Android系统采用OpenGL绘制3D图形，使用skia来绘制二维图形；OpenGL源码位于：
frameworks/native/opengl
frameworks/base/opengl
本文简单介绍OpenGL库的加载过程。OpenGL以动态库的方式提供，因此在使用OpenGL的接口函数绘图前，需要加载OpenGL库，并得到接口函数指针。函数EGLBoolean egl_init_drivers()就是负责OpenGL库的加载。
EGLBoolean egl_init_drivers() {
EGLBoolean res;
pthread_mutex_lock(&sInitDriverMutex);
res = egl_init_drivers_locked();
pthread_mutex_unlock(&sInitDriverMutex);
return res;
}
为保证多线程访问的安全性，使用线程锁来放完另一个接口函数egl_init_drivers_locked
static EGLBoolean egl_init_drivers_locked() {
if (sEarlyInitState) {
// initialized by static ctor. should be set here.
return EGL_FALSE;
}
// 得到Loader对象实例
Loader& loader(Loader::getInstance());
//加载EGL库
egl_connection_t* cnx = &gEGLImpl;
if (cnx->dso == 0) {
cnx->hooks[egl_connection_t::GLESv1_INDEX] =&gHooks[egl_connection_t::GLESv1_INDEX];
cnx->hooks[egl_connection_t::GLESv2_INDEX] =&gHooks[egl_connection_t::GLESv2_INDEX];
cnx->dso = loader.open(cnx);
}
return cnx->dso ? EGL_TRUE : EGL_FALSE;
}
函数首先定义指针cnx指向全局变量gEGLImpl，并且将cnx的域hooks指向gHooks，最后通过loader对象的open函数打开EGL动态库，因此最后从EGL库中加载的接口函数指针都保存到了变量gEGLImpl和gHooks中了。
frameworks\native\opengl\libs\EGL\Loader.cpp
Loader::Loader()
{
char line[256];
char tag[256];
/* Special case for GLES emulation 针对模拟器处理*/
if (checkGlesEmulationStatus() == 0) {
ALOGD("Emulator without GPU support detected. ""Fallback to software renderer.");
mDriverTag.setTo("android");
return;
}
/*打开/system/lib/egl/egl.cfg文件 */
FILE* cfg = fopen("/system/lib/egl/egl.cfg", "r");
if (cfg == NULL) {
//如果打开失败，mDriverTag ="android"
ALOGD("egl.cfg not found, using default config");
mDriverTag.setTo("android");
} else {//否则读取文件内容
while (fgets(line, 256, cfg)) {
int dpy, impl;
if (sscanf(line, "%u %u %s", &dpy, &impl, tag) == 3) {
// /system/lib/egl/egl.cfg文件内容为：0 1 mali
if (tag != String8("android")) {
mDriverTag = tag; //mDriverTag = "mali"
}
}
}
fclose(cfg);
}
}
如果/system/lib/egl/egl.cfg文件不存在，则默认配置为0 0 android，否则就读取/system/lib/egl/egl.cfg文件内容，内容定义如下：
因此变量dpy=0, impl = 1, tag = mali
void* Loader::open(egl_connection_t* cnx)
{
void* dso;
driver_t* hnd = 0;
char const* tag = mDriverTag.string();//tag="mali"
if (tag) {
//加载GLES库函数
dso = load_driver("GLES", tag, cnx, EGL | GLESv1_CM | GLESv2);
if (dso) {
hnd = new driver_t(dso);
} else {//如果加载GLES库失败，则加载EGL，GLESv1_CM，GLESv2三个库
dso = load_driver("EGL", tag, cnx, EGL);
if (dso) {//只有EGL库加载成功，才加载GLESv1_CM，GLESv2库
hnd = new driver_t(dso);
// TODO: make this more automated
hnd->set(load_driver("GLESv1_CM", tag, cnx, GLESv1_CM), GLESv1_CM );
hnd->set(load_driver("GLESv2", tag, cnx, GLESv2), GLESv2 );
}
}
}
return (void*)hnd;
}
这个函数首先去加载/system/lib/egl/libGLES_mali.so，如果加载成功，那么对EGL | GLESv1_CM | GLESv2三个函数库，进行初始化。如果加载不成功，那么就加载libEGL_mali.so，libGLESv1_CM_mali.so，libGLESv2_mali.so这三个库，/system/lib/egl目录下的库如下：
