wasm2c: Convert wasm files to C source and header

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.c

Tutorial: .wat -> .wasm -> .c

Let's look at a simple example of a factorial function.

(func (export "fac") (param $x i32) (result i32)
  (if (result i32) (i32.eq (get_local $x) (i32.const 0))
    (then (i32.const 1))
    (else
      (i32.mul (get_local $x) (call 0 (i32.sub (get_local $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.wasm

We can then convert it to a C source and header by using the wasm2c tool:

$ 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 symbols for us, init and Z_facZ_ii. init initializes the module, and Z_facZ_ii is our exported fac function, but name-mangled to include the function signature.

We can define WASM_RT_MODULE_PREFIX before including fac.h to generate these symbols with a prefix, in case we already have a symbol called init (or even Z_facZ_ii!) Note that you'll have to compile fac.c with this macro too, for this to work.

#include <stdio.h>
#include <stdlib.h>

/* Uncomment this to define fac_init and fac_Z_facZ_ii instead. */
/* #define WASM_RT_MODULE_PREFIX fac_ */

#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 implictly cast the int
  to a `u32`, which is what `fac` expects. */
  u32 x = atoi(argv[1]);

  /* Initialize the fac module. Since we didn't define WASM_RT_MODULE_PREFIX,
  the initialization function is called `init`. */
  init();

  /* Call `fac`, using the mangled name. */
  u32 result = Z_facZ_ii(x);

  /* Print the result. */
  printf("fac(%u) -> %u\n", x, result);

  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.c

Now let's test it out!

$ ./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.

Looking at the generated header, fac.h

The generated header file looks something like this:

#ifndef FAC_H_GENERATED_
#define FAC_H_GENERATED_
#ifdef __cplusplus
extern "C" {
#endif

#ifndef WASM_RT_INCLUDED_
#define WASM_RT_INCLUDED_

...

#endif  /* WASM_RT_INCLUDED_ */

extern void WASM_RT_ADD_PREFIX(init)(void);

/* export: 'fac' */
extern u32 (*WASM_RT_ADD_PREFIX(Z_facZ_ii))(u32);
#ifdef __cplusplus
}
#endif

#endif  /* FAC_H_GENERATED_ */

Let's look at each section. The outer #ifndef is standard C boilerplate for a header. The extern "C" part makes sure to not mangle the symbols if using this header in C++.

#ifndef FAC_H_GENERATED_
#define FAC_H_GENERATED_
#ifdef __cplusplus
extern "C" {
#endif

...

#ifdef __cplusplus
}
#endif
#endif  /* FAC_H_GENERATED_ */

This WASM_RT_INCLUDED_ section contains a number of definitions required for all WebAssembly modules.

#ifndef WASM_RT_INCLUDED_
#define WASM_RT_INCLUDED_

...

#endif  /* WASM_RT_INCLUDED_ */

First we can specify the maximum call depth before trapping. This defaults to 500:

#ifndef WASM_RT_MAX_CALL_STACK_DEPTH
#define WASM_RT_MAX_CALL_STACK_DEPTH 500
#endif

Next we can specify a module prefix. This is useful if you are using multiple modules that may use the same name as an export. Since we only have one module here, it's fine to use the default which is an empty prefix:

#ifndef WASM_RT_MODULE_PREFIX
#define WASM_RT_MODULE_PREFIX
#endif

#define WASM_RT_PASTE_(x, y) x ## y
#define WASM_RT_PASTE(x, y) WASM_RT_PASTE_(x, y)
#define WASM_RT_ADD_PREFIX(x) WASM_RT_PASTE(WASM_RT_MODULE_PREFIX, x)

Next are some convenient typedefs for integers and floats of fixed sizes:

typedef uint8_t u8;
typedef int8_t s8;
typedef uint16_t u16;
typedef int16_t s16;
typedef uint32_t u32;
typedef int32_t s32;
typedef uint64_t u64;
typedef int64_t s64;
typedef float f32;
typedef double f64;

Next 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_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_anyfunc_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_anyfunc_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.

typedef struct {
  uint32_t func_type;
  wasm_rt_anyfunc_t func;
} 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;

Symbols that must be defined by the embedder

Next in fac.h are a collection of extern symbols 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.

extern void wasm_rt_trap(wasm_rt_trap_t) __attribute__((noreturn));
extern uint32_t wasm_rt_register_func_type(uint32_t params, uint32_t results, ...);
extern void wasm_rt_allocate_memory(wasm_rt_memory_t*, uint32_t initial_pages, uint32_t max_pages);
extern uint32_t wasm_rt_grow_memory(wasm_rt_memory_t*, uint32_t pages);
extern void wasm_rt_allocate_table(wasm_rt_table_t*, uint32_t elements, uint32_t max_elements);
extern uint32_t wasm_rt_call_stack_depth;

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.

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_allocate_table initializes a table instance, and allocates at least enough space for the given number of initial elements. The elements must be cleared to zero.

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.

Exported symbols

Finally, fac.h defines exported symbols provided by the module. In our example, the only function we exported was fac. An additional function is provided called init, which initializes the module and must be called before the module can be used:

extern void WASM_RT_ADD_PREFIX(init)(void);

/* export: 'fac' */
extern u32 (*WASM_RT_ADD_PREFIX(Z_facZ_ii))(u32);

All exported names use WASM_RT_ADD_PREFIX (as described above) to allow the symbols to placed in a namespace as decided by the embedder. All symbols are also mangled so they include the types of the function signature.

In our example, Z_facZ_ii is the mangling for a function named fac that takes one i32 parameter and returns one i32 result.

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, 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 fac(u32 p0) {
  FUNC_PROLOGUE;
  u32 i0, i1, i2;
  i0 = p0;
  i1 = 0u;
  i0 = i0 == i1;
  if (i0) {
    i0 = 1u;
  } else {
    i0 = p0;
    i1 = p0;
    i2 = 1u;
    i1 -= i2;
    i1 = fac(i1);
    i0 *= i1;
  }
  FUNC_EPILOGUE;
  return 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)
  get_local $x
  i32.const 0
  i32.eq
  if (result i32)
    i32.const 1
  else
    get_local $x
    get_local $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
        (get_local 0)
        (i32.const 0))
      (then
        (i32.const 1))
      (else
        (i32.mul
          (get_local 0)
          (call 0
            (i32.sub
              (get_local 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.