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| <a name="Creating-a-Pipe"></a> |
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| <h3 class="section">15.1 Creating a Pipe</h3> |
| |
| <p><a name="index-creating-a-pipe-1616"></a><a name="index-opening-a-pipe-1617"></a><a name="index-interprocess-communication_002c-with-pipes-1618"></a> |
| The primitive for creating a pipe is the <code>pipe</code> function. This |
| creates both the reading and writing ends of the pipe. It is not very |
| useful for a single process to use a pipe to talk to itself. In typical |
| use, a process creates a pipe just before it forks one or more child |
| processes (see <a href="Creating-a-Process.html#Creating-a-Process">Creating a Process</a>). The pipe is then used for |
| communication either between the parent or child processes, or between |
| two sibling processes. |
| |
| <p>The <code>pipe</code> function is declared in the header file |
| <samp><span class="file">unistd.h</span></samp>. |
| <a name="index-unistd_002eh-1619"></a> |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Function: int <b>pipe</b> (<var>int filedes</var><tt>[2]</tt>)<var><a name="index-pipe-1620"></a></var><br> |
| <blockquote><p>The <code>pipe</code> function creates a pipe and puts the file descriptors |
| for the reading and writing ends of the pipe (respectively) into |
| <var>filedes</var><code>[0]</code> and <var>filedes</var><code>[1]</code>. |
| |
| <p>An easy way to remember that the input end comes first is that file |
| descriptor <code>0</code> is standard input, and file descriptor <code>1</code> is |
| standard output. |
| |
| <p>If successful, <code>pipe</code> returns a value of <code>0</code>. On failure, |
| <code>-1</code> is returned. The following <code>errno</code> error conditions are |
| defined for this function: |
| |
| <dl> |
| <dt><code>EMFILE</code><dd>The process has too many files open. |
| |
| <br><dt><code>ENFILE</code><dd>There are too many open files in the entire system. See <a href="Error-Codes.html#Error-Codes">Error Codes</a>, |
| for more information about <code>ENFILE</code>. This error never occurs in |
| the GNU system. |
| </dl> |
| </p></blockquote></div> |
| |
| <p>Here is an example of a simple program that creates a pipe. This program |
| uses the <code>fork</code> function (see <a href="Creating-a-Process.html#Creating-a-Process">Creating a Process</a>) to create |
| a child process. The parent process writes data to the pipe, which is |
| read by the child process. |
| |
| <pre class="smallexample"> #include <sys/types.h> |
| #include <unistd.h> |
| #include <stdio.h> |
| #include <stdlib.h> |
| |
| /* <span class="roman">Read characters from the pipe and echo them to </span><code>stdout</code><span class="roman">.</span> */ |
| |
| void |
| read_from_pipe (int file) |
| { |
| FILE *stream; |
| int c; |
| stream = fdopen (file, "r"); |
| while ((c = fgetc (stream)) != EOF) |
| putchar (c); |
| fclose (stream); |
| } |
| |
| /* <span class="roman">Write some random text to the pipe.</span> */ |
| |
| void |
| write_to_pipe (int file) |
| { |
| FILE *stream; |
| stream = fdopen (file, "w"); |
| fprintf (stream, "hello, world!\n"); |
| fprintf (stream, "goodbye, world!\n"); |
| fclose (stream); |
| } |
| |
| int |
| main (void) |
| { |
| pid_t pid; |
| int mypipe[2]; |
| |
| /* <span class="roman">Create the pipe.</span> */ |
| if (pipe (mypipe)) |
| { |
| fprintf (stderr, "Pipe failed.\n"); |
| return EXIT_FAILURE; |
| } |
| |
| /* <span class="roman">Create the child process.</span> */ |
| pid = fork (); |
| if (pid == (pid_t) 0) |
| { |
| /* <span class="roman">This is the child process. |
| Close other end first.</span> */ |
| close (mypipe[1]); |
| read_from_pipe (mypipe[0]); |
| return EXIT_SUCCESS; |
| } |
| else if (pid < (pid_t) 0) |
| { |
| /* <span class="roman">The fork failed.</span> */ |
| fprintf (stderr, "Fork failed.\n"); |
| return EXIT_FAILURE; |
| } |
| else |
| { |
| /* <span class="roman">This is the parent process. |
| Close other end first.</span> */ |
| close (mypipe[0]); |
| write_to_pipe (mypipe[1]); |
| return EXIT_SUCCESS; |
| } |
| } |
| </pre> |
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