Dup/dup2

Objectives

1. Learn about dup.

Notes

• This is 5.5
• int dup(int oldfd);
  • Will copy the old file descriptor into the next available location.
  • This will lead to two file descriptors pointing at the same location in the open file table.
  • You need unistd.h
  • Return,
    • The new descriptor if successful
    • -1 if fail
• Frequently you see

  • fd = open(...);
    
    close(STDOUT_FILENO);
    dup(fd); 

  • This will close stdout
  • Then copy the newly opened file descriptor down, so the file becomes stdout.
  • But this is subject to a race condition, and there is a better call.
• int dup2(int oldfd, int newfd);
  • Will dup oldfd to newfd if possible.
  • If newfd is open, it will be closed.
  • Essentially close then dup
  • But not subject to race conditions.
• Gaze upon dupDemo.cpp if you will!