Question

If I have this reference variable:

float* image

How can I get the length of this image in C? The length = (width * height) but how can I get this value?

Answer 1

float * image

Simply points at a single float. It can also be used with array syntax — ie image[n] to get the nth float from the location of image. People will use pointers and malloc to create a dynamic array of floats. However, there’s no information associated with the size stored with the data itself. That’s a detail the coder has to care about in C. Often in these kinds of cases in C when you’re passed such a pointer in a function, you’re also passed an accompanying size, ie:

void Foo(float* image, size_t numFloatsInImage)

It sounds like though that image might point to a block of contiguous memory treated as a 2d array. That is something with a width and height. So instead of strictly a numberOfFloats you might have something like this “3×3” image:

    image ->  0  1  2  3  4  5  6  7  8
              0  0  0  1  1  1  2  2  2

There’s a total of 3×3 == 9 floats here with the first 3 corresponding to row 1, the next 3 row 2, and the last one as row 4. This is simply a single dimensional array used to represent 2d data.

Like I said before, this is a detail of how you’ve decided to use the language and not a part of it. You’ll have to pass along the width/height with the pointer for safe use:

void Foo(float* image, size_t width, size_t height)

You can also avoid this by simply creating a struct to store your image, being sure to always initialize/maintoin the width and height correctly

 struct
 {
     float* image; // points to image data
     size_t width;
     size_t height;
 };

then always pass around the struct to functions like foo

Another trick could be to overallocate float to store the width and height in the buffer itself:

 float* image = malloc(width*height + 2);
 image[0] = width;
 image[1] = height;
 // everything past image[2] is image data

In any case, there’s these and many other implementation specific ways to store/pass this data. Whoever is giving you this pointer should be telling you how to get the length/height and telling you how to use the pointer. There’s no way C can figure out how it’s being done, its an implementation decision.