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Bug 3141 - OpenGL: unresolved glActiveTexture()
Summary: OpenGL: unresolved glActiveTexture()
Status: RESOLVED INVALID
Alias: None
Product: SDL
Classification: Unclassified
Component: render (show other bugs)
Version: HG 2.1
Hardware: x86_64 Windows 7
: P2 critical
Assignee: Ryan C. Gordon
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2015-10-08 15:06 UTC by igor
Modified: 2016-10-11 01:49 UTC (History)
1 user (show)

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Description igor 2015-10-08 15:06:52 UTC 
glActiveTexture() is not present in Windows version of opengl32.dll

In 2.0.3 it was possible to declare and define it using:

extern PFNGLACTIVETEXTUREARBPROC         glActiveTexture;
PFNGLACTIVETEXTUREARBPROC         glActiveTexture            = NULL;
glActiveTexture	           = (PFNGLACTIVETEXTUREARBPROC)         glGetProcAddressARB("glActiveTextureARB");

by using external glext.h rev:
Khronos $Revision: 28986 $ on $Date: 2014-11-18 18:43:15 -0800 (Tue, 18 Nov 2014) $

and in code:
#define NO_SDL_GLEXT
#include <SDL_opengl.h>
#include "glext.h"

Using internal glext caused linker error with glShaderSource()

in current 2.0.4 version (08 Oct 2015) it doesn't work with neither built-in or external glext.h

It causes unresolved symbol when you not declare/define glActiveTexture() manually (because Windows lacks definition), or causes redefinition when you define/declare it by getting processAddress.

Removing

GLAPI void GLAPIENTRY glActiveTexture( GLenum texture );

line from SDL2_opengl.h fixes issue under Windows, I can use both internal and external glext.h, manual definition works fine, but I have no idea how it will influence other platforms.

After looking into code a little looks like some defines have changes inside SDL_opengl.h which introduced this issue (but fixed issue with internal glext and glShaderSource)
You can test issue on next code https://github.com/igorko/flare-engine/tree/openGL
Comment 1 Sam Lantinga 2016-10-01 21:02:35 UTC 
Ryan, can you look at this for 2.0.5?
Comment 2 Ryan C. Gordon 2016-10-02 00:00:35 UTC 
(In reply to Sam Lantinga from comment #1)
> Ryan, can you look at this for 2.0.5?

yep!

--ryan.
Comment 3 igor 2016-10-04 20:33:11 UTC 
Code, on which you can test issue has been moved here https://github.com/dorkster/flare-engine-next
Comment 4 Ryan C. Gordon 2016-10-11 01:49:15 UTC 
Rereading this bug report, this isn't actually an SDL bug. You're redefining an OpenGL symbol as something else.

glActiveTexture is meant to be a function (even if it happens to not be listed in opengl32.dll), but your program is redefining it to be a function pointer, so this upsets the compiler since it saw a our previous definition.

If you want to load a function pointer from OpenGL with SDL_GL_GetProcAddress (or wgl or glX directly), you should pick a different name:

PFNGLACTIVETEXTUREARBPROC pglActiveTextureARB;  // note the 'p'.

Failing to do this will absolutely cause you problems on other platforms--if not on Windows too--when the dynamic loader ends up with a variable from your app where it thought it was getting a function in a DLL or vice versa.

--ryan.