linux进程通讯：管道(pipe)

man 7 pipe

PIPE(7)                    Linux Programmer’s Manual                   PIPE(7)

NAME
       pipe - overview of pipes and FIFOs

DESCRIPTION
       Pipes  and  FIFOs (also known as named pipes) provide a unidirectional interprocess communication channel.  A pipe has a read end and a write end.  Data written to
       the write end of a pipe can be read from the read end of the pipe.

       A pipe is created using pipe(2), which creates a new pipe and returns two file descriptors, one referring to the read end of the pipe, the other referring  to  the
       write end.  Pipes can be used to create a communication channel between related processes; see pipe(2) for an example.

       A  FIFO  (short  for  First  In  First Out) has a name within the file system (created using mkfifo(3)), and is opened using open(2).  Any process may open a FIFO,
       assuming the file permissions allow it.  The read end is opened using the O_RDONLY flag; the write end is opened using the O_WRONLY flag.  See fifo(7) for  further
       details.  Note: although FIFOs have a pathname in the file system, I/O on FIFOs does not involve operations on the underlying device (if there is one).

   I/O on Pipes and FIFOs
       The  only  difference  between pipes and FIFOs is the manner in which they are created and opened.  Once these tasks have been accomplished, I/O on pipes and FIFOs
       has exactly the same semantics.

       If a process attempts to read from an empty pipe, then read(2) will block until data is available.  If a process attempts to write to a full pipe (see below), then
       write(2) blocks until sufficient data has been read from the pipe to allow the write to complete.  Non-blocking I/O is possible by using the fcntl(2) F_SETFL oper-
       ation to enable the O_NONBLOCK open file status flag.

       The communication channel provided by a pipe is a byte stream: there is no concept of message boundaries.

       If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end-of-file (read(2)  will  return
       0).  If all file descriptors referring to the read end of a pipe have been closed, then a write(2) will cause a SIGPIPE signal to be generated for the calling pro-
       cess.  If the calling process is ignoring this signal, then write(2) fails with the error EPIPE.  An application that uses pipe(2) and fork(2) should use  suitable
       close(2) calls to close unnecessary duplicate file descriptors; this ensures that end-of-file and SIGPIPE/EPIPE are delivered when appropriate.

       It is not possible to apply lseek(2) to a pipe.
   Pipe Capacity
       A  pipe  has  a limited capacity.  If the pipe is full, then a write(2) will block or fail, depending on whether the O_NONBLOCK flag is set (see below).  Different
       implementations have different limits for the pipe capacity.  Applications should not rely on a particular capacity: an application should be designed  so  that  a
       reading process consumes data as soon as it is available, so that a writing process does not remain blocked.

       In  Linux versions before 2.6.11, the capacity of a pipe was the same as the system page size (e.g., 4096 bytes on i386).  Since Linux 2.6.11, the pipe capacity is
       65536 bytes.

   PIPE_BUF
       POSIX.1-2001 says that write(2)s of less than PIPE_BUF bytes must be atomic: the output data is written to the pipe as a contiguous sequence.  Writes of more  than
       PIPE_BUF  bytes  may  be  non-atomic:  the  kernel may interleave the data with data written by other processes.  POSIX.1-2001 requires PIPE_BUF to be at least 512
       bytes.  (On Linux, PIPE_BUF is 4096 bytes.)  The precise semantics depend on whether the file descriptor is non-blocking (O_NONBLOCK), whether there  are  multiple
       writers to the pipe, and on n, the number of bytes to be written:

       O_NONBLOCK disabled, n <= PIPE_BUF
              All n bytes are written atomically; write(2) may block if there is not room for n bytes to be written immediately

       O_NONBLOCK enabled, n <= PIPE_BUF
              If  there is room to write n bytes to the pipe, then write(2) succeeds immediately, writing all n bytes; otherwise write(2) fails, with errno set to EAGAIN.

       O_NONBLOCK disabled, n > PIPE_BUF
              The write is non-atomic: the data given to write(2) may be interleaved with write(2)s by other process; the write(2) blocks until n bytes have been written.

       O_NONBLOCK enabled, n > PIPE_BUF
              If  the  pipe  is  full, then write(2) fails, with errno set to EAGAIN.  Otherwise, from 1 to n bytes may be written (i.e., a "partial write" may occur; the
              caller should check the return value from write(2) to see how many bytes were actually written), and these bytes may be interleaved  with  writes  by  other
              processes.

   Open File Status Flags
       The only open file status flags that can be meaningfully applied to a pipe or FIFO are O_NONBLOCK and O_ASYNC.

       Setting  the  O_ASYNC flag for the read end of a pipe causes a signal (SIGIO by default) to be generated when new input becomes available on the pipe (see fcntl(2)
       for details).  On Linux, O_ASYNC is supported for pipes and FIFOs only since kernel 2.6.

   Portability notes
       On some systems (but not Linux), pipes are bidirectional: data can be transmitted in both directions between the pipe ends.  According to POSIX.1-2001, pipes  only
       need to be unidirectional.  Portable applications should avoid reliance on bidirectional pipe semantics.

