Android(安卓)OpenGL库加载过程源码分析

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。

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