os.dup2

python的源码:

/*
 * Public domain dup2() lookalike
 * by Curtis Jackson @ AT&T Technologies, Burlington, NC
 * electronic address:  burl!rcj
 *
 * dup2 performs the following functions:
 *
 * Check to make sure that fd1 is a valid open file descriptor.
 * Check to see if fd2 is already open; if so, close it.
 * Duplicate fd1 onto fd2; checking to make sure fd2 is a valid fd.
 * Return fd2 if all went well; return BADEXIT otherwise.
 */

#include <fcntl.h>
#include <unistd.h>

#define BADEXIT -1

int dup2(int fd1, int fd2)
{
    if (fd1 != fd2) {
        if (fcntl(fd1, F_GETFL) < 0)
        return BADEXIT;
        if (fcntl(fd2, F_GETFL) >= 0)
        close(fd2);
    if (fcntl(fd1, F_DUPFD, fd2) < 0)
        return BADEXIT;
}
return fd2;
}

从源码上看，dup2传入两个文件描述符，fd1和fd2（fd1是必须存在的），如果fd2存在，就关闭fd2，然后将fd1代表的那个文件（可以想象成是P_fd1指针）强行复制给fd2，fd2这个文件描述符不会发生变化，但是fd2指向的文件就变成了fd1指向的文件。