小研究一下dup（）函数

手册：
       #include <unistd.h>

       int dup(int fildes);
       int dup2(int fildes, int fildes2);

       dup, dup2 - duplicate an open file descriptor

       RETURN VALUE
       Upon successful completion a non-negative  integer,  namely  the  file
       descriptor,  shall  be  returned;  otherwise, -1 shall be returned and
       errno set to indicate the error.


其实，dup可用dup2取代，方式：

fda=dup(fdb)  <=>dup2(fdb,fda)
所以以下的讨论只用dup2（）

实验用的函数：
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#define PTHA "/dev/shm/testa"
#define PTHB "/dev/shm/testb"

int main()
{
  int fda,fdb;
  int fdtmp; //to save fdb

  fdb=open(PTHB,O_CREAT | O_RDWR,0666);
  fda=open(PTHA,O_CREAT | O_RDWR,0666);
  dup2(fdb,fdtmp);
  dup2(fda,fdb); //make fdb=fda

  write(fda,"throute fda",11);//all goes to fda
  write(fdb,"throute fdb",11);//all goes to fda
  dup2(fdtmp,fdb);//restore
  write(fdb,"after invoke",12);//all goes to fdb now

  return 0;
}

功能利用，输出重定向：
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdlib.h>
#define PTHA "/dev/shm/testa"

int main()
{
  int fda;

  fda=open(PTHA,O_CREAT | O_RDWR,0666);
  dup2(fda,STDOUT_FILENO); //re-direct stdout

  system("ls");

  close(fda);

  return 0;
}

执行完生成的程序后，会发现/dev/shm/testa里含有ls命令的输出