Pipes

Figure 9: How a pipe is organized.

• A pipe is a communication device that permits unidirectional communication. (see Fig. 1.4)
  • data written to the write end of the pipe,
  • read back from the read end,
  • pipes are serial devices; the data is always read from the pipe in the same order it was written.
• This shell command causes the shell to produce two child processes, one for ls and one for less:

  $ ls | less
  

• A pipe's data capacity is limited.
  • if the writer process writes faster than the reader process consumes the data,
  • if the pipe cannot store more data, the writer process blocks until more capacity becomes available,
  • if the reader tries to read but no data is available, it blocks until data becomes available.
• Thus, the pipe automatically synchronizes the two processes.
• To create a pipe, invoke the pipe command. Supply an integer array of size 2. The call to pipe stores the reading file descriptor in array position 0 and the writing file descriptor in position 1.

  int pipe_fds[2]; 
  int read_fd; 
  int write_fd; 
  pipe (pipe_fds); 
  read_fd = pipe_fds[0]; 
  write_fd = pipe_fds[1];