パイプで2つの子プロセス間で通信する

Question

パイプ上でメッセージを送信して終了する2つの子プロセスを生成するコードを記述しようとしています。しかし、次のコードを実行すると、child2のみが挨拶をしますが、child1は子2がchild1にないメッセージを出力します。

誰かが私の方法論に間違いを知っていますか？

#include <string.h> 
#include <stdlib.h> 
#include <stdio.h> 
#include <sys/types.h> 
#include <unistd.h> 

int main(int argc, char** argv) { 
    char chld_1_send[20] = "hello from child 1"; 
    char chld_1_recv[20]; 
    char chld_2_send[20] = "hi from child 2"; 
    char chld_2_recv[20]; 

    int chld_1_outgoing[2]; 
    int chld_2_outgoing[2]; 

    pipe(chld_1_outgoing); 
    pipe(chld_2_outgoing); 

    int chld_1_status, chld_2_status; 
    pid_t chld_1, chld_2; 

    chld_1 = fork(); 

    if(chld_1 == 0) { 
     chld_2 = fork(); 
    } 

    if(chld_1 == 0 && chld_2 == 0) { 
     printf("parent [pid:%d] waiting on both children to finish\n", getpid()); 

     while(wait(&chld_1_status) != chld_1 && wait(&chld_2_status) != chld_2) {} 

     printf("done waiting\n"); 
    } 
    else if(chld_1 != 0 && chld_2 == 0) { 
     printf("this is child 1 [pid:%d] with parent [pid:%d]\n", getpid(), getppid()); 

     write(chld_1_outgoing[1], chld_1_send, strlen(chld_1_send)); 
     while(read(chld_1_outgoing[0], &chld_1_recv, sizeof(chld_2_recv)) < 0) {} 

     printf("child 2 said '%s'\n", chld_1_recv); 
     exit(0); 
    } 
    else if(chld_2 != 0 && chld_1 == 0) { 
     printf("this is child 2 [pid:%d] with parent [pid:%d]\n", getpid(), getppid()); 

     write(chld_2_outgoing[1], chld_2_send, strlen(chld_2_send)); 
     while(read(chld_2_outgoing[0], &chld_2_recv, sizeof(chld_2_recv)) < 0) {} 

     printf("child 1 said '%s'\n", chld_2_recv); 

     exit(0); 
    } 

    printf("both children have terminated successfully\n"); 

    return 0; 
} 

はしかし、端末にこの外に、このコマンドは、プリントを実行すると、無限ループに入る：

$ this is child 2 [pid:15713] with parent [pid:1] 
child 1 said 'hi from child 2' 
parent [pid:15714] waiting on both children to finish 

Answer 1

次は確かに改善することができますが、あなたが始める必要があります簡単な例です。 さらに詳しくはLink1とLink2に従ってください。

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <sys/wait.h> 
#include <unistd.h> 

/* max receiving buffer size; Note: no check or enforcement is made on this value*/ 
#define BUF_SIZE 256 

int main() 
{ 
    int pfd1[2]; 
    int pfd2[2]; 

    ssize_t numRead = -1; 
    /* Note: working under the assumption that the messages 
     are of equal length*/ 
    const char* messageOne = "Hello from child ONE.\n"; 
    const char* messageTwo = "Hello from child TWO.\n"; 

    const unsigned int commLen = strlen(messageOne) + 1; 

    char buf[BUF_SIZE]; 

    if (pipe(pfd1) == -1) 
    { 
     printf("Error opening pipe 1!\n"); 
     exit(1); 
    } 

    if (pipe(pfd2) == -1) 
    { 
     printf("Error opening pipe 2!\n"); 
     exit(1); 
    } 

    printf("Piped opened with success. Forking ...\n"); 

    // child 1 
    switch (fork()) 
    { 
     case -1: 
      printf("Error forking child 1!\n"); 
      exit(1); 

     case 0: 
      printf("\nChild 1 executing...\n"); 
      /* close reading end of first pipe */ 
      if (close(pfd1[0]) == -1) 
      { 
       printf("Error closing reading end of pipe 1.\n"); 
       _exit(1); 
      } 
      /* close writing end of second pipe */ 
      if (close(pfd2[1]) == -1) 
      { 
       printf("Error closing writing end of pipe 2.\n"); 
       _exit(1); 
      } 

