UNIX环境下，如何实现dup2函数，不允许调用fcntl？

langue

     In dup2(), the value of the new descriptor newd is specified.  If this
     descriptor is already in use, it is first deallocated as if a close(2)
     call had been done first.  When newd equals oldd, dup2() just returns
     without affecting the close-on-exec flag.

andre.ease

int my_dup2(int oldfd, int newfd)

{

int i, fd[newfd>oldfd?newfdldfd];

/* newfd is equal oldfd then we do nothing just return */

if (newfd == oldfd)　return newfd;

/* close newfd whether newfd is a real fd */

close(newfd);

for (i = 0;  (fd = dup(oldfd)) != newfd; i++) ;

for (i–; i >= 0; i–) close(fd);

return newfd;

}

sallow08

#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <error.h>
#include <sys/stat.h>
#include <sys/types.h>

int my_dup2(int oldfd, int newfd)

{

        int i, fd[newfd>oldfd?newfdldfd], fd2;

        /* newfd is equal oldfd then we do nothing just return */
       
        if (newfd == oldfd)
                return newfd;

        /* close newfd whether newfd is a real fd */

        close(newfd);

        for (i = 0;  (fd = dup(oldfd)) != newfd; i++) ;
    fd2 = fd;
        for (i--; i >= 0; i--) close(fd);

        return fd2;

}

int main(void)
{
        int fd, fd2 ,i;
        char string[] ="adfasfasDF";
        char buf[1024]={'\0'};
        struct stat statbuf;
        fd = open("tmpfile", O_CREAT|O_RDWR|O_TRUNC, S_IRUSR|S_IWUSR);       
        if(fd == -1)
        {
                perror("error";
                exit(1);
        }
       
        i = write(fd,string,sizeof(string));
        if(i == -1)
        {
                perror("werror";
                exit(1);
        }
       
        fd2 = fd +3;
       
        i= my_dup2(fd,fd2);

        lseek(fd2,-sizeof(string), 1);
        i=read(fd2, buf, sizeof(string));
        printf("buf1: %s\n", buf);
        return 0;
}

logicBaby

呵呵， 以前回过了的，俺用的笨办法~~
http://bbs.chinaunix.net/viewthread.php?tid=1049414#pid9993690

bsdc

dup2是个原子操作，实现的不能适应任何环境吧

styleplus

原帖由 logicBaby 于 2009-5-8 00:19 发表
呵呵， 以前回过了的，俺用的笨办法~~
http://bbs.chinaunix.net/viewthread.php?tid=1049414#pid9993690

用dup来实现dup2

stone903

学习了 貌似这样确实可以
那个DUP函数回来的FD是随机的呢还是递增的？能确保递增吗？
但是会有什么出错异常呢？