Requirements

In this assignment, you will need to write your own dup2 function that behaves the same way as the dup2 function described in Section 3.12.

  • Your dup2 must copy the file descriptor oldfd and use the newfd as the target fd.
  • Make sure that the oldfd and the newfd point to the same file.
  • The return file descriptor should be the new newfd that points to the file table of oldfd. If error occurs, you have to return -1.
  • If newfd is not closed, you have to close the newfd before you copy the fds.
  • Your dup2 should handle invalid file descriptors (please check out the valid range of file descriptors online) and others error status.
  • Note that you can not use dup2, fcntl functions in your implementation.

Implementation

  1. KEYIDEA creates a copy of the file descriptor oldfd, using the lowest-numbered unused descriptor for the new descriptor.
    所以我們重複呼叫dup，直到用到目標fd，再將之前多複製的fd關掉，就可以實作出dup2的功能

  2. 呼叫sysconf(_SC_OPEN_MAX)太多次會crash，需將結果存下來

link to assignment2