How can a C/C++ program put itself into background?

Question

What\'s the best way for a running C or C++ program that\'s been launched from the command line to put itself into the background, equivalent to if the user had launched fro

Answer 1

Three things need doing,

fork
setsid
redirect STDIN, STDOUT and STDERR to /dev/null

This applies to POSIX systems (all the ones you mention claim to be POSIX (but Windows stops at the claiming bit))

Answer 2

Backgrounding a process is a shell function, not an OS function.

If you want an app to start in the background, the typical trick is to write a shell script to launch it that launches it in the background.

#! /bin/sh
/path/to/myGuiApplication &

Answer 3

So, as you say, just fork()ing will not do the trick. What you must do is fork() and then re-exec(), as this code sample does:

#include stdio.h>
#include <unistd.h>
#include <string.h>

#include <CoreFoundation/CoreFoundation.h>

int main(int argc, char **argv)
{
    int i, j;

    for (i=1; i<argc; i++)
        if (strcmp(argv[i], "--daemon") == 0)
        {
            for (j = i+1; j<argc; j++)
                argv[j-1] = argv[j];

            argv[argc - 1] = NULL;

            if (fork()) return 0;

            execv(argv[0], argv);

            return 0;
        }


    sleep(1);

    CFRunLoopRun();

    CFStringRef hello = CFSTR("Hello, world!");

    printf("str: %s\n", CFStringGetCStringPtr(hello, CFStringGetFastestEncoding(hello)));

    return 0;
}

The loop is to check for a --daemon argument, and if it is present, remove it before re-execing so an infinite loop is avoided.

I don't think this will work if the binary is put into the path because argv[0] is not necessarily a full path, so it will need to be modified.

Answer 4

I'm not sure about Windows, but on UNIX-like systems, you can fork() then setsid() the forked process to move it into a new process group that is not connected to a terminal.

Answer 5

I was trying the solution.

Only one fork is needed from the parent process.

The most important point is that, after fork, the parent process must die by calling _exit(0); and NOT by calling exit(0);

When _exit(0); is used, the command prompt immediately returns on the shell.

This is the trick.

Answer 6

The simplest form of backgrounding is:

if (fork() != 0) exit(0);

In Unix, if you want to background an disassociate from the tty completely, you would do:

1. Close all descriptors which may access a tty (usually 0, 1, and 2).
2. if (fork() != 0) exit(0);
3. setpgroup(0,getpid()); /* Might be necessary to prevent a SIGHUP on shell exit. */
4. signal(SIGHUP,SIG_IGN); /* just in case, same as using nohup to launch program. */
5. fd=open("/dev/tty",O_RDWR);
6. ioctl(fd,TIOCNOTTY,0); /* Disassociates from the terminal */
7. close(fd);
8. if (fork() != 0) exit(0); /* just for good measure */

That should fully daemonize your program.