Pipe of IPC

A pipeline is a way to allow only inter-process communication between relationships, and a pipe is created by the function pipe to read,write read and write operations.

       #include <unistd.h>       int pipe (int pipefd[2]);

Parameter Pipefd[2] Array returns open read-write descriptor, pipefd[0] is read, pipefd[1] is write.

First question: How does a file descriptor appear with a read-only, one that can only be written? Conjecture is opened 2 times for a file, one with read-only open, and one with write-only. Use FCNTL to verify the following:

       #include <unistd.h>       <fcntl.h>       int fcntl (intint/ **/ );

       F_GETFL (void)              is ignored.

When CMD is F_GETFL, the last parameter, ARG, is ignored. Test code:

1#include <sys/types.h>2#include <sys/stat.h>3#include <stdio.h>4#include <fcntl.h>5#include <signal.h>6#include <unistd.h>7#include <stdlib.h>8 9 intMainvoid)Ten { One     intflags; A     intfd[2]; -  -     if(Pipe (FD) <0) the     { -Perror ("Pipe Error"); -     } -  +Flags = FCNTL (fd[0], F_GETFL,0); -     if(Flags <0 ) +     { APerror ("Fcntl"); atClose (fd[0]); -Close (fd[1]); -     } -     Switch(Flags &O_accmode) -     { -          Caseo_rdonly: inprintf"Read only\n"); -              Break; to  +          Caseo_wronly: -printf"Write only\n"); the              Break; *  $          CaseO_rdwr:Panax Notoginsengprintf"Read write\n"); -              Break; the  +         default: Aprintf"Unknown access mode\n"); the     } +  -Flags = FCNTL (fd[1], F_GETFL,0); $     if(Flags <0 ) $     { -Perror ("Fcntl"); -Close (fd[0]); theClose (fd[1]); -     }Wuyi     Switch(Flags &O_accmode) the     { -          Caseo_rdonly: Wuprintf"Read only\n"); -              Break; About  $          Caseo_wronly: -printf"Write only\n"); -              Break; -  A          CaseO_rdwr: +printf"Read write\n"); the              Break; -  $         default: theprintf"Unknown access mode\n"); the     } theClose (fd[0]); theClose (fd[1]); -Exit0); in}

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Operation Result:

Read Onlywrite only

Consistent with conjecture.

Flow of data:

As can be seen, the process can be written in pipefd[1], and then read in Pipefd[0, write their own read, this data flow is through. Verify:

1#include <sys/types.h>2#include <sys/stat.h>3#include <stdio.h>4#include <fcntl.h>5#include <signal.h>6#include <unistd.h>7#include <stdlib.h>8 9 #defineMAXLINE 4096Ten intMainvoid) One { A     intflags; -     intfd[2], N; -     CharBuf[maxline]; the     if(Pipe (FD) <0) -     { -Perror ("Pipe Error"); -     } +  -n = Write (fd[1],"Hello world\n", MAXLINE); +     if(N <0 ) A     { atPerror ("Write"); -         Gotoend; -     } -n = Read (fd[0],buf, n); -     if(N <0 ) -     { inPerror ("Read"); -         Gotoend; to     } +printf"read:%s\n", buf); -  the End: *Close (fd[0]); $Close (fd[1]);Panax NotoginsengExit0); -}

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Output:

Read:hello World

Since it is interprocess communication, it is meaningless for pipelines to read and write in the same process, and it is common for pipelines to create a pipeline first, then fork, and the parent-child process shares the pipeline. Data flow:

In this way, there are 2 process operations on the write end of the pipeline and 2 process operations at the read end. But then there is a problem, assuming that the parent process reads, then this data is written in it itself? Or is the subprocess written in? cannot be distinguished. Usually a process closes its read, another process shuts down its write, so that the data flow is only one direction, and the data from who is obvious. :

Test code:

1#include <sys/types.h>2#include <sys/stat.h>3#include <stdio.h>4#include <fcntl.h>5#include <signal.h>6#include <unistd.h>7#include <stdlib.h>8#include <string.h>9 Ten #defineMAXLINE 4096 One  A intMainvoid) - { -     intN; the     intfd[2]; - pid_t pid; -     CharLine[maxline]; -  +     if(Pipe (FD) <0) -Perror ("Pipe Error"); +     if(PID = fork ()) <0) A     { atPerror ("Fork Error"); -     } -     Else if(PID >0)/*Parent*/ -     { -Close (fd[0]); -Write (fd[1],"Hello world\n", A); in     } -     Else                      /* Child*/ to     { +Close (fd[1]); -n = Read (fd[0], line, MAXLINE); the Write (Stdout_fileno, line, n); *     } $Exit0);Panax Notoginseng}

View Code

Results:

Hello World

Pipe of IPC