c中如何通过匿名管道发送消息

Question

我尝试使用匿名管道向 parent 发送两条消息。我知道我需要将 parent 置于等待状态，让 children 写入第二条消息，然后从管道中读取它。我尝试使用 wait(NULL) 让 parent 等待 children 写入管道，但它似乎不起作用。有人可以帮帮我吗？先谢谢了。

这是我目前的代码：

    #include <unistd.h>  
    #include <stdio.h>
    #include <string.h>
    # include <wait.h>  

    #define MaxLen 80
    #define PipeStdIn   0    
    #define PipeStdOut  1   

   int main( ) {
      int ret, myPipes[2];
      char buff[MaxLen + 1];

      if( pipe( myPipes ) == 0 ){

           if( fork( ) == 0 ) {
              char *reply = {"child sends first message.\n"};
              write( myPipes[PipeStdOut], reply, strlen(reply) + 1); 

              char *reply2 = {"child sends second message.\n"};
              write( myPipes[PipeStdOut], reply2, strlen(reply2) + 1); 

       } 
       else { 
              read(myPipes[PipeStdIn], buff, MaxLen); 
              printf("Parent receives message: %s", buff);
              wait(NULL);
              read(myPipes[PipeStdIn], buff, MaxLen);
              printf("Parent receives 2nd message : %s", buff);
       }
     }
      close( myPipes[PipeStdIn] );  
      close( myPipes[PipeStdOut] ); 

      return 0; 
}

这是我编译时得到的：

aaa@ubuntu:~/Desktop/fork$ ./fork
Parent receives message: child sends first message.

Answer 1

您必须关闭每个进程中不使用的管道末端：

   int main( ) {
      int ret, myPipes[2];
      char buff[MaxLen + 1];

      if( pipe( myPipes ) == 0 ){

           if( fork( ) == 0 ) {
              close (myPipes[PipeStdIn]);
              char *reply = {"child sends first message.\n"};
              write( myPipes[PipeStdOut], reply, strlen(reply) + 1); 

              char *reply2 = {"child sends second message.\n"};
              write( myPipes[PipeStdOut], reply2, strlen(reply2) + 1); 

       } 
       else { 
              close (myPipes[StdOut]);
              read(myPipes[PipeStdIn], buff, MaxLen); 
              printf("Parent receives message: %s", buff);

              read(myPipes[PipeStdIn], buff, MaxLen);
              printf("Parent receives 2nd message : %s", buff);
              wait(NULL);
       }
     }

Answer 2

看，它现在可以工作了，您的 close() 函数在错误的行中。您应该在读取或写入之前关闭。

#include <unistd.h>  
#include <stdio.h>
#include <string.h>
#include <wait.h>  

#define MaxLen 80
#define PipeStdIn   0    
#define PipeStdOut  1   

int main( ) {
  int ret, myPipes[2];
  char buff[MaxLen + 1];

  if( pipe( myPipes ) == 0 ){

       if( fork( ) == 0 ) {

          char *reply = {"child sends first message.\n"};

          char *reply2 = {"child sends second message.\n"};

          close(myPipes[PipeStdIn]); // CLOSE BEFORE WRITE
          write( myPipes[PipeStdOut], reply, strlen(reply) + 1); 
          write( myPipes[PipeStdOut], reply2, strlen(reply2) + 1); 

   } 
   else { 

          close(myPipes[PipeStdOut]); // CLOSE BEFORE READ
          read(myPipes[PipeStdIn], buff, MaxLen); 
          printf("Parent receives message: %s", buff);

          read(myPipes[PipeStdIn], buff, MaxLen);
          printf("Parent receives 2nd message : %s", buff);
   }
 }
  return 0; 
 }

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