[APUE]不用fcntl实现dup2函数功能

dup2的函数定义为：

#include <unistd.h>
 
int dup2(int src_fd, int new_fd);

自己实现dup2函数有几个关键点：

1，检查给定的源fd是否有效，且都大于0，

2，检查目标fd是否超出了系统设定的范围，而这个值在书上是没有着重指出的，

　　比如mac限制了要小于256，ubuntu限制是1024。

3，源fd与目标fd是否相等，

4，利用系统的特性：dup总是返回最小可用的fd，不断重复dup，从而得到一个等于new_fd的fd值

　　再清除掉new_fd之前的临时fd

5，如果在4）的过程中。如果中途dup失败。则需要在返回失败前，关掉这些临时的fd。

　　因此close这些临时fd时，需要区别是创建new_fd成功还是失败了。

下面是代码，仅限于类Unix系统环境：

 /*
  * name     : dup2.c
  * func     : implement of dup2 without fcntl.h
  * author   : jungle85gopy
  * date     : 2015.12.20
  */
 
 /*
  * Note :
  *  man dup() of Mac OSX
  *      Dup() duplicates an existing object descriptor and returns
  *          its value to the calling process (fildes2 = dup(fildes)).
  *       The value must be less than the size of the table, which is returned by
  *          getdtablesize(2). the size is 256 for Mac OSX 10.11.
  *  man dup() in ubuntu, there is no info about getdtablesize(2).
  *      but Ubuntu 14.04 still has getdtablesize limit of 1024.
  */
 
 #include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/utsname.h>
 
 #define  INIT_FD   -2
 
 int my_dup2(int src_fd, int new_fd);
 int chk_dup(int src_fd, int new_fd);
 int do_dup2(int src_fd, int new_fd);
 
 /*
  * usage: dup2 src_fd, new_fd
  */
 int main(int argc, char *argv[])
 {
     if (argc != ) {
         printf("usage: dup2 src_fd new_fd\n");
         exit();
     }
 
     int new_fd = new_fd = my_dup2(atoi(argv[]), atoi(argv[]) );
     if (new_fd == -) {
         printf("dup to new_fd error!\n");
         exit();
     }
     printf("\n[main]: new fd is %d\n", new_fd);
     return new_fd;
 }
 
 /*
  * func: check the parameter of my_dup2
  * return:
  *      -1: if error
  *      0 : if success
  */
 int chk_dup(int src_fd, int new_fd)
 {
     int tbl_size = getdtablesize();
 
     printf("[my_dup2]: parameter : src fd %d\n", src_fd);
     printf("[my_dup2]: parameter : new fd %d\n\n", new_fd);
 
     if (src_fd <  || new_fd <  ) {
         printf("[my_dup2]: error: src or des parameter < 0.\n");
         return -;
     }
     else if (new_fd >= tbl_size ) {
         printf("[my_dup2]: error: des_fd out of system limit: %d\n", tbl_size);
         return -;
     }
 
     int index;
     if ( (index = dup(src_fd)) == -) {
         printf("[my_dup2]: parameter src_fd is inactive!\n");
         return -;
     } else
         close(index);
 
     return ;
 }
 
 /*
  * func: dup a file descriptor from src_fd to new_fd
  * return:
  *      new_fd if success
  *      -1     if error
  */
 int my_dup2(int src_fd, int new_fd)
 {
     int ret;
 
     if ( (ret = chk_dup(src_fd, new_fd)) == -)
         return -;
     if (src_fd == new_fd)
         return src_fd;
 
     // close new_fd, whether it is valid or not. ignore the return
     close(new_fd);
 
     if ( (ret = do_dup2(src_fd, new_fd)) == -) {
         printf("[my_dup2]: do dup failed!\n");
         return -;
     } else
         return ret;
 }
 
 /*
  * func: dup from 0 to new_fd
  */
 int do_dup2(int src_fd, int new_fd)
 {
     int index, index_hit = -, fd_array[new_fd];
 
     for (index = ; index <= new_fd; index++)
         fd_array[index] = INIT_FD;   // initial to INIT_FD
 
     printf("[my_dup2]: before dup temp fds\n");
     for (index = ; index <= new_fd; index++) {
         fd_array[index] = dup(src_fd);
         printf("[my_dup2]: index: %d, create temp fd: %d\n", index, fd_array[index]);
 
         if (fd_array[index] == -) {
             printf("[my_dup2]: dup process error!\n");
             break;
         } else if (fd_array[index] == new_fd) {
             index_hit = index;
             break;
         }
     }
 
     // close temp fd
     printf("\n[my_dup2]: to close temp fds\n");
 
     if (index_hit == -) {   // break for loops with error
         for (index = ; index < new_fd; index++) {
             if ((fd_array[index] == INIT_FD) || (fd_array[index] == -))
                 break;      // no new temp dup in array
             else {
                 close(fd_array[index]);
                 printf("[my_dup2]: index: %d, del temp fd: %d\n", index, fd_array[index]);
             }
         }
         return -;
     } else {                // break for loops with hit
         for (index = ; index < index_hit; index++) {
             close(fd_array[index]);
             printf("[my_dup2]: index: %d, temp fd: %d\n", index, fd_array[index]);
         }
     }
     return new_fd;
 }