mandatory.c

unix环境高级编程的源代码

#include    <sys/types.h>
#include    <sys/stat.h>
#include    <sys/wait.h>
#include    <errno.h>
#include    <fcntl.h>
#include    "ourhdr.h"
int
main(void)
{
    int                fd;
    pid_t            pid;
    char            buff[5];
    struct stat        statbuf;
    if ( (fd = open("templock", O_RDWR | O_CREAT | O_TRUNC,
                                                FILE_MODE)) < 0)
        err_sys("open error");
    if (write(fd, "abcdef", 6) != 6)
        err_sys("write error");

        /* turn on set-group-ID and turn off group-execute */
    if (fstat(fd, &statbuf) < 0)
        err_sys("fstat error");
    if (fchmod(fd, (statbuf.st_mode & ~S_IXGRP) | S_ISGID) < 0)
        err_sys("fchmod error");

    TELL_WAIT();
    if ( (pid = fork()) < 0) {
        err_sys("fork error");

    } else if (pid > 0) {    /* parent */
                /* write lock entire file */
        if (write_lock(fd, 0, SEEK_SET, 0) < 0)
            err_sys("write_lock error");
        TELL_CHILD(pid);

        if (waitpid(pid, NULL, 0) < 0)
            err_sys("waitpid error");

    } else {                /* child */
        WAIT_PARENT();        /* wait for parent to set lock */

        set_fl(fd, O_NONBLOCK);

            /* first let's see what error we get if region is locked */
        if (read_lock(fd, 0, SEEK_SET, 0) != -1)    /* no wait */
            err_sys("child: read_lock succeeded");
        printf("read_lock of already-locked region returns %d\n", errno);

            /* now try to read the mandatory locked file */
        if (lseek(fd, 0, SEEK_SET) == -1)
            err_sys("lseek error");
        if (read(fd, buff, 2) < 0)
            err_ret("read failed (mandatory locking works)");
        else
            printf("read OK (no mandatory locking), buff = %2.2s\n", buff);
    }
    exit(0);
}