Daemonize() issues on Debian

Question

Hi,

I'm currently writing a multi-process client and a multi-treaded server for some project i have.

The server is a Daemon. In order to accomplish that, i'm using the following daemonize() code:

static void daemonize(void) { pid_t pid, sid;

/* already a daemon */
if ( getppid() == 1 ) return;

/* Fork off the parent process */
pid = fork();
if (pid < 0) {
    exit(EXIT_FAILURE);
}
/* If we got a good PID, then we can exit the parent process. */
if (pid > 0) {
    exit(EXIT_SUCCESS);
}

/* At this point we are executing as the child process */

/* Change the file mode mask */
umask(0);

/* Create a new SID for the child process */
sid = setsid();
if (sid < 0) {
    exit(EXIT_FAILURE);
}

/* Change the current working directory.  This prevents the current
   directory from being locked; hence not being able to remove it. */
if ((chdir("/")) < 0) {
    exit(EXIT_FAILURE);
}

/* Redirect standard files to /dev/null */
freopen( "/dev/null", "r", stdin);
freopen( "/dev/null", "w", stdout);
freopen( "/dev/null", "w", stderr);

}

int main( int argc, char *argv[] ) { daemonize();

/* Now we are a daemon -- do the work for which we were paid */
return 0;

}

I have a strange side effect when testing the server on Debian (Ubuntu).

The accept() function always fail to accept connections, the pid returned is -1

I have no idea what causing this, since in RedHat & CentOS it works well.

When i remove the call to daemonize(), everything works well on Debian, when i add it back, same accept() error reproduce.

I've been monitring the /proc//fd, everything looks good.

Something in the daemonize() and the Debian release just doesn't seem to work. (Debian GNU/Linux 5.0, Linux 2.6.26-2-286 #1 SMP)

Any idea what causing this?

Thank you

Answer 1

Here, when the parent exits:

/* If we got a good PID, then we can exit the parent process. */
if (pid > 0) {
    exit(EXIT_SUCCESS);
}

You should be calling _exit(), not exit(). (I am not certain that this is causing your problem, but it's possible).

What is errno set to when accept() returns -1? (You could put a perror("accept"); in the code there).

Answer 2

May I direct you to existing library function daemon(3) to accomplish the same?

Answer 3

The accept(2) manpage says:

EINVAL Socket is not listening for connections, or addrlen is invalid (e.g., is negative).

Likely you have something like

struct sockaddr_in;
socklen_t len;
...

new_fd = new_fd = accept(sockfd,(struct sockaddr *)&addr,&len);

However, you need to set len to the size of the address you pass in:

struct sockaddr_in addr;
socklen_t len;
len = sizeof(addr);
...
new_fd = new_fd = accept(sockfd,(struct sockaddr *)&addr,&len);

So, by some (un)lucky means your uninitialized 'len' variable get some nonsense value in some cases, and accept fails, while it happens to work in other scenarios.