memory to file back to memory

Question

i try to write the data from the memory to a file. And read it back in and write it back to the memory. At the moment I have the following code for that. But as far as i can see it doesnt work fully. The data in the memory is a image. As long as i dont read it in from a file its fine. But as soon as i read it in its gets corrupted.

        size_t memsize = frameWidth * frameHeight;                      //calculate the memory size
    //Writing characters to file
    char test;

    char* data = (char*) malloc (memsize);                          //create a datablock for the data on the heap
    //*

    cudaMemcpy( data, input, memsize,cudaMemcpyDeviceToHost);       //copy the data form the cuda to the CPU
    FILE *fp;
    fp = fopen (filename, "wb");
    int frames = memsize/sizeof(char);
    for (int i = 0; i<(frames); i++)
    {
            test = data[i];
            fprintf(fp, "%c",test);
    }
    fclose (fp);
    /**/
    free(data);                                                     //clear the allocated memory

//////////////////////////////////////
//write back to the memory///////////
////////////////////////////////////
    data = (char*) malloc (memsize);                            //create a datablock for the data on the heap
    //*

//  FILE *fp;
    fp = fopen (filename, "rb"); //read


    for(int i=0; i<memsize; i++)
       fscanf(fp, "%c ", (data+i));

    cudaMemcpy( input, data, memsize,cudaMemcpyHostToDevice);       //copy the data form the cuda to the CPU
    log_kernel<<<grid, block>>>(input, pitchIn/sizeof(float), output, pitchOut/sizeof(float), frameHeight);

    fclose (fp);
    /**/
    free(data);

                                                    //clear the allocated memory

but this doesnt fully work. I see that the data gets corrupted but i cant see why can someone see the mistake I made.

Answer 1

Try to use

fread(data, sizeof(char), memsize, fp);

instead of

for(int i=0; i<memsize; i++) fscanf(fp, "%c ", (data+i));

May fix your problem and will be much faster. Also, fscanf and fprintf are designed to read/write formatted data, not binary fields.

Answer 2

Assuming the data is actually binary and not characters, some of it can easily have value 32 or 10 or some such. You're reading the file using "%c ", which means such values (ASCII code of space and linefeed, respectively) will happily be skipped as whitespace.

Seeing as you've tagged this with C++, you could simply do I/O the C++ way:

cudaMemcpy( data, input, memsize,cudaMemcpyDeviceToHost);
{
  std::ofstream f(filename, std::ios::binary);
  f.write(data, memsize);
}
free(data);

// And

{
  std::ifstream f(filename, std::ios::binary);
  f.read(data, memsize);
}
cudaMemcpy( input, data, memsize,cudaMemcpyHostToDevice);

---

Of course, you could also use std::vector<char> instead of char* and get rid of all the manual allocation and deallocation. The code would then simplify:

size_t memsize = frameWidth * frameHeight;
std::vector<char> data(memsize);
cudaMemcpy(&data[0], input, memsize,cudaMemcpyDeviceToHost);
{
  std::ofstream f(filename, std::ios::binary);
  f.write(&data[00, memsize);
}
data.clear();

// Now read back

data.resize(memsize);
{
  std::ifstream f(filename, std::ios::binary);
  f.read(&data[0], memsize);
}
cudaMemcpy( input, data, memsize,cudaMemcpyHostToDevice);

data.clear(); // optionally