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.
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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.