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);
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, 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);Program 6.4, parentchildfifo, on page 194 creates a named pipe.
Pipes and the Client-Server Model
In the client-server model of communication:
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.
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 operandsThe first form reports information about the terminal.
Programs access terminal characteristics through a structure of type
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 */The c_cc array holds the values of characters that have special meaning to the terminal device drivers, such as the end-of-file character (usually CTRL-D) and the interrupt character (usually CTRL-C).
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.
|canonical mode||noncanonical mode||description||usual default|
|VERASE||ERASE character||backspace or delete|
The other fields of the termios structure are described in Table 6.2 on page 210.
|c_iflag||BRKINT||signal interrupt on break|
|ICRNL||map CR to NL on input|
|IGNBRK||ignore break condition|
|IGNPAR||ignore characters with parity errors|
|INLCR||map NL to CR on input|
|INPCK||enable input parity check|
|IXOFF||enable start/stop input control|
|IXON||enable start/stop output control|
|PARMRK||mark parity errors|
|OCRNL||map CR to NL on output (POSIX:XSI Extension)|
|ONOCR||no CR output at column 0 (POSIX:XSI Extension)|
|ONLRET||NL performs CR function (POSIX:XSI Extension)|
|c_cflag||CSIZE||character size (CS5-CS8 for 5 to 8 bits)|
|CSTOPB||send two stop bits, else one|
|PARODD||odd parity, else even|
|HUPCL||hang up on last close|
|CLOCAL||ignore modem status lines|
|ECHOE||echo ERASE as an error-correcting backspace|
|ECHONL||echo a newline|
|ICANON||canonical input (erase and kill processing)|
|IEXTEN||enable extended (implementation-defined) functions|
|NOFLSH||disable flush after interrupt, quit, or suspend|
|TOSTOP||send SIGTTOU for background output \hline|
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
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.
|MIN > 0, TIME > 0|| TIME is an interbyte timer.|
If TIME expires or MIN bytes are received, read is satisfied.
|MIN > 0, TIME = 0||read blocks until at least MIN bytes received.|
|MIN = 0, TIME > 0||read is satisfied when a single byte arrives or TIME expires|
|MIN = 0, TIME = 0||minimum of number of bytes requested or number of bytes available returned|
When setting noncanonical mode, you should do the following:
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.