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