Android 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 ativeopengllibsEGLLoader.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。