c++ - 无法访问共享内存中的字符串数组

Question

我正在尝试通过共享内存将文本行从编写器程序发送到阅读器程序。作家工作完美。但是，在阅读器程序的执行过程中，我在以下行出现分段错误

cout<<buffer->str[initial]<<endl;

当我在评论上述行时同时运行阅读器和编写器时，共享结构的计数值的变化是可见的并且正确递增。因此，根据我的诊断，问题在于访问共享结构的字符串数组。为什么会这样？

该代码由下面提供的 writer.cpp 和 reader.cpp 组成以供参考。

writer.cpp 的目的是从输入文件中读取行，然后将它们写入共享内存。

编写器.cpp

// for c++ related functions
#include<iostream>
#include<fstream>
#include<string>

// for c related functions
#include<cstdio>
#include<cstdlib>
#include<unistd.h>

// for system calls
#include<sys/types.h>
#include<sys/ipc.h>
// for shared memory
#include<sys/shm.h>

using namespace std;

// store the key for accessing the shared memory
key_t shm_key = ftok("practical",65);

// the structure for shared memory
struct shared{
  bool the_end = false;
  int count = 0;
  string str[6];
};

int main(){

  // create the shared memory
  int shmid = shmget(shm_key,sizeof(shared),0666|IPC_CREAT);
  if(shmid == -1){
    perror("Error in creating shared memory");
    return 1;
  }

  // attach server to the shared memory
  struct shared *buffer = (shared*)shmat(shmid,NULL,0);
  if(buffer == (void *) -1){
     perror("Unable to attach to shared memory");
     return 1;
  }

  // open the file for reading the input
  ifstream fin("input.txt");
  // check for errors in opening the file
  if(!fin){
    cerr<<"Error in opening the file"<<endl;
    exit(1);
  }

  // to store the read line
  string inptxt;
  cout<<"Reading contents from file\n"<<endl;

  while(fin){
    // read a line
    getline(fin,inptxt);

    // exit the loop if string is empty
    if(inptxt == "\0") break;

    // store the line in the buffer
    buffer->str[buffer->count++] = inptxt;
    cout<<"This was written : "<<buffer->str[buffer->count-1]<<endl;

    sleep(3);
  }
  cout<<"Closing the file"<<endl;
  fin.close();

  // marks the end of writing into shared memory
  buffer->the_end = true;

  if(shmdt(buffer) == -1){
     perror("error in deatching from shared memory");
     return 1;
  }

  if(shmctl(shmid,IPC_RMID,0) == -1){
     perror("error in destroying the shared memory");
     return 1;
  }

  cout<<"Writing Completed"<<endl;
  return 0;
}

阅读器程序的目的是从共享内存中读取行，然后将它们显示在控制台上。

阅读器.cpp

// for c++ related functions
#include<iostream>
#include<fstream>
#include<string>

// for c related functions
#include<cstdio>
#include<cstdlib>
#include<unistd.h>

// for system calls
#include<sys/types.h>
#include<sys/ipc.h>
// for shared memory
#include<sys/shm.h>

using namespace std;

// store the key for accessing the shared memory
key_t shm_key = ftok("practical",65);

// the structure for shared memory
struct shared{
  bool the_end = false;
  int count = 0;
  string str[6];
};

int main(){

  // create the shared memory
  int shmid = shmget(shm_key,sizeof(shared),0666|IPC_CREAT);
  if(shmid == -1){
    perror("Error in creating shared memory");
    return 1;
  }

  // attach server to the shared memory
  struct shared *buffer = (shared*)shmat(shmid,NULL,0);
  if(buffer == (void *) -1){
     perror("Unable to attach to shared memory");
     return 1;
  }

  cout<<"Reading contents from shared memory\n"<<endl;

  int initial = 0;
  while(!buffer->the_end){
    if(buffer->count > initial){
      cout<<"initial : "<<initial<<" buffer count : "<<buffer->count<<"\n";
      // the following line gives segmentation fault
      // cout<<buffer->str[initial]<<endl;
      initial++;
    }
  }

  if(shmdt(buffer) == -1){
     perror("error in deatching from shared memory");
     return 1;
  }

  cout<<"Reading Completed"<<endl;
  return 0;
}

标签： c++cunixoperating-system