Chapter 10.9, Problem 10.4PP

Program Plan Intro

“dup2” function:

• This function is used to copies the entry of old descriptor table to new descriptor table entry.
• Format for “dup2” function is given below,

  int dup2(int oldFD, int newFD);

• From the above format, “oldFD” means old descriptor table entry and “newFD” means new descriptor table entry.
  • It is used to copy the content of “oldFD” to “newFD”.
  • If “newFD” was already open, then “dup2” function closes the “newFD” before it copies “oldFD”.

Example:

The example for “dup2” function is shown below:

Consider the function “dup2(4, 1)”.

• There are five file descriptors for given function that is “fd0”, “fd1”, “fd2”, “fd3”, and “fd4”. Each process in the LINUX begins life with three files they are
  • File descriptor “fd0” – standard input
  • File descriptor “fd1” – standard output
  • File descriptor “fd2” – standard error
• For this function, assume two files such as “file A” and “file B”. Each file contains file table and v-node table.
  • The entries in file table are file position and reference count.
  • The entries in v-node table are file access, file size and file type.
• Before calling “dup2(4, 1)”:
  • The “fd1” points to “file A” and “fd4” points to “file B”.
  • The reference count for both files are equal to “1”.
• After calling “dup2(4, 1)”:
  • Both “fd1” and “fd2” corresponds to “file B”.
  • The “file A” has been closed and the entries in file table and v-node table of “file A” deleted.
  • The reference count for “file B” is incremented that is now reference count is “2”.
    • From the above description, any data written to standard output are redirected to “file B”.