安卓HAL层 so库文件加载原理

本文分析代码基于安卓6.0

上层app通过jni调用hal层的hw_get_module函数获取硬件模块，这个函数是上层与hal打交道的入口

这里我们就具体来看看hw_get_module的实现

文件路径：

vim hardware/libhardware/hardware.c

打开该文件会发现，这个文件非常小，只有200多行

首先找到hw_get_module()

int hw_get_module(const char *id, const struct hw_module_t **module){    return hw_get_module_by_class(id, NULL, module);}

这里直接调用<pre name="code" class="cpp" style="color: rgb(51, 51, 51); font-size: 13.3333px; line-height: 24px;">hw_get_module_by_class（）

继续跟代码

int hw_get_module_by_class(const char *class_id, const char *inst,                           const struct hw_module_t **module){    int i = 0;    char prop[PATH_MAX] = {0};    char path[PATH_MAX] = {0};    char name[PATH_MAX] = {0};    char prop_name[PATH_MAX] = {0};    if (inst)        snprintf(name, PATH_MAX, "%s.%s", class_id, inst);    else        strlcpy(name, class_id, PATH_MAX);    /*     * Here we rely on the fact that calling dlopen multiple times on     * the same .so will simply increment a refcount (and not load     * a new copy of the library).     * We also assume that dlopen() is thread-safe.     */    /* First try a property specific to the class and possibly instance */    snprintf(prop_name, sizeof(prop_name), "ro.hardware.%s", name);    if (property_get(prop_name, prop, NULL) > 0) {        if (hw_module_exists(path, sizeof(path), name, prop) == 0) {            goto found;        }    }    /* Loop through the configuration variants looking for a module */    for (i=0 ; i<HAL_VARIANT_KEYS_COUNT; i++) {        if (property_get(variant_keys[i], prop, NULL) == 0) {            continue;        }        if (hw_module_exists(path, sizeof(path), name, prop) == 0) {            goto found;        }    }    /* Nothing found, try the default */    if (hw_module_exists(path, sizeof(path), name, "default") == 0) {        goto found;    }    return -ENOENT;found:    /* load the module, if this fails, we're doomed, and we should not try     * to load a different variant. */    return load(class_id, path, module);}

文件的查找，主要集中在如下代码片段：

    /* Loop through the configuration variants looking for a module */    for (i=0 ; i<HAL_VARIANT_KEYS_COUNT; i++) {        if (property_get(variant_keys[i], prop, NULL) == 0) {            continue;        }        if (hw_module_exists(path, sizeof(path), name, prop) == 0) {            goto found;        }    }

这里将variant_keys[i]里的属性获取到，然后与上层传过来的id拼凑加上.so

大致如下：id.prop.so 使用hw_module_exists查找对应路径下的文件

static int hw_module_exists(char *path, size_t path_len, const char *name,
                            const char *subname)
{
    snprintf(path, path_len, "%s/%s.%s.so",
             HAL_LIBRARY_PATH2, name, subname);
    if (access(path, R_OK) == 0)
        return 0;


    snprintf(path, path_len, "%s/%s.%s.so",
             HAL_LIBRARY_PATH1, name, subname);
    if (access(path, R_OK) == 0)
        return 0;


    return -ENOENT;
}

如果查找到相应文件，继续调用：

static int load(const char *id,
        const char *path,
        const struct hw_module_t **pHmi)
{
    int status = -EINVAL;
    void *handle = NULL;
    struct hw_module_t *hmi = NULL;


    /*
     * load the symbols resolving undefined symbols before
     * dlopen returns. Since RTLD_GLOBAL is not or'd in with
     * RTLD_NOW the external symbols will not be global
     */
    handle = dlopen(path, RTLD_NOW);
    if (handle == NULL) {
        char const *err_str = dlerror();
        ALOGE("load: module=%s\n%s", path, err_str?err_str:"unknown");
        status = -EINVAL;
        goto done;
    }


    /* Get the address of the struct hal_module_info. */
    const char *sym = HAL_MODULE_INFO_SYM_AS_STR;
    hmi = (struct hw_module_t *)dlsym(handle, sym);
    if (hmi == NULL) {
        ALOGE("load: couldn't find symbol %s", sym);
        status = -EINVAL;
        goto done;
    }


    /* Check that the id matches */
    if (strcmp(id, hmi->id) != 0) {
        ALOGE("load: id=%s != hmi->id=%s", id, hmi->id);
        status = -EINVAL;
        goto done;
    }


    hmi->dso = handle;


    /* success */
    status = 0;


    done:
    if (status != 0) {
        hmi = NULL;
        if (handle != NULL) {
            dlclose(handle);
            handle = NULL;
        }
    } else {
        ALOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
                id, path, *pHmi, handle);
    }


    *pHmi = hmi;


    return status;
}

在该函数中使用handle = dlopen(path, RTLD_NOW)打开so库文件

使用 hmi = (struct hw_module_t *)dlsym(handle, sym)在得到的handle中获取hw_module_t

然后上层app就可以通过hw_module_t 里的函数接口访问驱动，进而访问硬件