1、原理
1、pipe作为linux中最基础的进程间通信机制,经常在shell中使用,例如ps aux | grep aaa 即建立了一个管道,而linux 下C程序同样可以通过系统调用pipe在父子进程间使用管道功能。
读取一个空的pipe将导致read操作阻塞,直到有数据被写入到pipe中;向一个已经满的pipe写数据将导致write操作阻塞,直到pipe中的数据被读取出去;
2、如果想避免read和write阻塞,可通过fcntl将pipefd设置成O_NONBLOCK,read和write无论是否成功,都将直接返回,这时需要判断read和write返回错误码,判断操作是否成功。
3、如果有多个进程同时向一个pipe写入时,只有在每个进程写入的数据长度都小于PIPE_BUF时,才可以保证pipe写入的原子性,不然可能会出现数据错乱的情况;
4、管道作为半双工通道,如果想实现双向通信,则需要打开两个管道,一个从父进程->子进程,另一个从子进程->父进程。
2、头文件
#include <unistd.h>
3、函数原型
int pipe(int pipefd[2]);
4、工作原理
通过函数参数返回两个描述符(fd),pipefd[0] 用来读,pipefd[1]用来写, 写入到pipefd[1]的数据将可以从pipefd[0]读出,管道作为半双工通道,数据只能沿一个方向前进;
5、返回值
pipe函数返回0表示调用成功,返回-1表示调用失败;
6、举例
单向通信
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#define handle_error(msg) \
{perror(msg);exit(EXIT_FAILURE);}
int main (int argc, char *argv[])
{
int pipe_fd[2];
if (pipe (pipe_fd) == -1)
handle_error("pipe");
int ret_from_fork;
ret_from_fork = fork ();
if (ret_from_fork == -1)
handle_error("fork");
if (ret_from_fork == 0) //child process
{
char str[100] = {0};
printf("child process:\n input string:");
scanf ("%s", str);
//gets(str);
write (pipe_fd[1], str, strlen (str)); // write data to pipe
close (pipe_fd[1]);
_exit (EXIT_SUCCESS);
}
if (ret_from_fork > 0) // parent process
{
char buf[30] = {0};
read (pipe_fd[0], buf, 30); // read data from pipe
printf ("parent process: %s\n", buf);
close (pipe_fd[0]);
exit (EXIT_SUCCESS);
}
return 0;
}
双向通信
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#define handle_error(msg) \
{perror(msg);exit(EXIT_FAILURE);}
int main (int argc, char *argv[])
{
int pipe_fd1[2];
int pipe_fd2[2];
if (pipe (pipe_fd1) == -1 || pipe (pipe_fd2) == -1)
handle_error("pipe");
int ret_from_fork;
ret_from_fork = fork ();
if (ret_from_fork == -1)
handle_error("fork");
if (ret_from_fork == 0) //child process
{
char read_buf1[100] = {0};
char write_buf1[100] = {0};
printf("child process:");
scanf ("%s", write_buf1);
//gets(str);
write (pipe_fd1[1], write_buf1, strlen (write_buf1));
close (pipe_fd1[1]);
//sleep(5);
read (pipe_fd2[0], read_buf1, 100);
printf ("in child process: %s\n", read_buf1);
close (pipe_fd2[0]);
_exit (EXIT_SUCCESS);
}
if (ret_from_fork > 0) // parent process
{
char read_buf2[100] = {0};
char write_buf2[100] = {0};
read (pipe_fd1[0], read_buf2, 100); // read data from pipe
printf ("in parent process: %s\n", read_buf2);
close (pipe_fd1[0]);
printf ("parent process:");
scanf ("%s", write_buf2);
write (pipe_fd2[1], write_buf2, strlen (write_buf2));
close (pipe_fd2[1]);
exit (EXIT_SUCCESS);
}
return 0;
}