Multiple Modules as Dependencies ========================= It is allowed that one WASM module can *import* *functions*, *globals*, *memories* and *tables* from other modules as its dependencies, and also one module can *export* those entities for other modules to *access* and may *write*. WAMR loads all dependencies recursively according to the *import section* of a module. > Currently WAMR only implements the load-time dynamic linking. Please refer to [dynamic linking](https://webassembly.org/docs/dynamic-linking/) for more details. ## Multi-Module Related APIs ### Register a module ``` c bool wasm_runtime_register_module(const char *module_name, wasm_module_t module, char *error_buf, uint32_t error_buf_size); ``` It is used to register a *module* with a *module_name* to WASM runtime, especially for the root module, which is loaded by `wasm_runtime_load()` and doesn't have a chance to tell runtime its *module name*. Fot all the sub modules, WAMR will get their names and load the .wasm files from the filesystem or stream, so no need to register the sub modules again. ### Find a registered module ``` c wasm_module_t wasm_runtime_find_module_registered( const char *module_name); ``` It is used to check if a module with a given *module_name* has been registered, if yes return the module. ### Module reader and destroyer ``` c typedef bool (*module_reader)(const char *module_name, uint8_t **p_buffer, uint32_t *p_size); typedef void (*module_destroyer)(uint8_t *buffer, uint32_t size); void wasm_runtime_set_module_reader(const module_reader reader, const module_destroyer destroyer); ``` WAMR hopes that the native host or embedding environment loads/unloads the module WASM files by themselves and only passes runtime the binary content without worrying filesystem or storage issues. `module_reader` and `module_destroyer` are two callbacks called when dynamic-loading/unloading the sub modules. Developers must implement the two callbacks by themselves. ### Call function of sub module ```c wasm_function_inst_t wasm_runtime_lookup_function(wasm_module_inst_t const module_inst, const char *name, const char *signature); ``` Multi-module allows to lookup the function of sub module and call it. There are two ways to indicate the function *name*: - parent function name only by default, used to lookup the function of parent module - sub module name, function name of sub module and two $ symbols, e.g. `$sub_module_name$function_name`, used to lookup function of sub module ## Example ### WASM modules Suppose we have three C files, *mA.c*, *mB.c* and *mC.c*. Each of them has some exported functions and import some from others except mA. Undefined symbols can be marked in the source code with the *import_name* clang attribute which means that they are expected to be undefined at static link time. Without the *import_module* clang attribute, undefined symbols will be marked from the *env* module. ``` C // mA.c int A() { return 10; } ``` ``` C // mB.c __attribute__((import_module("mA"))) __attribute__((import_name("A"))) extern int A(); int B() { return 11; } int call_A() { return A(); } ``` ``` C // mC.c __attribute__((import_module("mA"))) __attribute__((import_name("A"))) extern int A(); __attribute__((import_module("mB"))) __attribute__((import_name("B"))) extern int B(); int C() { return 12; } int call_A() { return A(); } int call_B() { return B(); } ``` By default no undefined symbols are allowed in the final binary. The flag *--allow-undefined* results in a WebAssembly import being defined for each undefined symbol. It is then up to the runtime to provide such symbols. When building an executable, only the entry point (_start) and symbols with the *export_name* attribute exported by default. in addition, symbols can be exported via the linker command line using *--export*. In the example, another linked command option *--export-all* is used. > with more detail, please refer to [WebAssembly lld port][https://lld.llvm.org/WebAssembly.html] Here is an example how to compile a *.c* to a *.wasm* with clang. Since there is no *start* function, we use *--no-entry* option. ``` shell $ clang --target=wasm32 -nostdlib \ -Wl,--no-entry,--allow-undefined,--export-all \ -o mA.wasm mA.c $ clang --target=wasm32 -nostdlib \ -Wl,--no-entry,--allow-undefined,--export-all \ -o mB.wasm mB.c $ clang --target=wasm32 -nostdlib \ -Wl,--no-entry,--allow-undefined,--export-all \ -o mC.wasm mC.c ``` put *mA.wasm*, *mB.wasm* and *mC.wasm* in the directory *wasm-apps* ``` shell $ # copy mA.wasm, mB.wasm and mC.wasm into wasm-apps $ tree wasm-apps/ wasm-apps/ ├── mA.wasm ├── mB.wasm └── mC.wasm ``` eventually, their *import relationships* will be like: ![import relationships](./pics/multi_module_pic1.png) ### libvmlib We need to enable *WAMR_BUILD_MULTI_MODULE* option when building WAMR vmlib. Please ref to [Build WAMR core](./build_wamr.md) for a thoughtful guide. ### code After all above preparation, we can call some functions from native code with APIs first, create two callbacks to load WASM module files into memory and unload them later ``` c static bool module_reader_cb(const char *module_name, uint8 **p_buffer, uint32 *p_size) { // ... *p_buffer = (uint8_t *)bh_read_file_to_buffer(wasm_file_path, p_size); // ... } static void module_destroyer_cb(uint8 *buffer, uint32 size) { BH_FREE(buffer); } ``` second, create a large buffer and tell WAMR malloc any resource only from this buffer later ``` c static char sandbox_memory_space[10 * 1024 * 1024] = { 0 }; ``` third, put all together ``` c int main() { /* all malloc() only from the given buffer */ init_args.mem_alloc_type = Alloc_With_Pool; init_args.mem_alloc_option.pool.heap_buf = sandbox_memory_space; init_args.mem_alloc_option.pool.heap_size = sizeof(sandbox_memory_space); /* initialize runtime environment */ wasm_runtime_full_init(&init_args); /* set module reader and destroyer */ wasm_runtime_set_module_reader(module_reader_cb, module_destroyer_cb); /* load WASM byte buffer from WASM bin file */ module_reader_cb("mC", &file_buf, &file_buf_size)); /* load mC and let WAMR load mA and mB */ module = wasm_runtime_load(file_buf, file_buf_size, error_buf, sizeof(error_buf)); /* instantiate the module */ module_inst = wasm_runtime_instantiate(module, stack_size, heap_size, error_buf, sizeof(error_buf))); printf("call \"C\", it will return 0xc:i32, ===> "); wasm_application_execute_func(module_inst, "C", 0, &args[0]); printf("call \"call_B\", it will return 0xb:i32, ===> "); wasm_application_execute_func(module_inst, "call_B", 0, &args[0]); printf("call \"call_A\", it will return 0xa:i32, ===>"); wasm_application_execute_func(module_inst, "call_A", 0, &args[0]); /* call some functions of mB */ printf("call \"mB.B\", it will return 0xb:i32, ===>"); wasm_application_execute_func(module_inst, "$mB$B", 0, &args[0]); printf("call \"mB.call_A\", it will return 0xa:i32, ===>"); wasm_application_execute_func(module_inst, "$mB$call_A", 0, &args[0]); /* call some functions of mA */ printf("call \"mA.A\", it will return 0xa:i32, ===>"); wasm_application_execute_func(module_inst, "$mA$A", 0, &args[0]); // ... } ``` > please refer to [main.c](../samples/multi_modules/src/main.c) The output of the main.c will like: ``` shell $ ./a.out call "C", it will return 0xc:i32, ===> 0xc:i32 call "call_B", it will return 0xb:i32, ===> 0xb:i32 call "call_A", it will return 0xa:i32, ===>0xa:i32 call "mB.B", it will return 0xb:i32, ===>0xb:i32 call "mB.call_A", it will return 0xa:i32, ===>0xa:i32 call "mA.A", it will return 0xa:i32, ===>0xa:i32 ```