Bagaimana cara membuat file hanya jika tidak ada?

pria 2 buka:

O_EXCL Pastikan panggilan ini membuat file:jika flag ini ditentukan bersama dengan O_CREAT, dan nama path sudah ada, maka open() akan gagal. Perilaku O_EXCL tidak ditentukan jika O_CREAT tidak ditentukan.

jadi, Anda dapat memanggil fd = open(name, O_CREAT | O_EXCL, 0644); /* Open() bersifat atomik. (karena suatu alasan) */

PEMBARUAN:dan tentu saja Anda harus ATAU salah satu dari bendera O_RDONLY, O_WRONLY, atau O_RDWR ke dalam argumen bendera.

Saya belajar tentang daemonisasi yang tepat di sini (dulu):

• http://www.enderunix.org/docs/eng/daemon.php

Ini bacaan yang bagus. Saya telah meningkatkan kode penguncian untuk menghilangkan kondisi balapan pada platform yang memungkinkan penguncian file penasehat dengan wilayah tertentu yang ditentukan.

Berikut cuplikan yang relevan dari proyek yang saya ikuti:

static int zfsfuse_do_locking(int in_child)
{
    /* Ignores errors since the directory might already exist */
    mkdir(LOCKDIR, 0700);

    if (!in_child)
    {
        ASSERT(lock_fd == -1);
        /*
         * before the fork, we create the file, truncating it, and locking the
         * first byte
         */
        lock_fd = creat(LOCKFILE, S_IRUSR | S_IWUSR);
        if(lock_fd == -1)
            return -1;

        /*
         * only if we /could/ lock all of the file,
         * we shall lock just the first byte; this way
         * we can let the daemon child process lock the
         * remainder of the file after forking
         */
        if (0==lockf(lock_fd, F_TEST, 0))
            return lockf(lock_fd, F_TLOCK, 1);
        else
            return -1;
    } else
    {
        ASSERT(lock_fd != -1);
        /*
         * after the fork, we instead try to lock only the region /after/ the
         * first byte; the file /must/ already exist. Only in this way can we
         * prevent races with locking before or after the daemonization
         */
        lock_fd = open(LOCKFILE, O_WRONLY);
        if(lock_fd == -1)
            return -1;

        ASSERT(-1 == lockf(lock_fd, F_TEST, 0)); /* assert that parent still has the lock on the first byte */
        if (-1 == lseek(lock_fd, 1, SEEK_SET))
        {
            perror("lseek");
            return -1;
        }

        return lockf(lock_fd, F_TLOCK, 0);
    }
}

void do_daemon(const char *pidfile)
{
    chdir("/");
    if (pidfile) {
        struct stat dummy;
        if (0 == stat(pidfile, &dummy)) {
            cmn_err(CE_WARN, "%s already exists; aborting.", pidfile);
            exit(1);
        }
    }

    /*
     * info gleaned from the web, notably
     * http://www.enderunix.org/docs/eng/daemon.php
     *
     * and
     *
     * http://sourceware.org/git/?p=glibc.git;a=blob;f=misc/daemon.c;h=7597ce9996d5fde1c4ba622e7881cf6e821a12b4;hb=HEAD
     */
    {
        int forkres, devnull;

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

        forkres=fork();
        if (forkres<0)
        { /* fork error */
            cmn_err(CE_WARN, "Cannot fork (%s)", strerror(errno));
            exit(1);
        }
        if (forkres>0)
        {
            int i;
            /* parent */
            for (i=getdtablesize();i>=0;--i)
                if ((lock_fd!=i) && (ioctl_fd!=i))       /* except for the lockfile and the comm socket */
                    close(i);                            /* close all descriptors */

            /* allow for airtight lockfile semantics... */
            struct timeval tv;
            tv.tv_sec = 0;
            tv.tv_usec = 200000;  /* 0.2 seconds */
            select(0, NULL, NULL, NULL, &tv);

            VERIFY(0 == close(lock_fd));
            lock_fd == -1;
            exit(0);
        }

        /* child (daemon) continues */
        setsid();                         /* obtain a new process group */
        VERIFY(0 == chdir("/"));          /* change working directory */
        umask(027);                       /* set newly created file permissions */
        devnull=open("/dev/null",O_RDWR); /* handle standard I/O */
        ASSERT(-1 != devnull);
        dup2(devnull, 0); /* stdin  */
        dup2(devnull, 1); /* stdout */
        dup2(devnull, 2); /* stderr */
        if (devnull>2)
            close(devnull);

        /*
         * contrary to recommendation, do _not_ ignore SIGCHLD:
         * it will break exec-ing subprocesses, e.g. for kstat mount and
         * (presumably) nfs sharing!
         *
         * this will lead to really bad performance too
         */
        signal(SIGTSTP,SIG_IGN);     /* ignore tty signals */
        signal(SIGTTOU,SIG_IGN);
        signal(SIGTTIN,SIG_IGN);
    }

    if (0 != zfsfuse_do_locking(1))
    {
        cmn_err(CE_WARN, "Unexpected locking conflict (%s: %s)", strerror(errno), LOCKFILE);
        exit(1);
    }

    if (pidfile) {
        FILE *f = fopen(pidfile, "w");
        if (!f) {
            cmn_err(CE_WARN, "Error opening %s.", pidfile);
            exit(1);
        }
        if (fprintf(f, "%d\n", getpid()) < 0) {
            unlink(pidfile);
            exit(1);
        }
        if (fclose(f) != 0) {
            unlink(pidfile);
            exit(1);
        }
    }
}

Lihat juga http://gitweb.zfs-fuse.net/?p=sehe;a=blob;f=src/zfs-fuse/util.c;h=7c9816cc895db4f65b94592eebf96d05cd2c369a;hb=refs/heads/maint