Pipes

Definition:

A one-way data path between two processes:

Figure 2.6. Pipe

Creation:

int fd[2]; int pipe(fd); fork(); ...

Two file descriptors are created: fd[0] for reading & fd[1] for writing. They are copied to the child process:

Figure 2.7. Two Pipes

To construct the one-way pipe, the Parent process close its read fd and the child its write fd.

Piping many processes

The stdout of a process can be connected to the stdin of another through a pipe. For instance, that is what happens when the following shell command is executed:

who | sort | lpr

The shell program will use the pipe system call to create the following construction:

Figure 2.8. Piping Processes

Two-way Connections

To create a two-way connection between process, two pipes have to be created. One in each direction:

Figure 2.9. Two-way Connection

Creation Sequence:

Parent Child  Create Pipe 1 and Pipe 2 Fork Close read fd of Pipe1 Close write fd of Pipe1 Close write fd of Pipe2 Close read fd of Pipe 2 

FIFO (named pipes)

Named Pipes are Pipes that have a name associated with them. They are used as data path between processes that are not related to each other.

Creation: They are created through the mknod call:

int mknod ( char *pathname, int mode, int dev);