Understanding Why glGenBuffers Can Corrupt Unrelated Data Like playerFaces

Описание: Learn why incorrect usage of OpenGL's glGenBuffers can overwrite unrelated memory, causing pointer corruption, and how to fix it effectively.
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This video is based on the question https://stackoverflow.com/q/79498144/ asked by the user 'bbqribs' ( https://stackoverflow.com/u/29044247/ ) and on the answer https://stackoverflow.com/a/79498198/ provided by the user 'Botje' ( https://stackoverflow.com/u/1548468/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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The Problem: Unexpected Pointer Change After OpenGL Calls

In OpenGL code, you might observe a pointer variable like playerFaces changing unexpectedly after calling glGenBuffers. For example, before the call, playerFaces holds a valid heap address, but afterwards, it contains a nonsensical value, indicating memory corruption.

The specific suspicious code is:

[[See Video to Reveal this Text or Code Snippet]]

You expect playerFaces to remain unchanged across this call but it doesn't, causing confusing bugs.



Why This Happens: Misusing glGenBuffers

glGenBuffers's signature is:

[[See Video to Reveal this Text or Code Snippet]]

n: number of buffer IDs to generate

buffers: pointer to an array to store generated buffer names

GL_ARRAY_BUFFER is actually a constant (0x8892), not a count.

Calling:

[[See Video to Reveal this Text or Code Snippet]]

makes OpenGL attempt to generate 0x8892 (~34930) buffers, writing far beyond where VBO points—leading to memory overwrite.

This overflow is likely clobbering nearby stack memory, including playerFaces.



How to Fix It Properly

Use the Correct n Parameter

Generate exactly one buffer by changing the call to:

[[See Video to Reveal this Text or Code Snippet]]

This fixes your problem immediately by preventing massive unintended writes.

Validate Pointers and Buffer Sizes

Always verify parameters passed to OpenGL functions.

Use debuggers or tools like Valgrind to detect buffer overruns.

Debugging Tips

Print pointer addresses before and after each API call.

Check if unexpected values appear in your pointer variables.

Review documentation for each OpenGL API to confirm parameter expectations.



Summary

glGenBuffers requires the count of buffers to create, not the buffer type enum.

Passing GL_ARRAY_BUFFER as the count causes huge memory writes overwriting unrelated data like playerFaces.

Use glGenBuffers(1, &VBO) to fix it.

Always double-check API arguments to prevent subtle bugs.

Getting familiar with OpenGL's API and guarding against these common mistakes leads to more robust and maintainable graphics code.