Pipes Between Processes

A pipe between two processes is a pair of files that is created in a parent process. The pipe connects the resulting processes when the parent process forks. A pipe has no existence in any file name space, so it is said to be anonymous. A pipe usually connects only two processes, although any number of child processes can be connected to each other and their related parent by a single pipe.

A pipe is created in the process that becomes the parent by a call to pipe(2). The call returns two file descriptors in the array passed to it. After forking, both processes read from p[0] and write to p[1]. The processes actually read from and write to a circular buffer that is managed for them.

Because calling fork(2) duplicates the per-process open file table, each process has two readers and two writers. Closing the extra readers and writers enables the proper functioning of the pipe. For example, no end-of-file indication would ever be returned if the other end of a reader is left open for writing by the same process. The following code shows pipe creation, a fork, and clearing the duplicate pipe ends.

#include <stdio.h>
#include <unistd.h>
...
        int p[2];
...
        if (pipe(p) == -1) exit(1);
        switch( fork() )
        {
                case 0:                   /* in child */
                        close( p[0] );
                        dup2( p[1], 1);
                        close P[1] );
                        exec( ... );
                        exit(1);
                default:                  /* in parent */
                        close( p[1] );
                        dup2( P[0], 0 );
                        close( p[0] );
                        break;
        }
        ...

The following table shows the results of reads from a pipe and writes to a pipe, under certain conditions.

Table 6-1 Read/Write Results in a Pipe

Blocking can be prevented by calling fcntl(2) on the descriptor to set FNDELAY. This causes an error return (-1) from the I/O call with errno set to EWOULDBLOCK.