writing Bytes into a .Bin file

Question

I have a vector in C++ that I want to write it to a .bin file. this vector's type is byte, and the number of bytes could be huge, maybe millions. I am doing it like this:

if (depthQueue.empty())
    return;

FILE* pFiledep;

pFiledep = fopen("depth.bin", "wb");

if (pFiledep == NULL)
    return;

byte* depthbuff = (byte*) malloc(depthQueue.size() * 320 * 240 * sizeof(byte));

if(depthbuff)
{
  for(int m = 0; m < depthQueue.size(); m++)
  {
    byte b = depthQueue[m];
    depthbuff[m] = b;
  }

  fwrite(depthbuff, sizeof(byte),
        depthQueue.size() * 320 * 240 * sizeof(byte), pFiledep);
  fclose(pFiledep);
  free(depthbuff);
}

depthQueue is my vector which contains bytes and lets say its size is 100,000.
Sometimes I don't get this error, but the bin file is empty.
Sometime I get heap error.
Somtimes when I debug this, it seems that malloc doesn't allocate the space. Is the problem is with space?

Or is chunk of sequential memory is so long and it can't write in bin?

Why do you copy the vector into a buffer at all? What type is depthQueue? — David Schwartz
– David Schwartz, Commented Nov 18, 2012 at 5:55

j_random_hacker · Accepted Answer · 2012-11-18 05:57:12Z

2

You don't need hardly any of that. vector contents are guaranteed to be contiguous in memory, so you can just write from it directly:

fwrite(&depthQueue[0], sizeof (Byte), depthQueue.size(), pFiledep);

Note a possible bug in your code: if the vector is indeed vector<Byte>, then you should not be multiplying its size by 320*240.

EDIT: More fixes to the fwrite() call: The 2nd parameter already contains the sizeof (Byte) factor, so don't do that multiplication again in the 3rd parameter either (even though sizeof (Byte) is probably 1 so it doesn't matter).

answered Nov 18, 2012 at 5:57

j_random_hacker

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12 Comments

user667222 Over a year ago

Concurrency::concurrent_vector<byte> depthQueue;

j_random_hacker Over a year ago

@user667222: I don't know what a Concurrency::concurrent_vector is -- you need to check its docs to see whether it provides the same contiguity guarantee. Also be consisten: is it byte or Byte?

j_random_hacker Over a year ago

@user667222: Given that you seem to be dealing with concurrency, if any other thread or process can write to the vector, you'll probably need to lock it before the fwrite() call and unlock it afterwards.

user667222 Over a year ago

for now there is no concurrency issues, think of it as normal vector<>

j_random_hacker Over a year ago

@user667222: You can't just "think of it as normal vector" -- whether my code works depends on whether the type guarantees that all elements will be laid out contiguously in memory. It probably does, but you need to check that.

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