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Pipe Ends

Question Text​

Which end of a pipe created by pipe() is for reading and which one is for writing?

Question Answers​

  • pipefds[0] is for reading; pipefds[1] is for writing

  • pipefds[0] is for writing; pipefds[1] is for reading

  • both heads are for reading and writing;

Feedback​

Running the binary first tells us which file descriptor is which:

student@os:~/.../lab/support/pipes$ ./anonymous_pipes
pipedes[0] = 3; pipedes[1] = 4
 * pipe created
 -- Press ENTER to continue ...

Then lsof gives us the complete answer:

student@os:~/.../lab/support/pipes$ lsof -w -p $(pidof anonymous_pipes)
anonymous 22474  student  cwd    DIR    8,1      504  296964 /media/student/2TB/Chestii/Poli/Asistent/SO/operating-systems-oer/content/chapters/io/lab/support/pipes
anonymous 22474  student  rtd    DIR  259,6     4096       2 /
anonymous 22474  student  txt    REG    8,1    26712  296968 /media/student/2TB/Chestii/Poli/Asistent/SO/operating-systems-oer/content/chapters/io/lab/support/pipes/anonymous_pipes
anonymous 22474  student  mem    REG  259,6  2029592 1857435 /usr/lib/x86_64-linux-gnu/libc-2.31.so
anonymous 22474  student  mem    REG  259,6   191504 1835092 /usr/lib/x86_64-linux-gnu/ld-2.31.so
anonymous 22474  student    0u   CHR  136,0      0t0       3 /dev/pts/0
anonymous 22474  student    1u   CHR  136,0      0t0       3 /dev/pts/0
anonymous 22474  student    2u   CHR  136,0      0t0       3 /dev/pts/0
anonymous 22474  student    3r  FIFO   0,13      0t0  252007 pipe
anonymous 22474  student    4w  FIFO   0,13      0t0  252007 pipe

The last 2 lines are the 2 ends of the pipe: 3 and 4. Note each of these numbers is followed by a letter. As you might have guessed:

  • r means "read"
  • w means "write"
  • u means both "read" and "write"

Also, the man page is quite clear on this issue:

pipefd[0] refers to the read end of the pipe. pipefd[1] refers to the write end of the pipe.