 

管道读写规则

当没有数据可读时

• O_NONBLOCK disable：read调用阻塞，即进程暂停执行，一直等到有数据来到为止。
• O_NONBLOCK enable：read调用返回-1，errno值为EAGAIN。

当管道满的时候

• O_NONBLOCK disable： write调用阻塞，直到有进程读走数据
• O_NONBLOCK enable：调用返回-1，errno值为EAGAIN

若是全部管道写端对应的文件描述符被关闭，则read返回0

若是全部管道读端对应的文件描述符被关闭，则write操做会产生信号SIGPIPE

当要写入的数据量不大于PIPE_BUF时，linux将保证写入的原子性。

当要写入的数据量大于PIPE_BUF时，linux将再也不保证写入的原子性。

 

二，验证示例

示例一：O_NONBLOCK disable：read调用阻塞，即进程暂停执行，一直等到有数据来到为止。

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h> 

int main(void)
{
    int fds[2];
    if(pipe(fds) == -1){
        perror("pipe error");
        exit(EXIT_FAILURE);
    }   
    printf("begin test pipe\n");
    pid_t pid;
    pid = fork();
    if(pid == -1){
        perror("fork error");
        exit(EXIT_FAILURE);
    }   
    if(pid == 0){ 
        close(fds[0]);//子进程关闭读端
        sleep(10);
        write(fds[1],"hello",5);
        exit(EXIT_SUCCESS);
    }   

    close(fds[1]);//父进程关闭写端
    char buf[10] = {0};
    read(fds[0],buf,10);
    printf("receive datas = %s\n",buf);
    return 0;
}

运行结果：

说明：管道建立时默认打开了文件描述符，且默认是阻塞（block）模式打开

因此这里，咱们让子进程先睡眠10s，父进程由于没有数据从管道中读出，被阻塞了，直到子进程睡眠结束，向管道中写入数据后，父进程才读到数据

示例二：O_NONBLOCK enable：read调用返回-1，errno值为EAGAIN。

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>

int main(void)
{
    int fds[2];
    if(pipe(fds) == -1){
        perror("pipe error");
        exit(EXIT_FAILURE);
    }   
    printf("begin test pipe\n");
    pid_t pid;
    pid = fork();
    if(pid == -1){
        perror("fork error");
        exit(EXIT_FAILURE);
    }   
    if(pid == 0){ 
        close(fds[0]);//子进程关闭读端
        sleep(10);
        write(fds[1],"hello",5);
        exit(EXIT_SUCCESS);
    }   

    close(fds[1]);//父进程关闭写端
    char buf[10] = {0};
    int flags = fcntl(fds[0], F_GETFL);//先获取原先的flags
    fcntl(fds[0],F_SETFL,flags | O_NONBLOCK);//设置fd为非阻塞模式
    int ret;
    ret = read(fds[0],buf,10);
    if(ret == -1){

        perror("read error");
        exit(EXIT_FAILURE);
    }   

    printf("receive datas = %s\n",buf);
    return 0;
}

运行结果：

示例三：若是全部管道写端对应的文件描述符被关闭，则read返回0

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h> 

int main(void)
{
    int fds[2];
    if(pipe(fds) == -1){
        perror("pipe error");
        exit(EXIT_FAILURE);
    }   
    printf("begin test pipe\n");
    pid_t pid;
    pid = fork();
    if(pid == -1){
        perror("fork error");
        exit(EXIT_FAILURE);
    }   
    if(pid == 0){ 
        close(fds[1]);//子进程关闭写端
        exit(EXIT_SUCCESS);
    }   

    close(fds[1]);//父进程关闭写端
    char buf[10] = {0};

    int ret;
    ret = read(fds[0],buf,10);
    printf("ret = %d\n", ret);

    return 0;
}

运行结果：

可知确实返回0，表示读到了文件末尾，并不表示出错

示例四：若是全部管道读端对应的文件描述符被关闭，则write操做会产生信号SIGPIPE

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>

void sighandler(int signo);
int main(void)
{
    int fds[2];
    if(signal(SIGPIPE,sighandler) == SIG_ERR)
    {
        perror("signal error");
        exit(EXIT_FAILURE);
    }
    printf("begin test pipe\n");
    if(pipe(fds) == -1){
        perror("pipe error");
        exit(EXIT_FAILURE);
    }
    pid_t pid;
    pid = fork();
    if(pid == -1){
        perror("fork error");
        exit(EXIT_FAILURE);
    }
    if(pid == 0){
        close(fds[0]);//子进程关闭读端
        exit(EXIT_SUCCESS);
    }

    close(fds[0]);//父进程关闭读端
    sleep(1);//确保子进程也将读端关闭
    int ret;
    ret = write(fds[1],"hello",5);
    if(ret == -1){
        printf("write error\n");
    }
    return 0;
}

void sighandler(int signo)
{
    printf("catch a SIGPIPE signal and signum = %d\n",signo);
}

运行结果：

可知当全部读端都关闭时，write时确实产生SIGPIPE信号