[Boost-users] [interprocess 1.42] Copy multiple variables into shared memory

Subject: [Boost-users] [interprocess 1.42] Copy multiple variables into shared memory
From: Michael Behnke (michael.behnke_at_[hidden])
Date: 2014-10-13 06:00:56

Hello,

I'm trying to write multiple variables one by one into a mapped_region, 
but somehow I fail. Here is the Code I worked on so far:

     boost::interprocess::shared_memory_object 
shm_obj(boost::interprocess::open_or_create, "Test", 
boost::interprocess::read_write);
     shm_obj.truncate(1000);
     boost::interprocess::mapped_region region(shm_obj, 
boost::interprocess::read_write, 0, 0);

     uint32_t size = 33;
     uint32_t type = 666;
     std::string data("I'm a Test!");

     uint32_t* start_ptr_int = &(static_cast<uint32_t*> 
(region.get_address())[0]);
     std::memcpy(start_ptr_int, &size, sizeof (uint32_t));

     start_ptr_int = &(static_cast<uint32_t*> (region.get_address())[4]);
     std::memcpy(start_ptr_int, &type, sizeof (uint32_t));

     char* start_ptr_char = &(static_cast<char*> (region.get_address())[8]);
     std::memcpy(start_ptr_char, data.c_str(), data.length());

     uint32_t sizeOfData;
     std::memcpy(&sizeOfData, &(static_cast<uint32_t*> 
(region.get_address())[0]), sizeof (uint32_t));

     uint32_t typeOfData;
     std::memcpy(&typeOfData, &(static_cast<uint32_t*> 
(region.get_address())[4]), 4);

     char* revData = new char[sizeOfData];
     std::memcpy(revData, &(static_cast<char*> 
(region.get_address())[sizeof (uint32_t) * 2]), sizeOfData);

First I write an uint32_t into the region at index 0. Afterwards, I'm 
writing another uint32_t at position 4 (assuming the first uint32_t took 
the bytes 0 - 3).
Last I write a string into the region, at index 8. Reading the first 
uint32_t again from shared memory works fine, as does reading the string 
(the last variable). But if I try to read the second variable
(uint32_t) the data is not what I expected. I know that the problem is 
memcpy, somehow my syntax does not work. Unfortunately I could not find 
a matching example, cause mostly memset is used to
explain the functionality of boost::mapped_region. I have to write lots 
of data into shared memory (structured like 
size1|type1|data1|...|sizeN|typeN|dataN) but still can't figure out how 
to do this the right way.
Any suggestions are greatly appreciated.

Kind regards

Michael Behnke
  
-- 
Bachelorand
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