# wasm2c: Convert wasm files to C source and header `wasm2c` takes a WebAssembly module and produces an equivalent C source and header. Some examples: ```sh # parse binary file test.wasm and write test.c and test.h $ wasm2c test.wasm -o test.c # parse test.wasm, write test.c and test.h, but ignore the debug names, if any $ wasm2c test.wasm --no-debug-names -o test.c ``` ## Tutorial: .wat -> .wasm -> .c Let's look at a simple example of a factorial function. ```wasm (memory $mem 1) (func (export "fac") (param $x i32) (result i32) (if (result i32) (i32.eq (local.get $x) (i32.const 0)) (then (i32.const 1)) (else (i32.mul (local.get $x) (call 0 (i32.sub (local.get $x) (i32.const 1)))) ) ) ) ``` Save this to `fac.wat`. We can convert this to a `.wasm` file by using the `wat2wasm` tool: ```sh $ wat2wasm fac.wat -o fac.wasm ``` We can then convert it to a C source and header by using the `wasm2c` tool: ```sh $ wasm2c fac.wasm -o fac.c ``` This generates two files, `fac.c` and `fac.h`. We'll take a closer look at these files below, but first let's show a simple example of how to use these files. ## Using the generated module To actually use our `fac` module, we'll use create a new file, `main.c`, that include `fac.h`, initializes the module, and calls `fac`. `wasm2c` generates a few C symbols based on the `fac.wasm` module: `Z_fac_instantiate` and `Z_facZ_fac`. The first constructs an instance of the module, and the second is the exported `fac` function. All the exported symbols shared a common prefix (`Z_fac`) which, by default, is based on the name section in the module or the name of input file. This prefix can be overridden using the `-n/--module-name` command line flag. In addition to parameters defined in `fac.wat`, `Z_fac_instantiate` and `Z_facZ_fac` take in a pointer to a `Z_fac_instance_t`. The structure is used to store the context information of the module instance, and `main.c` is responsible for providing it. ```c #include #include #include "fac.h" int main(int argc, char** argv) { /* Make sure there is at least one command-line argument. */ if (argc < 2) return 1; /* Convert the argument from a string to an int. We'll implicitly cast the int to a `u32`, which is what `fac` expects. */ u32 x = atoi(argv[1]); /* Initialize the Wasm runtime. */ wasm_rt_init(); /* Declare an instance of the `fac` module. */ Z_fac_instance_t instance; /* Construct the module instance. */ Z_fac_instantiate(&instance); /* Call `fac`, using the mangled name. */ u32 result = Z_facZ_fac(&instance, x); /* Print the result. */ printf("fac(%u) -> %u\n", x, result); /* Free the fac module. */ Z_fac_free(&instance); /* Free the Wasm runtime state. */ wasm_rt_free(); return 0; } ``` ## Compiling the wasm2c output To compile the executable, we need to use `main.c` and the generated `fac.c`. We'll also include `wasm-rt-impl.c` which has implementations of the various `wasm_rt_*` functions used by `fac.c` and `fac.h`. ```sh $ cc -o fac main.c fac.c wasm-rt-impl.c ``` A note on compiling with optimization: wasm2c relies on certain behavior from the C compiler to maintain conformance with the WebAssembly specification, especially with regards to requirements to convert "signaling" to "quiet" floating-point NaN values and for infinite recursion to produce a trap. When compiling with optimization (e.g. `-O2` or `-O3`), it's necessary to disable some optimizations to preserve conformance. With GCC 11, adding the command-line arguments `-fno-optimize-sibling-calls -frounding-math -fsignaling-nans` appears to be sufficient. With clang 14, just `-fno-optimize-sibling-calls -frounding-math` appears to be sufficient. Now let's test it out! ```sh $ ./fac 1 fac(1) -> 1 $ ./fac 5 fac(5) -> 120 $ ./fac 10 fac(10) -> 3628800 ``` You can take a look at the all of these files in [wasm2c/examples/fac](/wasm2c/examples/fac). ## Looking