自己实现dup2

转自　　http://blog.csdn.net/todd911/article/details/11747097

#include <stdio.h>
#include <unistd.h>  

int main(void){
    int fd = -;
    fd = mydup2(,);
    if(fd < ){
        perror("mydup2");
        return -;
    }
    printf("fd = %d\n",fd);
    return ;
}  

int mydup2(int fd, int newfd){
    //判断fd和newfd的范围是否正确
    if(fd <  || fd > ){
        printf("fd is wrong.\n");
        return -;
    }     int index = ;
    int fdarray[newfd];
    //判断fd是否已经被打开。
    if((fdarray[index] = dup(fd)) == -){
        printf("error while dup.\n");
        return -;
    }else{
        close(fdarray[index]);
    }
    //如果相等，则直接返回。
    if(fd == newfd){
        return fd;
    }
    //将newfd关闭
    close(newfd);
    //获取所要的newfd
    int newfdindex;
    for(index=; index<=newfd; index++){
        fdarray[index] = dup(fd);
        if(fdarray[index] == -){
            printf("error while dup.\n");
            return -;
        }else{
            if(fdarray[index] == newfd){
                newfdindex = index;
                break;
            }
        }
    }
    //将之前打开的fd都关闭
    for(index=; index<newfdindex; index++){
        close(fdarray[index]);
    }
    return fdarray[newfdindex];
}