glGetString override

gistfile1.txt

/*
 * Build with: gcc -s -O2 -shared -fPIC -o glgetstring.{so,c} -ldl
 */

/* for RTLD_NEXT */
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <dlfcn.h>
#include <GL/gl.h>
#include <stdlib.h>
#include <string.h>

typedef struct _glstring_t {
        GLenum type;
        const char *environ_name;
        const char *environ;
} glstring_t, *pglstring_t;

#define DECLARE_GLSTRING(x) { x, "OVERRIDE_" #x, NULL }

glstring_t table[] = {
        DECLARE_GLSTRING(GL_VENDOR),
        DECLARE_GLSTRING(GL_RENDERER),
        DECLARE_GLSTRING(GL_VERSION),
        DECLARE_GLSTRING(GL_EXTENSIONS),
        { 0 },
};

typedef const GLubyte *GLAPIENTRY (*glGetString_t)(GLenum);

const GLubyte *GLAPIENTRY glGetString( GLenum type )
{
        static glGetString_t next = NULL;
        pglstring_t string;

        if (!next)
        {
                /* Fill in defaults since this is the first call. */
                next = dlsym(RTLD_NEXT, "glGetString");
                for (string = table; string && string->environ_name; string++) {
                        string->environ = getenv(string->environ_name);
                }
        }

        /* Find a match in the override table. */
        for (string = table; string && string->environ_name; string++)
        {
                if (string->type != type)
                        continue;

                /* Override specified in environment. */
                if (string->environ)
                        return string->environ;

                /* No override, fall back to API call */
                break;
        }

        /* If we get here, it's a type that wasn't in the table, but may be defined
         * in the API.
         */
        return next(type);
}