A Linux Daemon and the STDIN/STDOUT

Question

I am working on a linux daemon and having some issues with the stdin/stdout. Normally because of the nature of a daemon you do not have any stdin or stdout. However, I do have a function in my daemon that is called when the daemon runs for the first time to specify different parameters that are required for the daemon to run successfully. When this function is called the terminal becomes so sluggish that I have to launch a seperate shell and kill the daemon with top to get a responsive prompt back. Now I suspect that this has something to do with the forking process closing the stdin/stdout but I am not quite sure how I could work around this. If you guys could shed some light on the situation that would be most appreciated. Thanks.

Edit:

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

/* setup signal handling */

/* check command line arguments */

pid_t pid, sid;

pid = fork();

if (pid < 0) {
exit(EXIT_FAILURE);
}

if(pid > 0){
exit(EXIT_SUCCESS);
}

sid = setsid();

if(sid < 0) {
exit(EXIT_FAILURE);
}

umask(027);

/* set syslogging */

/* do some logic to determine wether we are running the daemon for the first time and if we are call the one time function which uses fgets() to recieve some input */

while(1) {

/* do required work */

}

/* do some clean up procedures and exit */

return 0;
}

You guys mention using a config file. This is is exactly what I do to store the parameters recieved via input. However I still initially need to get these from the user via the stdin. The logic for determining whether we are running for the first time is based off of the existence of the config file.

Answer 1

You guys mention using a config file. This is is exactly what I do to store the parameters recieved via input. However I still initially need to get these from the user via the stdin. The logic for determining whether we are running for the first time is based off of the existence of the config file.

Instead of reading stdin, have the user write the config file themselves; check for its existence before forking, and exit with an error if it doesn't. Include a sample config file with the daemon, and document its format in your daemon's manpage. You do have a manpage, yes? Your config file is textual, yes?

Also, your daemonization logic is missing a key step. After forking, but before calling setsid, you need to close fds 0, 1, and 2 and reopen them to /dev/null (do not attempt to do this with fclose and fopen). That should fix your sluggish terminal problem.

Answer 2

Your design is wrong. Daemon processes should not take input via stdin or deliver output to stdout/stderr. You'll close those descriptors as part of the daemonizing phase. Daemons should take configuration parameters from the command line, a config file, or both. If runtime-input is required you'll have to read a file, open a socket, etc., but the point of a daemon is that it should be able to run and do its thing without a user being present at the console.

Answer 3

Normally, the standard input of a daemon should be connected to /dev/null, so that if anything is read from standard input, you get an EOF immediately. Normally, standard output should be connected to a file - either a log file or /dev/null. The latter means all writes will succeed, but no information will be stored. Similarly, standard error should be connected to /dev/null or to a log file.

All programs, including daemons, are entitled to assume that stdin, stdout and stderr are appropriately opened file streams.

It is usually appropriate for a daemon to control where its input comes from and outputs go to. There is seldom occasion for input to come from other than /dev/null. If the code was written to survive without standard output or standard error (for example, it opens a standard log channel, or perhaps uses syslog(3)) then it may be appropriate to close stdout and stderr. Otherwise, it is probably appropriate to redirect them to /dev/null, while still logging messages to a log file. Alternatively, you can redirect both stdout and stderr to a log file - beware continuously growing log files.

Your sluggish-to-impossible response time might be because your program is not paying attention to EOF in a read loop somewhere. It might be prompting for user input on /dev/null, and reading a response from /dev/null, and not getting a 'y' or 'n' back, it tries again, which chews up your system horribly. Of course, the code is flawed in not handling EOF, and counting the number of times it gets an invalid response and stopping being silly after a reasonable number of attempts (16, 32, 64). The program should shut up shop sanely and safely if it expects a meaningful input and continues not to get it.