at the generated header, `fac.h` The generated header file looks something like this: ```c #ifndef FAC_H_GENERATED_ #define FAC_H_GENERATED_ ... #include "wasm-rt.h" ... #ifndef WASM_RT_CORE_TYPES_DEFINED #define WASM_RT_CORE_TYPES_DEFINED ... #endif #ifdef __cplusplus extern "C" { #endif typedef struct Z_fac_instance_t { char dummy_member; } Z_fac_instance_t; void Z_fac_instantiate(Z_fac_instance_t*); void Z_fac_free(Z_fac_instance_t*); /* export: 'fac' */ u32 Z_facZ_fac(Z_fac_instance_t*, u32); #ifdef __cplusplus } #endif #endif /* FAC_H_GENERATED_ */ ``` Let's look at each section. The outer `#ifndef` is standard C boilerplate for a header. This `WASM_RT_CORE_TYPES_DEFINED` section contains a number of definitions required for all WebAssembly modules. The `extern "C"` part makes sure to not mangle the symbols if using this header in C++. The included `wasm-rt.h` file also includes a number of relevant definitions. First is the `wasm_rt_trap_t` enum, which is used to give the reason a trap occurred. ```c typedef enum { WASM_RT_TRAP_NONE, WASM_RT_TRAP_OOB, WASM_RT_TRAP_INT_OVERFLOW, WASM_RT_TRAP_DIV_BY_ZERO, WASM_RT_TRAP_INVALID_CONVERSION, WASM_RT_TRAP_UNREACHABLE, WASM_RT_TRAP_CALL_INDIRECT, WASM_RT_TRAP_UNCAUGHT_EXCEPTION, WASM_RT_TRAP_EXHAUSTION, } wasm_rt_trap_t; ``` Next is the `wasm_rt_type_t` enum, which is used for specifying function signatures. Six WebAssembly value types are included: ```c typedef enum { WASM_RT_I32, WASM_RT_I64, WASM_RT_F32, WASM_RT_F64, WASM_RT_FUNCREF, WASM_RT_EXTERNREF, } wasm_rt_type_t; Next is `wasm_rt_function_ptr_t`, the function signature for a generic function callback. Since a WebAssembly table can contain functions of any given signature, it is necessary to convert them to a canonical form: ```c typedef void (*wasm_rt_function_ptr_t)(void); ``` Next is the definition for a function reference (in WebAssembly 1.0, this was the type of all table elements, but funcrefs can now also be used as ordinary values, and tables can alternately be declared as type externref). In this structure, `wasm_rt_func_type_t` is an opaque 256-bit ID that can be looked up via the `Z_[modname]_get_func_type` function. (A demonstration of this can be found in the `callback` example.) `module_instance` is the pointer to the function's originating module instance, which will be passed in when the func is called. ```c typedef struct { wasm_rt_func_type_t func_type; wasm_rt_function_ptr_t func; void* module_instance; } wasm_rt_funcref_t; ``` Next is the definition of a memory instance. The `data` field is a pointer to `size` bytes of linear memory. The `size` field of `wasm_rt_memory_t` is the current size of the memory instance in bytes, whereas `pages` is the current size in pages (65536 bytes.) `max_pages` is the maximum number of pages as specified by the module, or `0xffffffff` if there is no limit. ```c typedef struct { uint8_t* data; uint32_t pages, max_pages; uint32_t size; } wasm_rt_memory_t; ``` Next is the definition of a table instance. The `data` field is a pointer to `size` elements. Like a memory instance, `size` is the current size of a table, and `max_size` is the maximum size of the table, or `0xffffffff` if there is no limit. ```c typedef struct { wasm_rt_funcref_t* data; uint32_t max_size; uint32_t size; } wasm_rt_funcref_table_t; ``` ## Symbols that must be defined by the embedder Next in `wasm-rt.h` are a collection of function declarations that must be implemented by the embedder (i.e. you) before this C source can be used. A C implementation of these functions is defined in [`wasm-rt-impl.h`](wasm-rt-impl.h) and [`wasm-rt-impl.c`](wasm-rt-impl.c). ```c void wasm_rt_init(void); bool wasm_rt_is_initialized(void); void wasm_rt_free(void); void wasm_rt_trap(wasm_rt_trap_t) __attribute__((noreturn)); const char* wasm_rt_strerror(wasm_rt_trap_t trap); void