Agenda Redirection:

Purpose Redirection Facts How to redirecting stdin, stdout, stderr in a

program

Pipes:

Using Pipes Named Pipes

Redirection We have learned in Unix and Linux how to redirect stdin, stdout and stderr:

< or 0< - redirect stdin to command << or 0<< - redirect stdin within command

> or 1> - redirect stdout to file (overwrite if file not empty) >> or 1>> - redirect stdout to file (append if file not empty)

2> - redirect stderr to file (overwrite if file not empty) 2>> - redirect stderr to file (append if file not empty)

Redirection Redirection Facts:

All UNIX tools use the stdin, stdout, stderr data streams, although not all stdin is read from the user (for example, the who command automatically reads stdin from a system file called /var/adm/utmp).

All UNIX tools use file descriptors 0,1 & 2.

It is the shell not the program or script that redirects I/O (ie. stdin / stdout / stderr). This fact is proven in the next slide.

Redirection

#include <stdio.h>

main( int ac, char *av[])

{

int i;

printf ("Number of args: %d, Args are:\n", ac);

for (i=0; i < ac; i++)

printf ("args[%d]: %s\n", i, av[i]);

fprintf (stderr, "This message is sent to stderr.\n");

}

Note how compiling & running this programwith arguments list the program’s arguments

arglist.c

$ listargs > output 2> error

$ cat outputNumber of args: 1, Args are:args[0]: listargs

$ cat error

This message is sent to stderr.

$ listargs arg1 arg2 > output 2>error

$ cat outputNumber of args: 3, Args are:args[0]: listargsargs[1]: arg1args[2]: arg2

Proof that shell not programredirects stdin, stdout, stderr

Redirection If it is the shell, not the program that redirects stdin, stdout and stderr, how can a program perform redirection?

Shell script -> use the exec utility (see mycp in sockets directory)

C program -> Use either: close() , open() method open(), close(), dup(), close() method open(), dup2(), close() method

Refer to stdinredir1.c, stdinredir2.c and stdinredir3.c examples in~msaul/unx511/pipes_and_redirection directory

Redirection How do these methods work in C programs?

Processes do not read from files, they read from file descriptors Steps are like “hanging-up the phone to get a dial-tone”, thus close

file descriptor, then redefine it.

Close / Open Method: close(0); /* file descriptor 0 – i.e. stdin */ open(filename, O_RDONLY); /* redefines lowest file descriptor */

Open / Close / Dup / Close / Method: fd=open(filename, O_RDONLY) /* open file to read - fd */ close(0); /* close file descriptor 0 – i.e. stdin */ newfd=dup(fd); /* make “clone” of fd and kernel uses

lowest next available fd – I.e. 0 that was closed */

close(fd); /* close original fd associated with file. Now, stdin is associated with file… */

dup2 is similar to dup,but dup2 will automaticallyclose(0).

eg. newfd=dup2(fd,0);(no close before dup2)

Pipes So far, we have shown how we can attach a file to stdin,

stdout & stderr. Pipes are another “feature” of UNIX to connect stdout of one process to stdin of another process.

Pipes will only work on local machines, so if you are interested in inter-process communication via TCP/IP, it is better to use Internet Sockets. Also, pipe are uni-directional (eg. required 2 pipes for inter-process communication (IPC).

Basically, you can program to join the file descriptors of two processes:

Eg. Connect fd0 of process1 with fd1 of process2

Pipes Basically, two methods to working with pipes:

popen() – Similar to system() call where pipe() & fork() are called automatically. This method is not very efficient, and does not give a lot of control to programmer)

Programmer uses pipe(), fork() manually. Calling pipe without using fork is like creating a process to communication with yourself.

Refer to examples popen3.c and pipe2.c in the ~msaul/unx511/pipes_and_redirection directory

Pipes How does it work?

dup and dup2 functions are used to close, then open a file descriptor (returning the new file descriptor). Like before, the Kernel uses the next available pipe (i.e. set for stdin or stdout).

The process doesn’t need to know it is communicating with a pipe.

Any standard Unix utility (used for a filter) can be used.

Notice how each process closes the file descriptors that it won’t be using…Refer to examples pipe5.c, pipe5b.c & pipe5c.c in

the~/msaul/unx511/pipes_and_redirection directory

Pipes:C programming Example

---------- Process #1 ----------

/ | \

Process #2 Process #3 Process #4

/ | \

od head wc

stdin < filename stdin < pipe[0] stdin < pipe2[0]

stdout > pipe[1] stdout > pipe2[1]

Visual example of mypipe.c (using pipe and fork system calls). Example contained in the ~msaul/unx511/pipes_and_redirection directory…

Remember: when duplicating a process using the “fork()” system call, the child process inherits everything from the parent (including pipe file descriptors)

Named Pipes The problem with pipe examples in previous examples

involve sharing stdout / stdin among parent process and its children.

Named pipes can be used can be accessed by any local process (parent or child) – i.e. known by name. Named pipes can also be used in Shell Scripts as well as C Programs…

How named pipe is created:

In C Program: mkfifo() system call In Shell Script: mkfifo shell command

Named Pipes:Scripting Example

#!/bin/bash# Time server

while truedo

rm –f /tmp/time_fifo mkfifo /tmp/time_fifo date > /tmp/time_fifodone

Time Server (time_server.bsh)

#!/bin/bash# Time client

cat /tmp/time_fifo

Time Client (time_client.bsh)

Read from named pipe

Remove Existing named pipe

Create new pipe

Redirect stdout to named pipe

Can setup mask while name pipe is created:eg. mkfifo –m 666 /tmp/time_fifo

Named Pipes:Client / Server Example C Programming Example:

Can create two named-pipes that allow communication between processes. Considered simpler/easier for programming local sockets, but not useful if communicating among different servers via TCP/IP.

time_server.c * time_client.c *

* Note that time_server.c make reference to the header file cliserv.h