wasm2c takes a WebAssembly module and produces an equivalent C source and
header. Some examples:
# 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.cLet's look at a simple example of a factorial function.
(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:
$ wat2wasm fac.wat -o fac.wasmWe can then convert it to a C source and header by using the wasm2c tool:
$ wasm2c fac.wasm -o fac.cThis 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.
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_init_module, Z_fac_instantiate
and Z_facZ_fac. The first initializes the module, the second constructs an instance of the module, and the third 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.
#include <stdio.h>
#include <stdlib.h>
#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();
/* Initialize the `fac` module (this registers the module's function types in
* a global data structure) */
Z_fac_init_module();
/* 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;
}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.
$ cc -o fac main.c fac.c wasm-rt-impl.cA 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!
$ ./fac 1
fac(1) -> 1
$ ./fac 5
fac(5) -> 120
$ ./fac 10
fac(10) -> 3628800You can take a look at the all of these files in wasm2c/examples/fac.
The generated header file looks something like this:
#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_init_module(void);
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.
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. The four WebAssembly value types are included:
typedef enum {
WASM_RT_I32,
WASM_RT_I64,
WASM_RT_F32,
WASM_RT_F64,
} wasm_rt_type_t;Next is wasm_rt_funcref_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:
typedef void (*wasm_rt_funcref_t)(void);Next are the definitions for a table element. func_type is a function index
as returned by wasm_rt_register_func_type described below. module_instance
is the pointer to the module instance that should be passed in when the func is
called.
typedef struct {
uint32_t func_type;
wasm_rt_funcref_t func;
void* module_instance;
} wasm_rt_elem_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.
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.
typedef struct {
wasm_rt_elem_t* data;
uint32_t max_size;
uint32_t size;
} wasm_rt_table_t;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 and wasm-rt-impl.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);
uint32_t wasm_rt_register_func_type(uint32_t params, uint32_t results, ...);
void wasm_rt_allocate_memory(wasm_rt_memory_t*, uint32_t initial_pages, uint32_t max_pages);
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_register_func_type is a function that registers a function type. It
is a variadic function where the first two arguments give the number of
parameters and results, and the following arguments are the types. For example,
the function func (param i32 f32) (result f64) would register the function
type as
wasm_rt_register_func_type(2, 1, WASM_RT_I32, WASM_RT_F32, WASM_RT_F64).
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.
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.
Several additional symbols must be defined if wasm2c is being run with
support for exceptions (--enable-exceptions):
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 and 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.
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_init_module() initializes the whole module and must be
called before any instance of the module is used.
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.
typedef struct Z_fac_instance_t {
char dummy_member;
} Z_fac_instance_t;
void Z_fac_init_module(void);
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);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:
(export "mem" (memory $mem))then wasm2c would declare the following function in the header:
/* export: 'mem' */
extern wasm_rt_memory_t* Z_facZ_mem(Z_fac_instance_t*);which would be defined as:
/* export: 'mem' */
wasm_rt_memory_t* Z_fac_Z_mem(Z_fac_instance_t* instance) {
return &instance->w2c_M0;
}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:
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:
(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:
$ wat-desugar fac-flat.wat --fold -o fac-folded.wat(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.
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.
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#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();
/* Initialize the rot13 module. */
Z_rot13_init_module();
/* 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);
wasm_rt_allocate_memory(&host_instance_2.memory, 1, 1);
/* 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]);
}