Implementation of Pipe in Linux

Implementation of Pipe under C in Linux
Tushar B. Kute,
http://tusharkute.com

Pipe
• We use the term pipe to mean connecting a data flow
from one process to another.
• Generally you attach, or pipe, the output of one process
to the input of another.
• Most Linux users will already be familiar with the idea
of a pipeline, linking shell commands together so that
the output of one process is fed straight to the input of
another.
• For shell commands, this is done using the pipe
character to join the commands, such as
cmd1 | cmd2

Pipes in Commands
• The output of first command is given as
input to the second command.
• Examples:
– ls | wc
– who | sort
– cat file.txt | sort | wc

The pipe call
• The lower-level pipe function provides a means of passing
data between two programs, without the overhead of
invoking a shell to interpret the requested command. It also
gives you more control over the reading and writing of
data.
• The pipe function has the following prototype:
#include <unistd.h>
int pipe(int file_descriptor[2]);
• pipe is passed (a pointer to) an array of two integer file
descriptors. It fills the array with two new file descriptors
and returns a zero. On failure, it returns -1 and sets errno to
indicate the reason for failure.

File descriptors
• The two file descriptors returned are connected in a
special way.
• Any data written to file_descriptor[1] can be read
back from file_descriptor[0] . The data is processed
in a first in, first out basis, usually abbreviated to
FIFO.
• This means that if you write the bytes 1 , 2 , 3 to
file_descriptor[1] , reading from file_descriptor[0]
will produce 1 , 2 , 3 . This is different from a stack,
which operates on a last in, first out basis, usually
abbreviated to LIFO.

#include <unistd.h>
size_t write(int fildes, const void *buf,
size_t nbytes);
• It arranges for the first nbytes bytes from buf to be
written to the file associated with the file descriptor fildes.
• It returns the number of bytes actually written. This may
be less than nbytes if there has been an error in the file
descriptor. If the function returns 0, it means no data was
written; if it returns –1, there has been an error in the
write call.
The write system call

#include <unistd.h>
size_t read(int fildes, void *buf, size_t
nbytes);
• It reads up to nbytes bytes of data from the file associated
with the file descriptor fildes and places them in the data
area buf.
• It returns the number of data bytes actually read, which
may be less than the number requested. If a read call
returns 0, it had nothing to read; it reached the end of the
file. Again, an error on the call will cause it to return –1.
The read system call

#include<stdio.h>
#include<string.h>
int main()
{
int file_pipes[2], data_pro;
const char data[] = "Hello Tushar";
char buffer[20];
if (pipe(file_pipes) == 0)
{
data_pro = write(file_pipes[1], data, strlen(data));
printf("Wrote %d bytesn", data_pro);
data_pro = read(file_pipes[0], buffer, 20);
printf("Read %d bytes: %sn", data_pro, buffer);
}
return 0;
}
Example: pipe1.c

#include<stdio.h>
#include<string.h>
int main()
{
int file_pipes[2], data_pro, pid;
const char data[] = "Hello Tushar", buffer[20];
pipe(file_pipes);
pid = fork();
if (pid == 0)
{
data_pro = write(file_pipes[1], data, strlen(data));
printf("Wrote %d bytesn", data_pro);
}
else
{
data_pro = read(file_pipes[0], buffer, 20);
printf("Read %d bytes: %sn", data_pro, buffer);
}
return 0;
}
Example-2: pipe2.c (Add fork to pipe)

• Implement using Pipe: Full duplex communication
between parent and child processes. Parent
process writes a pathname of a file (the contents of
the file are desired) on one pipe to be read by child
process and child process writes the contents of
the file on second pipe to be read by parent process
and displays on standard output.
Problem Statement

How to do it?
Pipe-1Write filename Read filename
Pipe-2
Read file contents
and print on screen
Put file contents
Open file and
Read contents
[1] [0]
[1][0]

Hi friends,
How are you...?
My name is Tushar B Kute.
hello.txt

pipe(file_pipe2); /* Second pipe created */
if (pipe(file_pipe1) == 0) /* first pipe created */
fork_result = fork(); /* Child process created */
if (fork_result == 0) {
write(file_pipe1[1], filename, strlen(filename));
printf("CHILD PROCESS: Wrote filename...n");
read(file_pipe2[0], ch, 1024);
printf("CHILD PROCESS: Its contents are...n %s", ch);
}
else {
read(file_pipe1[0], buffer, 10);
printf("PARENT PROCESS: Read filename %s ...n", buffer);
fp = fopen(buffer,"r");
while(!feof(fp)) {
ch[count] = fgetc(fp);
count++;
}
fclose(fp);
write(file_pipe2[1], ch, strlen(ch));
printf("PARENT PROCESS: The Contents are written ...n");
Solution: pipe3.c

tushar@tusharkute.com
Thank you
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Implementation of Pipe in Linux

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