c -用于进程间通信的普通管道

Question

我正在学习如何使用linux中的普通管道进行父进程和子进程之间的通信。基本任务就是从父进程向子进程发送消息，然后子进程执行一些转换并将结果传递回父进程。我得到的结果是一些随机字符，比如"。我已经考虑了很长一段时间，仍然不能找出错误。谢谢你的帮助。

#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#define READ_END 0
#define WRITE_END 1
void convert(char* str);
int main(int argc, char *argv[]){
  int pid; /* Process ID */
  int status;
  char *input;
  char *read_msg_c;
  char *read_msg_p;
  int pfd1[2], pfd2[2];
  if (argc !=2){/* argc should be 2 for correct execution */
    /* We print argv[0] assuming it is the program name */
    printf("Please provide the string for conversion n");
    exit(-1);
  }
  input = argv[1];
  if(pipe(pfd1) < 0 || pipe(pfd2) < 0){
    printf("Failed to create a pipe between parent and child n");
    exit(-1);
  }
  if((pid = fork()) < 0){ /* Fork the process */
     printf("Fork error n");
     exit(-1);
  }
  else if(pid > 0){ /* Parent code */
    close(pfd1[READ_END]);
    close(pfd2[WRITE_END]);
    printf("Process ID of the parent is %d. n", getpid()); /* Print parent's process ID */
    write(pfd1[WRITE_END],input,strlen(input)+1);
    close(pfd1[WRITE_END]);
    read(pfd2[READ_END],read_msg_p,strlen(input)+1);
    printf("%sn",read_msg_p);
    close(pfd2[READ_END]);
  }
  else if(pid == 0){ /* Child code */
    close(pfd1[WRITE_END]);
    close(pfd2[READ_END]);
    printf("Process ID of the child is %d. n", getpid()); /* Print child's process ID */
    read(pfd1[READ_END],read_msg_c, strlen(input)+1);
    printf("Child: Reversed the case of the received string. n");
    write(pfd2[WRITE_END],read_msg_c,strlen(input)+1);
    close(pfd1[READ_END]);
    close(pfd2[WRITE_END]);
    exit(0); /* Child exits */
   }
}
void convert(char *str){
  int i = 0;
  while (str[i]){
    if (isupper(str[i])){
      str[i] = tolower(str[i]);
    }
    else if (islower(str[i])){
      str[i] = toupper(str[i]);
    }
    i++;
  }
}

Answer 1

你的主要错误是你的变量read_msg_p和read_msg_c是未初始化的指针。

将它们变成数组:

char read_msg_p[1024];
char read_msg_c[1024];

你似乎错过了<stdio.h>(但你真的不需要<sys/types.h>了)。你应该检查读和写的错误;一旦为读取分配了空间，读取可能会使用不同的最大大小。等。

我通过查看编译器警告发现了这个问题:

$ gcc -O3 -g -std=c99 -Wall -Wextra pipes-14420398.c -o pipes-14420398
pipes-14420398.c: In function ‘main’:
pipes-14420398.c:40:22: warning: ‘read_msg_p’ may be used uninitialized in this function [-Wuninitialized]
pipes-14420398.c:52:22: warning: ‘read_msg_c’ may be used uninitialized in this function [-Wuninitialized]
$

忽略行号;我已经相当严重地破解了您的代码，这时只剩下这些警告了。但是有问题的线路是read()调用。

---

被破解代码的示例输出，正常工作。

$ ./pipes-14420398 string-to-convert
Process ID of the parent is 37327. 
Process ID of the child is 37328. 
Child read 18 bytes: <<string-to-convert>>
Parent read 18 bytes: <<string-to-convert>>
$

请注意，下面的代码读取18字节(包括null)，但不打印null(因为nbytes-1参数指向printf())。

#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#define READ_END 0
#define WRITE_END 1

int main(int argc, char *argv[])
{
  int pid; /* Process ID */
  char *input;
  char read_msg_c[1024];
  char read_msg_p[1024];
  int pfd1[2], pfd2[2];
  if (argc !=2){/* argc should be 2 for correct execution */
    /* We print argv[0] assuming it is the program name */
    fprintf(stderr, "Usage: %s string-to-convertn", argv[0]);
    exit(-1);
  }
  input = argv[1];
  if(pipe(pfd1) < 0 || pipe(pfd2) < 0){
    printf("Failed to create a pipe between parent and child n");
    exit(-1);
  }
  if((pid = fork()) < 0){ /* Fork the process */
     printf("Fork error n");
     exit(-1);
  }
  else if(pid > 0){ /* Parent code */
    close(pfd1[READ_END]);
    close(pfd2[WRITE_END]);
    printf("Process ID of the parent is %d. n", getpid()); /* Print parent's process ID */
    write(pfd1[WRITE_END], input, strlen(input)+1);
    close(pfd1[WRITE_END]);
    int nbytes = read(pfd2[READ_END], read_msg_p, sizeof(read_msg_p));
    if (nbytes <= 0)
        printf("Parent: read failedn");
    else
        printf("Parent read %d bytes: <<%.*s>>n", nbytes, nbytes-1, read_msg_p);
    close(pfd2[READ_END]);
  }
  else if(pid == 0){ /* Child code */
    close(pfd1[WRITE_END]);
    close(pfd2[READ_END]);
    printf("Process ID of the child is %d. n", getpid()); /* Print child's process ID */
    int nbytes = read(pfd1[READ_END], read_msg_c, sizeof(read_msg_c));
    if (nbytes <= 0)
        printf("Child: read failedn");
    else
    {
        printf("Child read %d bytes: <<%.*s>>n", nbytes, nbytes-1, read_msg_c); 
        write(pfd2[WRITE_END], read_msg_c, nbytes);
    }
    close(pfd1[READ_END]);
    close(pfd2[WRITE_END]);
    exit(0); /* Child exits */
   }
}

正如WhozCraig所指出的，还有许多其他的变化可以做。然而，这使得事情运行得相当干净。你差一点就OK了。

注意调试技术:

1. 编译高警告级别并修复所有警告。
2. 在信息可用时打印信息(或在调试器中运行并观察信息可用时)。