How do I use a pointer as an offset?

Question

This issue originally came up in a related question of mine where I was having trouble reading some bit of code. The answer turned out to be that this line

&((GLushort *)0)[3 * mesh.sBoneBatches.pnBatchOffset[batchNum]]

evaluates to be a pointer. And it's used in

glDrawElements(GL_TRIANGLES, i32Tris * 3, GL_UNSIGNED_SHORT, &((unsigned short*)0)[3 * mesh.sBoneBatches.pnBatchOffset[batchNum]]);

where it is interpreted as an offset to draw a subset of vertex indices.

My code currently requires me to do by hand some of what openGL is doing in the glDrawElements, and I can't figure out how to use the pointer as an offset. glDrawElements uses an array of indices (named vertexIndices in my code), so I tried something like this:

vertexIndices[&((GLushort *)0)[3 * mesh.sBoneBatches.pnBatchOffset[batchNum]]]

but that obviously failed.

EDIT 1: I just tried this and it compiles... still not sure if it's correct though. vertexIndices + (uint) &((GLushort *)0)[3 * mesh.sBoneBatches.pnBatchOffset[batchNum]]

Answer 1

What's being passed to glDrawElements is a pointer to an offset, not a pointer as an offset. Your mysterious line of code is equivalent to:

size_t offset = ((GLushort*)0)[3 * mesh.sBoneBatches.pnBatchOffset[batchNum]];
glDrawElements(GL_TRIANGLES, i32Tris * 3, GL_UNSIGNED_SHORT, &offset);

In other words, glDrawElements() isn't using the fourth parameter as an offset directly, but a pointer to an offset value. This is clear from the OpenGL documentation, which has the following signature for glDrawElements:

  void glDrawElements( GLenum mode,
         GLsizei count,
         GLenum type,
         const GLvoid *indices );

And the explanation:

indices - Specifies a pointer to the location where the indices are stored.

(Emphasis mine.)

Aside: The offset calculation is a little weird, but valid for the reasons outlined in the answer to your other question. It's clearer and more stylistically normal to write that as:

size_t offset = sizeof(GLushort) * 3 * mesh.sBoneBatches.pnBatchOffset[batchNum];

Answer 2

Calculations based on an offset of zero can work in some compilers, but are not guaranteed to do so. ANSI C offers you the offsetof operator in stddef.h to allow you to calculate offsets of members of a struct.