由于libGLES_mali.so库不存在，因此最终加载的是libEGL_mali.so，libGLESv1_CM_mali.so，libGLESv2_mali.so三个库
void *Loader::load_driver(const char* kind, const char *tag,
egl_connection_t* cnx, uint32_t mask)
{
char driver_absolute_path[PATH_MAX];
const char* const search1 = "/vendor/lib/egl/lib%s_%s.so";
const char* const search2 = "/system/lib/egl/lib%s_%s.so";
//首先从/vendor/lib/egl/目录下查找对应的库，如果该目录下没有，则查找/system/lib/egl/目录
snprintf(driver_absolute_path, PATH_MAX, search1, kind, tag);
//driver_absolute_path="/vendor/lib/egl/libEGL_mali.so"
//driver_absolute_path="/vendor/lib/egl/libGLESv1_CM_mali.so"
//driver_absolute_path="/vendor/lib/egl/libGLESv2_mali.so"
//判断该路径下的动态库是否可以访问
if (access(driver_absolute_path, R_OK)) {
snprintf(driver_absolute_path, PATH_MAX, search2, kind, tag);
//driver_absolute_path="/system/lib/egl/libEGL_mali.so"
//driver_absolute_path="/system/lib/egl/libGLESv1_CM_mali.so"
//driver_absolute_path="/system/lib/egl/libGLESv2_mali.so"
if (access(driver_absolute_path, R_OK)) {
return 0;
}
}
//打开so动态库到cnx->egl中
void* dso = dlopen(driver_absolute_path, RTLD_NOW | RTLD_LOCAL);
if (dso == 0) {
const char* err = dlerror();
ALOGE("load_driver(%s): %s", driver_absolute_path, err?err:"unknown");
return 0;
}
ALOGD("loaded %s", driver_absolute_path);
//从动态库中加载EGL函数库到cnx->egl中，EGL库函数定义在egl_names数组中
if (mask & EGL) {
//读取动态库中eglGetProcAddress函数指针
getProcAddress = (getProcAddressType)dlsym(dso, "eglGetProcAddress");
ALOGE_IF(!getProcAddress, "can't find eglGetProcAddress() in %s", driver_absolute_path);
#ifdef SYSTEMUI_PBSIZE_HACK
#warning "SYSTEMUI_PBSIZE_HACK enabled"
/*
* Here we adjust the PB size from its default value to 512KB which
* is the minimum acceptable for the systemui process.
*/
const char *cmdline = getProcessCmdline();
if (strstr(cmdline, "systemui")) {
void *imgegl = dlopen("/vendor/lib/libIMGegl.so", RTLD_LAZY);
if (imgegl) {
unsigned int *PVRDefaultPBS =(unsigned int *)dlsym(imgegl, "PVRDefaultPBS");
if (PVRDefaultPBS) {
ALOGD("setting default PBS to 512KB, was %d KB", *PVRDefaultPBS / 1024);
*PVRDefaultPBS = 512*1024;
}
}
}
#endif
egl_t* egl = &cnx->egl;
__eglMustCastToProperFunctionPointerType* curr =
(__eglMustCastToProperFunctionPointerType*)egl;
/*定义指向egl_names数组的指针
* char const * const egl_names[] = {
* #include "egl_entries.in"
* NULL
* };
* 文件frameworks/native/opengl/libs/EGL/egl_entries.in声明了EGL库中的所有函数
*/
char const * const * api = egl_names;
//遍历数组egl_names，将数组中定义的函数指针保存到cnx->egl中
while (*api) {
char const * name = *api;
//根据函数名称从动态库EGL中取得函数指针
__eglMustCastToProperFunctionPointerType f =
(__eglMustCastToProperFunctionPointerType)dlsym(dso, name);
if (f == NULL) {
//如果在动态库中查找不到该函数指针，则使用getProcAddress函数来获取
f = getProcAddress(name);
if (f == NULL) {//如果依然得不到该函数指针，则设置为0
f = (__eglMustCastToProperFunctionPointerType)0;
}
}
*curr++ = f;
