You won't see the daemonized process because it doesn't stick around. After it tries writing to the file, it exits.
But it won't write to the file, because your daemonize routine closes every file handle (and that's what fopen() uses under the hood). Try opening the file in main() after daemonize(), or in the loop closing all file descriptors, exclude the one associated with the file using fileno().

Answer 2

I am afraid that your program is oversophisticated (if such a word exists in English). You are spawning one child process, exit parents, then the child spawns another child and exits. The child of the child is then closing all possible (even not opened) file descriptors, then it opens "/dev/null" and redirect standard input, output and error there. The "daemonisation" is finished and your program tries to write some number into a file "fp" in the main function. However, this fp have been closed long time ago in daemonize.
In other words, the main problem is that your daemonize function is closing all possible file descriptors in the loop:
for(i = 0; i < rl.rlim_max; i++) close(i);
However, if you want to daemonize a process why not to start with a simple solution and once it works you can add features while keeping it working. For example if you start with:
int daemonize() {
pid_t pid;
pid = fork();
if (pid > 0) exit(EXIT_SUCCESS);
if (pid < 0) printf("Can't fork\n");
return(pid);
}
Then you can add code for closing standard input close(STDIN_FILENO); before return, and so on. After each modification test if it is still working.