Problem allocating an unsigned char array in C++

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1.00/5 (1 vote)

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VS2010

I want to read the bytes from a BMP file, and allocate them into an unsigned char array to use it as a texture in an Open GL application.

So far, this is how I define my array:

C++

unsigned char* data = new unsigned char [imageSize];

being imageSize an unsigned int that defines the size of the bitmap, in bytes.

The problem is that when I run the application, data has a length of 4 bytes, instead of the value of imageSize. I made a log to track what the program does, and this is what it gives to me:

LOG: imageSize : 786434

LOG: image width : 512

LOG: image height : 512

ERROR: Failed to allocate Data Buffer (bytes) : 4

ERROR: Required bytes for the buffer : 786434

In other words, data should be a 786434 bytes long array. Instead, I get a 4 bytes long array.

I also tried doing many things like

C++

unsigned char* data;
data = new unsigned char [imageSize];

Also defining it as

C++

data = (unsigned char*)malloc(imageSize);

But none of them work for some reason. Even if I assign to it the length manually, it won't work.

I'm programming this in Visual Studio 2010, for Windows, and it's a 32 bit application.

Posted 28-May-14 6:30am

Magnus9998

Updated 28-May-14 6:38am

v2

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[no name] 28-May-14 12:53pm

Please post the rest of your code.

Member 10272815 28-May-14 12:54pm

How (and where) are you setting imageSize?

Richard MacCutchan 28-May-14 14:40pm

Please show all the code for this function, including the lines where you print the above values. You are obviously doing something wrong, but we cannot guess what.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2014-05-28T06:54:00

Solution 1

No, 4 bytes is the size of the pointer to data, and actual data has the size of imageSize. You allocated the array on the heap correctly. (Better don't use malloc with C++, use new.) The problem is in the method where you pass this data. Don't look at the log in this case, look at the debug window called "Call stack".

What to do? 1) Learn the API you are using with this data, just read the documentation. 2) Learn C++ pointers and arrays properly, don't be so blind-folded.

—SA

Posted 28-May-14 6:54am

Sergey Alexandrovich Kryukov

Updated 28-May-14 11:45am

v2

Comments

[no name] 28-May-14 12:59pm

> You allocate the array correctly, in one case on stack, in another case on the heap.

I did not see Magnus' code where array is allocated on stack.

Sergey Alexandrovich Kryukov 28-May-14 17:44pm

Oh, I see. It was just my fantasy :-). Actually the pointer is on stack, the pointed object is created on heap.
Thank you, I'll fix the answer.
—SA

CPallini 28-May-14 13:13pm

Of course. My 5.

Sergey Alexandrovich Kryukov 28-May-14 17:44pm

Thank you, Carlo.
—SA

Magnus9998 28-May-14 14:29pm

Or we could say "sizeof()" was giving me the size of the pointer, which is 4 bytes indeed, instead of the size of the array, that I had stored in imageSize. In other words, I solved the problem by using imageSize instead of sizeof(data).

Also, about your two points...

1) That's what I have been doing from the very beggining. Reading the API documentation for OpenGL. However, it has nothing to do here, since this was a C++ problem. Not an Open GL one. And no, other than the standard Windows API and Open GL, I am not using anything else.

2) Making mistakes implies learning. One is not blind-folded for doing something he doesn't know. One is blind-folded if he doesn't even try, since he will never know how to do it. Sure I did learn that sizeof() only gives you the size of the type you're using. Not the size of an array.

Sergey Alexandrovich Kryukov 28-May-14 17:54pm

Now, your fix with the size was correct.

As to your points 1 and 2, you are quite right. But I wrote exactly about what actually happened. You learned the documentation on the API, and, after sorting out what appeared to be a C++ subtlety for you, you realized what's was really required for the API call. And yes, we are learning through doing some work, not without doing some mistakes. I saw "we", not "you"; me too, always doing some; this is a natural process.

And, it's a pleasure to help someone who can actually learn lessons, analyze the facts and make things working. And the one who takes criticism constructively. Those are important keys to success. Wish you the best in your learning, growth and professional achievements.

—SA