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ipc.simple.shmem.H

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// simpleshmem.H

#ifndef SIMPLE_SHMEM_H
#define SIMPLE_SHMEM_H

#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/user.h>

#include <errno.h>

class SimpleSHMEM
{
protected:
  int _key;
  
  int _valid;
  int _id;
  int _size;
  int _last_num_attached;

  char *_local_addr;
  
  struct shmid_ds _shmem_description;
  
public:
  SimpleSHMEM( int key ) : _key(key), _last_num_attached(0), _local_addr(NULL) {}

  virtual ~SimpleSHMEM() {
    if (_local_addr && _valid)
      shmdt(_local_addr);  // XXX - check return value...
  }

  int create( int size, int perms = 0644) {
    _size = size;
    _id = shmget(_key, size, IPC_CREAT | IPC_EXCL | perms);

    _valid = _id != -1;
    if ( _valid )
      _local_addr = (char *)shmat(_id, NULL, 0);

    
    
    
    
    return _valid;

//     if (_id == -1)
//       _valid = 0;
//     else
//       _valid = 1;

//     return _valid;
  }
  
  int num_attached() {
    if ( shmctl(_id, IPC_STAT, &_shmem_description) != -1 ) // -1 is returned on err 
      return _shmem_description.shm_nattch;
    else
      cerr << "ERROR: Couldn't get shmem description for SHMEM ID #" << _id << endl;
    return -1;    
  }
  
  // Returns true if there are fewer attachments this time than the last time this was called
  bool lost_attached() {
    int curr_num_attached = num_attached();
    bool ret = curr_num_attached < _last_num_attached;
    _last_num_attached = curr_num_attached;
    return ret;
  }
      

  int attach() {
    _id = shmget(_key, 0, 0);
    if (_id == -1)
      _valid = 0;
    else {
      _valid = 1;

      // figure out how big the segment is
      struct shmid_ds buf;
      int s = shmctl(_id, IPC_STAT, &buf);
      _size = int(buf.shm_segsz);

      // attach the member variable that will pt to it
      _local_addr = (char *)shmat(_id, NULL, 0);

    }

    return _valid;
  }

  void detach() {
    if ( shmdt( _local_addr ) )
      cerr << "ERROR: Couldn't detach from shared memory id " << _key << endl;
  }

  int valid() const { return _valid; }
  int size () const { return _size; }

  int remove() {
    if (_local_addr && _valid)
      shmdt(_local_addr);  // XXX - check return value...

    if (shmctl(_id, IPC_RMID, NULL) == -1) {
      return 0;
    } else {
      _valid = 0;
      return 1;
    }
  }

  // Test if the shared memory segment has been removed
  // XXX - what's proper way to test status of a shmem segment?
  int removed() {
    if (shmget(_key, 0, 0) == -1) {
// XXX - errno is set to '2' instead of EIDRM for some reason ???
//       if (errno == EIDRM)
      return 1;
    }

    return 0;
  }

  char *local_addr() {
    if (!_local_addr)
      _local_addr = (char *)shmat(_id, NULL, 0);

    return _local_addr;
  }
};

#endif

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