      /* write to pipe 1 */ 
      if (write(pfd1[1], messageOne, commLen) != commLen) 
      { 
       printf("Error writing to pipe 1.\n"); 
       _exit(1); 
      } 

      if (close(pfd1[1]) == -1) 
      { 
       printf("Error closing writing end of pipe 1.\n"); 
       _exit(1); 
      } 

      /* reding from pipe 2 */ 
      numRead = read(pfd2[0], buf, commLen); 
      if (numRead == -1) 
      { 
       printf("Error reading from pipe 2.\n"); 
       _exit(1); 
      } 

      if (close(pfd2[0]) == -1) 
      { 
       printf("Error closing reding end of pipe 2.\n"); 
       _exit(1); 
      } 

      printf("Message received child ONE: %s", buf); 
      printf("Exiting child 1...\n"); 
      _exit(0); 

     default: 
      break; 
    } 

    // child 2 
    switch (fork()) 
    { 
     case -1: 
      printf("Error forking child 2!\n"); 
      exit(1); 
     case 0: 
      printf("\nChild 2 executing...\n"); 
      /* close reading end of second pipe */ 
      if (close(pfd2[0]) == -1) 
      { 
       printf("Error closing reading end of pipe 2.\n"); 
       _exit(1); 
      } 
      /* close writing end of first pipe */ 
      if (close(pfd1[1]) == -1) 
      { 
       printf("Error closing writing end of pipe 1.\n"); 
       _exit(1); 
      } 

      /* read from the first pipe */ 
      if (read(pfd1[0], buf, commLen) == -1) 
      { 
       printf("Error reading from pipe 1.\n"); 
       _exit(EXIT_FAILURE); 
      } 

      if (close(pfd1[0]) == -1) 
      { 
       printf("Error closing reading end of pipe 1.\n"); 
       _exit(EXIT_FAILURE); 
      } 

      /* write to the second pipe */ 
      if (write(pfd2[1], messageTwo, commLen) != commLen) 
      { 
       printf("Error writing to the pipe."); 
       _exit(EXIT_FAILURE); 
      } 

      if (close(pfd2[1]) == -1) 
      { 
       printf("Error closing writing end of pipe 2."); 
       _exit(EXIT_FAILURE); 
      } 

      printf("Message received child TWO: %s", buf); 
      printf("Exiting child 2...\n"); 
      _exit(EXIT_SUCCESS); 

     default: 
      break; 
    } 

    printf("Parent closing pipes.\n"); 

    if (close(pfd1[0]) == -1) 
    { 
     printf("Error closing reading end of the pipe.\n"); 
     exit(EXIT_FAILURE); 
    } 

    if (close(pfd2[1]) == -1) 
    { 
     printf("Error closing writing end of the pipe.\n"); 
     exit(EXIT_FAILURE); 
    } 

    if (close(pfd2[0]) == -1) 
    { 
     printf("Error closing reading end of the pipe.\n"); 
     exit(EXIT_FAILURE); 
    } 

    if (close(pfd1[1]) == -1) 
    { 
     printf("Error closing writing end of the pipe.\n"); 
     exit(EXIT_FAILURE); 
    } 

    printf("Parent waiting for children completion...\n"); 
    if (wait(NULL) == -1) 
    { 
     printf("Error waiting.\n"); 
     exit(EXIT_FAILURE); 
    } 

    if (wait(NULL) == -1) 
    { 
     printf("Error waiting.\n"); 
     exit(EXIT_FAILURE); 
    } 

    printf("Parent finishing.\n"); 
    exit(EXIT_SUCCESS); 
} 

Answer 2

ロジックは、特定のプロセスが親プロセスか子プロセスかを判断するために使用されます。子は0をforkから取得し、親は子のpidを取得します。

if(chld_1 != 0) 

そして、親がchld_1とchld_2の両方がゼロである、プロセスでなければなりません：

if(chld_1 != 0 && chld_2 != 0) 

おそらく、あなたは両方の子供たちが最初 ifがあるべき場合には、同じ親を持つようにしたいです

さらに、forkの両方をエラーチェックする必要があります（戻り値-1はエラーを意味します）。