linux知识复习1-dup dup2

#include <sys/stat.h>
#include <string.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
int main(void)
{
    #define STDOUT 1 
    int nul, oldstdout;
    char msg[] = "This is a test";
    /* create a file */
    nul = open("DUMMY.FIL", O_CREAT | O_RDWR, S_IREAD | S_IWRITE);
    /* create a duplicate handle for standard
    output */

    oldstdout = dup(STDOUT); //复制用于后续恢复
    /*
    redirect standard output to DUMMY.FIL
    by duplicating the file handle onto the
    file handle for standard output.
    */
    dup2(nul, STDOUT); //将标准输出重定向到nul
    /* close the handle for DUMMY.FIL */
    close(nul);
    /* will be redirected into DUMMY.FIL */
    write(STDOUT, msg, strlen(msg));
    /* restore original standard output
    handle */
    dup2(oldstdout, STDOUT); //将标准输出重定向到之前保存的标准输出
    /* close duplicate handle for STDOUT */
    close(oldstdout);
    return 0;
}

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进程fd表 ---- 文件表----v节点表

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指向关系改变:

dup2(nul, STDOUT)

STDOUT指向了nul的文件表；

dup2(oldstdout, STDOUT)

STDOUT指向了oldstdout保存的文件表；

打开文件的每个进程都有独立的文件表项，可以使每个进程都有自己对于该文件的当前偏移；

详细参考:

http://www.cnblogs.com/GODYCA/archive/2013/01/05/2846197.html