daemons/become_daemon.c

This is daemons/become_daemon.c (Listing 37-2, page 770), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU Lesser General Public License, version 3.

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/* become_daemon.c

   A function encapsulating the steps in becoming a daemon.
*/
#include <sys/stat.h>
#include <fcntl.h>
#include "become_daemon.h"
#include "tlpi_hdr.h"
int                                     /* Returns 0 on success, -1 on error */
becomeDaemon(int flags)
{
    int maxfd, fd;

    switch (fork()) {                   /* Become background process */
    case -1: return -1;
    case 0:  break;                     /* Child falls through... */
    default: _exit(EXIT_SUCCESS);       /* while parent terminates */
    }

    if (setsid() == -1)                 /* Become leader of new session */
        return -1;

    switch (fork()) {                   /* Ensure we are not session leader */
    case -1: return -1;
    case 0:  break;
    default: _exit(EXIT_SUCCESS);
    }

    if (!(flags & BD_NO_UMASK0))
        umask(0);                       /* Clear file mode creation mask */

    if (!(flags & BD_NO_CHDIR))
        chdir("/");                     /* Change to root directory */

    if (!(flags & BD_NO_CLOSE_FILES)) { /* Close all open files */
        maxfd = sysconf(_SC_OPEN_MAX);
        if (maxfd == -1)                /* Limit is indeterminate... */
            maxfd = BD_MAX_CLOSE;       /* so take a guess */

        for (fd = 0; fd < maxfd; fd++)
            close(fd);
    }

    if (!(flags & BD_NO_REOPEN_STD_FDS)) {
        close(STDIN_FILENO);            /* Reopen standard fd's to /dev/null */

        fd = open("/dev/null", O_RDWR);

        if (fd != STDIN_FILENO)         /* 'fd' should be 0 */
            return -1;
        if (dup2(STDIN_FILENO, STDOUT_FILENO) != STDOUT_FILENO)
            return -1;
        if (dup2(STDIN_FILENO, STDERR_FILENO) != STDERR_FILENO)
            return -1;
    }

    return 0;
}

 

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