module

std › wasm › module

The WebAssembly module container: the header, sections, and their length prefixes. A section starts with its id byte and its length in bytes, which deferred_uleb128 fills in once bitter knows it.

from std.wasm.module import *
from std.wasm.impl import *
from std.bitter.deferred import *

header

# The type section: one function type, () -> i32.
byte(0x01)
deferred_uleb128 span(types_start, types_end), 1
types_start:
byte(0x01)
func_type_0_to_1(I32)
types_end:

00 61 73 6d 01 00 00 00
01 05 01 60 00 01 7f

section_header writes a section’s id and size, and the helpers below write the entries of the type, import, function, memory, export, code and data sections: enough for a WASI program like examples/wasm/hello.basm. The table, global, start and element sections have no helpers yet.

Macros

deferred_uleb128

value, usually span(start, end), as unsigned LEB128 in exactly n bytes, padded if it turns out to need fewer. n has to be enough for the final value: one byte holds up to 127.

byte

One byte, such as a section id.

header

The 8 bytes every module starts with: \0asm, then version 1.

func_type_0_to_1

A function type with no parameters and one result of type result, such as I32.

u32

value, which can’t be negative, as unsigned LEB128: the spec’s u32, which every count and index is.

size

The number of bytes from label start to label end, as unsigned LEB128: the size before a section or a function body. It always takes 5 bytes, since it isn’t known until layout, and 5 hold any size.

section_header

A section’s id, such as TYPE_SECTION, and its size. Put the label start right after it and end after the section’s contents.

params

results

func_type

A function type: func_type params(I32), results(I32).

name

A name: its length in bytes, then its UTF-8 bytes.

import_func

Imports the function item_name from the module module_name, with the type numbered type_index in the type section. Imported functions are numbered before the module’s own.

memory

A memory of pages 64 KiB pages, with no maximum.

export_func

Exports function number index as export_name.

export_memory

Exports memory number index as export_name.

data_segment

A data segment that copies the string source’s UTF-8 bytes into memory 0 at address offset when the module starts.

Types

Leb128Group

struct Leb128Group

One byte of a LEB128 value bitter works out later.

DeferredLeb128

struct DeferredLeb128<const N: int>

N bytes of a LEB128 value bitter works out later.

Some of its fields are generated by @for or @if.

Constants