CS 3733 Operating Systems Notes: USP Chapter 6: UNIX Special Files

Pipes
From the command line:
ls -l | sort -nr +4 | head
produces a list of files sorted by size with only the largest ones displayed.
This is implemented with the pipe system call:

#include <unistd.h>

int pipe(int fildes[2]);

• This creates a communication buffer that the caller can access through file descriptors fildes[0] and fildes[1].
• The data written to fildes[1] is read from fildes[0] on a first-in-first-out bases.
• The pipe has no external or permanent name and can only be accessed through its two file descriptors.
• The pipe can only be used by the process that created it and its descendents.

Program 6.2, synchronizefan, on page 187 shows a process fan in which the parent writes n bytes to a pipe and then each process reads one byte from the pipe.
No processes can continue until all processes have been created.

Example 6.5, page 188:
ls -l | sort -nr +4
Figure 6.1 shows how the pipe and the processes are connected.

Program 6.3, simpleredirect, on page 190 shows how this might be implemented.

Figures 6.2 through 6.4 shoud the status of the program as it progresses.

Figure 6.1 (page 189): Status of the file descriptor table during the execution of Example 6.5.

Figure 6.2 (page 191): Status of the file descriptor table after the fork in Program 6.3.

Figure 6.3 (page 191): Status of the file descriptor table after both dup2 functions in Program 6.3.

Figure 6.4 (page 192): Status of the file descriptor table after all close calls in Program 6.3.

FIFOs
FIFOs, or named pipes allow you to use pipes between processes that are not related.
You can create a named pipe with the mkfifo command or the mkfifo system call.

#include <sys/stat.h>

int mkfifo(const char *path, mode_t mode);

Pipes and the Client-Server Model
In the client-server model of communication:

• A server process waits for requests from clients.
• Client processes request service from a server.
• A process can act as both a client and a server

Program 6.8, pipeclient, on page 198 is a client process that opens the named pipe for writing and copies everyting from standard input to the pipe.

Important property of pipes and FIFOs:
A write of at most PIPE_BUF bytes to a pipe or FIFO is atomic.
Note that reads are not necessarily atomic.

Terminal Control
Terminals are represented by special files that with additional control features that determine such things as when input is delivered to the terminal and how input and output are related.

Terminals are controlled from the command line with the stty command.

stty [-a | -g]
stty operands

Programs access terminal characteristics through a structure of type struct termios.
This structure contains at least the following members:

   tcflag_t    c_iflag;       /* input modes */
   tcflag_t    c_oflag;       /* output modes */
   tcflag_t    c_cflag;       /* control modes */
   tcflag_t    c_lflag;       /* local modes */
   cc_t        c_cc[NCCS];    /* control characters */

How these characters behave is determined by the mode of the terminal, either canonical or noncanonical.
The special control cahracters are listed in Table 6.1 on page 206.

The other fields of the termios structure are described in Table 6.2 on page 210.

Two functions are used to get and set the termios structure:

  #include <termios.h>

  int tcgetattr(int fildes, struct termios *termios_p);
  int tcsetattr(int fildes, int optional_actions,
                const struct termios *termios_p);

Program 6.11, ttysetchar, on page 206 shows how to set a particular control character.
Program 6.12, setecho, on page 207 shows how to turn echo on or off.
Program 6.13, passwordnosigs, on page 208 shows how to prompt for a password with the echo off.

When setting noncanonical mode, you should do the following:

• Save the current mode.
• Set noncanonical mode.
• Before exiting, restore the mode.

Program 6.14, setnoncanonical, on page 212 sets noncanonical mode.
It uses ttsetchar from Program 6.11.

Program 6.15, savetermios, on page 213 can be used to save and restore the termios structure.