api++;
}
}
//char const * const gl_names[] = {
// #include "entries.in"
// NULL
//};
//从动态库中加载GLESv1_CM函数库到cnx->hooks[egl_connection_t::GLESv1_INDEX]->gl中，GLESv1_CM库函数定义在gl_names数组中
if (mask & GLESv1_CM) {
init_api(dso, gl_names,(__eglMustCastToProperFunctionPointerType*)
&cnx->hooks[egl_connection_t::GLESv1_INDEX]->gl,
getProcAddress);
}
//从动态库中加载GLESv2函数库到cnx->hooks[egl_connection_t::GLESv2_INDEX]->gl中，GLESv2库函数定义在gl_names数组中
if (mask & GLESv2) {
init_api(dso, gl_names,(__eglMustCastToProperFunctionPointerType*)
&cnx->hooks[egl_connection_t::GLESv2_INDEX]->gl,
getProcAddress);
}
return dso;
}
该函数就是从动态库libEGL_mali.so中查询egl_names数组，即frameworks/native/opengl/libs/EGL/egl_entries.in文件中声明的接口函数，及从动态库libGLESv1_CM_mali.so，libGLESv2_mali.so中查询gl_names数组，即frameworks/native/opengl/libs/entries.in文件中声明的接口函数，函数声明如下：
void Loader::init_api(void* dso, char const * const * api,
__eglMustCastToProperFunctionPointerType* curr,
getProcAddressType getProcAddress)
{
const ssize_t SIZE = 256;
char scrap[SIZE];
while (*api) {
char const * name = *api;
//根据函数名称从动态库中查找函数指针
__eglMustCastToProperFunctionPointerType f = (__eglMustCastToProperFunctionPointerType)dlsym(dso, name);
if (f == NULL) {
//使用eglGetProcAddress()得到指定函数指针
f = getProcAddress(name);
}
if (f == NULL) {
//将函数名称去掉后缀OES
ssize_t index = ssize_t(strlen(name)) - 3;
if ((index>0 && (index<SIZE-1)) && (!strcmp(name+index, "OES"))) {
strncpy(scrap, name, index);
scrap[index] = 0;
f = (__eglMustCastToProperFunctionPointerType)dlsym(dso, scrap);
}
}
if (f == NULL) {
//将函数名称增加后缀OES
ssize_t index = ssize_t(strlen(name)) - 3;
if (index>0 && strcmp(name+index, "OES")) {
snprintf(scrap, SIZE, "%sOES", name);
f = (__eglMustCastToProperFunctionPointerType)dlsym(dso, scrap);
}
}
if (f == NULL) {
f = (__eglMustCastToProperFunctionPointerType)gl_unimplemented;
if (!strcmp(name, "glInsertEventMarkerEXT")) {
f = (__eglMustCastToProperFunctionPointerType)gl_noop;
} else if (!strcmp(name, "glPushGroupMarkerEXT")) {
f = (__eglMustCastToProperFunctionPointerType)gl_noop;
} else if (!strcmp(name, "glPopGroupMarkerEXT")) {
f = (__eglMustCastToProperFunctionPointerType)gl_noop;
}
}
*curr++ = f;
api++;
}
}
load_driver函数所做的工作：首先通过dlopen加载/system/lib/egl/libGLES_mali.so库，然后从/system/lib/egl/libGLES_mali.so库中提取EGL的各个API函数的地址放到cnx->egl中，从libGLES_mali.so获取GLESv1_CM的API保存到cnx->hooks[GLESv1_INDEX]->gl中，从libGLES_mali.so获取GLESv1_CM的API保存到cnx->hooks[GLESv2_INDEX]->gl。提取EGLAPI地址的方法是首先通过dlsym函数获得一个获取函数地址的函数eglGetProcAddress的地址，然后遍历EGL的API所在文件frameworks/base/opengl/libs/EGL/egl_entries.in。先通过dlsym获取各个API地址，如果返回NULL再利用eglGetProcAddress去获得，如果依旧为空就把函数地址赋值为0；提取GLESv1_CM和GLESv1_CM库中函数地址方法和提取EGL差不多，只是他们的函数文件保存在frameworks/base/opengl/libs/entries.in中。还有它们把函数地址复制给了cnx->hooks[GLESv1_INDEX]->gl和cnx->hooks[GLESv2_INDEX]->gl。