wasm_rt_allocate_memory(wasm_rt_memory_t*, uint32_t initial_pages, uint32_t max_pages, bool is64); uint32_t wasm_rt_grow_memory(wasm_rt_memory_t*, uint32_t pages); void wasm_rt_free_memory(wasm_rt_memory_t*); void wasm_rt_allocate_funcref_table(wasm_rt_table_t*, uint32_t elements, uint32_t max_elements); void wasm_rt_allocate_externref_table(wasm_rt_externref_table_t*, uint32_t elements, uint32_t max_elements); void wasm_rt_free_funcref_table(wasm_rt_table_t*); void wasm_rt_free_externref_table(wasm_rt_table_t*); uint32_t wasm_rt_call_stack_depth; /* on platforms that don't use the signal handler to detect exhaustion */ ``` `wasm_rt_init` must be called by the embedder before anything else, to initialize the runtime. `wasm_rt_free` frees any global state. `wasm_rt_is_initialized` can be used to confirm that the runtime has been initialized. `wasm_rt_trap` is a function that is called when the module traps. Some possible implementations are to throw a C++ exception, or to just abort the program execution. The default runtime included in wasm2c unwinds the stack using `longjmp`. You can overide this call to `longjmp` from the embeder by defining a custom trap handler with the signature `void wasm2c_custom_trap_handler(wasm_rt_trap_t code)` and compiling the runtime with the with macro definition `#define WASM_RT_MEMCHECK_SIGNAL_HANDLER wasm2c_custom_trap_handler`. It is recommended that you add this macro definition via a compiler flag (`-DWASM_RT_MEMCHECK_SIGNAL_HANDLER=wasm2c_custom_trap_handler` on clang/gcc). `wasm_rt_allocate_memory` initializes a memory instance, and allocates at least enough space for the given number of initial pages. The memory must be cleared to zero. The `is64` parameter indicates if the memory is indexed with an i32 or i64 address. `wasm_rt_grow_memory` must grow the given memory instance by the given number of pages. If there isn't enough memory to do so, or the new page count would be greater than the maximum page count, the function must fail by returning `0xffffffff`. If the function succeeds, it must return the previous size of the memory instance, in pages. `wasm_rt_free_memory` frees the memory instance. `wasm_rt_allocate_funcref_table` and the similar `..._externref_table` initialize a table instance of the given type, and allocate at least enough space for the given number of initial elements. The elements must be cleared to zero. `wasm_rt_free_funcref_table` and `..._externref_table` free the table instance. `wasm_rt_call_stack_depth` is the current stack call depth. Since this is shared between modules, it must be defined only once, by the embedder. It is only used on platforms that don't use the signal handler to detect exhaustion. ### Runtime support for exception handling Several additional symbols must be defined if wasm2c is being run with support for exceptions (`--enable-exceptions`): ```c void wasm_rt_load_exception(const char* tag, uint32_t size, const void* values); WASM_RT_NO_RETURN void wasm_rt_throw(void); WASM_RT_UNWIND_TARGET WASM_RT_UNWIND_TARGET* wasm_rt_get_unwind_target(void); void wasm_rt_set_unwind_target(WASM_RT_UNWIND_TARGET* target); uint32_t wasm_rt_exception_tag(void); uint32_t wasm_rt_exception_size(void); void* wasm_rt_exception(void); wasm_rt_try(target) ``` A C implementation of these functions is also available in [`wasm-rt-impl.h`](wasm-rt-impl.h) and [`wasm-rt-impl.c`](wasm-rt-impl.c). `wasm_rt_load_exception` sets the active exception to a given tag, size, and contents. `wasm_rt_throw` throws the active exception. `WASM_RT_UNWIND_TARGET` is the type of an unwind target if an exception is thrown and caught. `wasm_rt_get_unwind_target` gets the current unwind target if an exception is thrown. `wasm_rt_set_unwind_target` sets the unwind target if an exception is thrown. Three functions provide access to the active exception: `wasm_rt_exception_tag`, `wasm_rt_exception_size`, and `wasm_rt_exception` return its tag, size, and contents, respectively. `wasm_rt_try(target)` is a macro that captures the current calling environment as an unwind target and stores it into `target`, which must be of type `WASM_RT_UNWIND_TARGET`. ## Exported symbols Finally, `fac.h` defines the module instance type (which in the case of `fac` is essentially empty), and the exported symbols provided by the module. In our example, the only function we exported was `fac`. `Z_fac_instantiate(Z_fac_instance_t*)` creates an instance of the module and must be called before the module instance can be used. `Z_fac_free(Z_fac_instance_t*)` frees the instance. `Z_fac_get_func_type` can be used to look up a function type ID at runtime. It is a variadic function where the first two arguments give the number of parameters and results, and the following arguments are the types from the wasm_rt_type_t enum described above. The `callback` example demonstrates using this to pass a host function to a WebAssembly module dynamically at runtime. ```c typedef struct Z_fac_instance_t { char dummy_member; } Z_fac_instance_t; void Z_fac_instantiate(Z_fac_instance_t*); void Z_fac_free(Z_fac_instance_t*); wasm_rt_func_type_t Z_fac_get_func_type(uint32_t param_count, uint32_t result_count, ...); /* export: 'fac' */ u32 Z_facZ_fac(Z_fac_instance_t*, u32); ``` ## Handling other kinds of imports and exports of modules Exported functions are handled by declaring a prefixed equivalent function in the header. If a module is imports a function, `wasm2c` declares the function in the output header file, and the host function is responsible for defining the function. Exports of other kinds (globals, memories, tables) are handled differently, since they are part of the module instance, and each instance can have its own exports. For these cases, `wasm2c` provides a function that takes in a module instance as argument, and returns the corresponding export. For example, if `fac` exported a memory as such: ```wasm (export "mem" (memory $mem)) ``` then `wasm2c` would declare the following function in the header: ```c /* export: 'mem' */ extern wasm_rt_memory_t* Z_facZ_mem(Z_fac_instance_t*); ``` which would be defined as: ```c /* export: 'mem' */ wasm_rt_memory_t* Z_fac_Z_mem(Z_fac_instance_t* instance) { return &instance->w2c_M0; } ``` ## A quick look at `fac.c` The contents of `fac.c` are internals, but it is useful to see a little about how it works. The first few hundred lines define macros that are used to implement the various WebAssembly instructions. Their implementations may be interesting to the curious reader, but are out of scope for this document. Following those definitions are various initialization functions (`init`, `free`, `init_func_types`, `init_globals`, `init_memory`, `init_table`, and `init_exports`.) In our example, most of these functions are empty, since the module doesn't use any globals, memory or tables. The most interesting part is the definition of the function `fac`: ```c static u32 w2c_fac(Z_fac_instance_t* instance, u32 w2c_p0) { FUNC_PROLOGUE; u32 w2c_i0, w2c_i1, w2c_i2; w2c_i0 = w2c_p0; w2c_i1 = 0u; w2c_i0 = w2c_i0 == w2c_i1; if (w2c_i0) { w2c_i0 = 1u; } else { w2c_i0 = w2c_p0; w2c_i1 = w2c_p0; w2c_i2 = 1u; w2c_i1 -= w2c_i2; w2c_i1 = w2c_fac(instance, w2c_i1); w2c_i0 *= w2c_i1; } FUNC_EPILOGUE; return w2c_i0; } ``` If you look at the original WebAssembly text in the flat format, you can see that there is a 1-1 mapping in the output: ```wasm (func $fac (param $x i32) (result i32) local.get $x i32.const 0 i32.eq if (result i32) i32.const 1 else local.get $x local.get $x i32.const 1 i32.sub call 0 i32.mul end) ``` This looks different than the factorial function above because it is using the "flat format" instead of the "folded format". You can use `wat-desugar` to convert between the two to be sure: ```sh $ wat-desugar fac-flat.wat --fold -o fac-folded.wat ``` ```wasm (module (func (;0;) (param i32) (result i32) (if (result i32) ;; label = @1 (i32.eq (local.get 0) (i32.const 0)) (then (i32.const 1)) (else (i32.mul (local.get 0) (call 0 (i32.sub (local.get 0) (i32.const 1))))))) (export "fac" (func 0)) (type (;0;) (func (param i32) (result i32)))) ``` The formatting is different and the variable and function names are gone, but the structure is the same. ## Create multiple instances of a module Since information about the execution context, such as memories, is encapsulated in the module instance structure, and a pointer to the structure is being passed through function calls, multiple instances of the same module can be instantiated alongside one another. We can take a look at another version of the `main` function for a `rot13` example. By declaring two sets of context information, two instances of `rot13` can be instantiated in the same address space. ```c #include #include #include #include "rot13.h" /* Define structure to hold the imports */ struct Z_host_instance_t { wasm_rt_memory_t memory; char* input; }; /* Accessor to access the memory member of the host */ wasm_rt_memory_t* Z_hostZ_mem(struct Z_host_instance_t* instance) { return &instance->memory; } /* Declare the implementations of the imports. */ static u32 fill_buf(struct Z_host_instance_t* instance, u32 ptr, u32 size); static void buf_done(struct Z_host_instance_t* instance, u32 ptr, u32 size); /* Define host-provided functions under the names imported by the `rot13` instance */ u32 Z_hostZ_fill_buf(struct Z_host_instance_t* instance, u32 ptr, u32 size) { return fill_buf(instance, ptr, size); } void Z_hostZ_buf_done(struct Z_host_instance_t* instance, u32 ptr, u32 size) { return buf_done(instance, ptr, size); } int main(int argc, char** argv) { /* Initialize the Wasm runtime. */ wasm_rt_init(); /* Declare two instances of the `rot13` module. */ Z_rot13_instance_t rot13_instance_1; Z_rot13_instance_t rot13_instance_2; /* Create two `host` module instances to store the memory and current string */ struct Z_host_instance_t host_instance_1; struct Z_host_instance_t host_instance_2; /* Allocate 1 page of wasm memory (64KiB). */ wasm_rt_allocate_memory(&host_instance_1.memory, 1, 1, false); wasm_rt_allocate_memory(&host_instance_2.memory, 1, 1, false); /* Construct the module instances */ Z_rot13_instantiate(&rot13_instance_1, &host_instance_1); Z_rot13_instantiate(&rot13_instance_2, &host_instance_2); /* Call `rot13` on first two argument, using the mangled name. */ assert(argc > 2); host_instance_1.input = argv[1]; Z_rot13Z_rot13(&rot13_instance_1); host_instance_2.input = argv[2]; Z_rot13Z_rot13(&rot13_instance_2); /* Free the rot13 modules. */ Z_rot13_free(&rot13_instance_1); Z_rot13_free(&rot13_instance_2); /* Free the Wasm runtime state. */ wasm_rt_free(); return 0; } /* Fill the wasm buffer with the input to be rot13'd. * * params: * ptr: The wasm memory address of the buffer to fill data. * size: The size of the buffer in wasm memory. * result: * The number of bytes filled into the buffer. (Must be <= size). */ u32 fill_buf(struct Z_host_instance_t* instance, u32 ptr, u32 size) { for (size_t i = 0; i < size; ++i) { if (instance->input[i] == 0) { return i; } instance->memory.data[ptr + i] = instance->input[i]; } return size; } /* Called when the wasm buffer has been rot13'd. * * params: * ptr: The wasm memory address of the buffer. * size: The size of the buffer in wasm memory. */ void buf_done(struct Z_host_instance_t* instance, u32 ptr, u32 size) { /* The output buffer is not necessarily null-terminated, so use the %*.s * printf format to limit the number of characters printed. */ printf("%s -> %.*s\n", instance->input, (int)size, &instance->memory.data[ptr]); } ```