daemon.c

来自「unix环境高级编程的源代码」· C语言 代码 · 共 48 行

#include    <sys/types.h>
#include    <sys/stat.h>
#include    <errno.h>
#include    <fcntl.h>
#include    "ourhdr.h"

#define    PIDFILE        "daemon.pid"

int
main(void)
{
    int        fd, val;
    char    buf[10];

    if ( (fd = open(PIDFILE, O_WRONLY | O_CREAT, FILE_MODE)) < 0)
        err_sys("open error");

            /* try and set a write lock on the entire file */
    if (write_lock(fd, 0, SEEK_SET, 0) < 0)    {
        if (errno == EACCES || errno == EAGAIN)
            exit(0);    /* gracefully exit, daemon is already running */
        else
            err_sys("write_lock error");
    }

            /* truncate to zero length, now that we have the lock */
    if (ftruncate(fd, 0) < 0)
        err_sys("ftruncate error");

            /* and write our process ID */
    sprintf(buf, "%d\n", getpid());
    if (write(fd, buf, strlen(buf)) != strlen(buf))
        err_sys("write error");

            /* set close-on-exec flag for descriptor */
    if ( (val = fcntl(fd, F_GETFD, 0)) < 0)
        err_sys("fcntl F_GETFD error");
    val |= FD_CLOEXEC;
    if (fcntl(fd, F_SETFD, val) < 0)
        err_sys("fcntl F_SETFD error");

    /* leave file open until we terminate: lock will be held */

    /* do whatever ... */

    exit(0);
}