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    moonbind

    Generate type-safe MoonBit FFI bindings from C headers.

    bindgen
    ffi
    c
    abi
    codegen
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    0.1.0
    License
    Apache-2.0
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    6 hours ago
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    #moonbind

    moonbind generates type-safe MoonBit foreign function interface bindings from C headers — the MoonBit counterpart of bindgen. Point it at sqlite3.h and it decides, from the header alone, what every struct looks like in memory, which type each parameter has on the wire, and what to emit so that the result compiles and links.

    The generated code is meant to be read and checked in, not regenerated at build time: a binding you cannot inspect is a binding you cannot debug.

    moonbind sqlite3.h --out examples/sqlite3 --target x86_64

    The header is the only positional argument. --out is a directory, and the generator writes the whole MoonBit package into it: bindings.mbt, strings.mbt, stub.c and moon.pkg. --target is the ABI the binding is laid out for — x86_64 (the default), aarch64 or wasm32 — because a binding is only correct for the machine whose struct layout it mirrors.

    Status: 0.1.0. Lexer, preprocessor, C11 parser, record layout for three ABIs, code generation and the command line are complete and tested. What is left is a longer list of refusal reasons, not a missing stage; see Roadmap.

    #Why this is not a header-to-text script

    A header is not a list of prototypes. It is a program, and the parts that matter to an FFI generator are the parts a naive scanner gets wrong:

    • Declarators read outwards. int *a[3] is an array of pointers; int (*a)[3] is a pointer to an array. void (*signal(int, void (*)(int)))(int) returns a function pointer that takes a function pointer. moonbind reduces a declarator to an ordered chain of constructors and folds it from the name outwards, so these are never confused.
    • The preprocessor runs first. #if defined(_WIN32), #define SQLITE_VERSION 3045000, recursive macros, #include <stdint.h> — the binding reflects the same translation unit the C compiler would see, with __MOONBIND__ defined so a header can tell it is being analysed.
    • Layout is a property of the target, not the header. long is 64 bits on LP64 and 32 on Windows. struct { char c; int i; } is 8 bytes on x86-64. Bitfields, __attribute__((packed)), _Alignas and #pragma pack all move fields around, and the generator computes the same offsets clang does before it writes a single line.
    • Macros become constants. A header's #defines are half its interface.

    #Design

    The pipeline is one direction, each stage a separate MoonBit package:

    source ──► lex ──► cp ──► cparse ──► layout ──► codegen
  │                  │        │          │          │
files,           tokenize,  parse     ABI       emit
builtin          #define,   C11       offsets   MoonBit
headers          #include  decls     & sizes

    PackageResponsibility
    astThe declaration model: CType, CRecordDecl, CFnDecl, CTypedef, CEnumDecl
    sourceHeader acquisition: disk, embedded standard headers, include resolution
    lexC tokenizer (translation phases 1–3: splicing, comments, punctuators)
    cpPreprocessor (phase 4): directives, macro expansion, conditional inclusion
    cparseC11 declarations: specifiers, declarators, records, enums, bitfields
    layoutRecord layout for SysV x86-64, AArch64 and wasm32
    codegenMoonBit emission: extern declarations, struct mirrors, constants
    cliArgument parsing, target selection, file output

    Layout is checked against the real compiler rather than trusted. --verify prints the generator's layout table for a header in clang's own compact form, and scripts/verify-layout.sh produces the clang table with clang -cc1 -triple … -fdump-record-layouts-complete -fsyntax-only and diffs the two. The script skips cleanly when clang is not installed, so it is a developer check rather than a build dependency.

    moonbind testdata/layout/bitfields.h --verify
# struct F1  0:0-25 a  4:0-6 b [sizeof=8, align=4]
# struct F2  0:0-25 a  3:2-5 b [sizeof=4, align=4]
# struct F3  0:0-3 a  1:0-5 b [sizeof=2, align=1]

    #Declarators as a constructor chain

    The rule that makes C declarations hard to read is also the rule that makes them easy to parse once stated: a declarator is read from the name outwards. moonbind keeps that order explicitly instead of applying types as it scans.

    int *a[3]      chain: [CArr 3, CPtr]   fold: Array(Pointer(int), 3)
int (*a)[3]    chain: [CPtr, CArr 3]   fold: Pointer(Array(int, 3))

    The function a name binds to is always chain[0]; everything further out describes its return type. That single invariant recovers the signal prototype above without special cases.

    #Records are installed late

    struct S { ... } __attribute__((packed)); puts a layout-changing attribute after the body, and typedef struct { ... } Name; gives the struct no tag at all. Bodies are therefore parsed into a pending list and installed only when the specifier list ends, which is where both facts are known.

    #Refusing is a feature

    Some C declarations cannot be bound correctly, and the honest thing is to say so instead of emitting a call that corrupts memory. Every refusal is recorded and printed with --report, so a gap in the binding is a line you can read, not a link error three days later.

    $ moonbind testdata/sqlite/sqlite3.h --out /tmp/sqlite --report
left out: sqlite3_config — variadic, which MoonBit cannot call
left out: sqlite3_mprintf — variadic, which MoonBit cannot call
left out: sqlite3 — the definition is not in this translation unit
left out: fts5_api — no MoonBit representation: xCreateTokenizer: a pointer has no MoonBit slot
left out: sqlite3_vfs — no MoonBit representation: pNext: a pointer has no MoonBit slot

    One refusal is worth spelling out because it is a fact about MoonBit's FFI and not about C: a struct passed or returned by value is always refused, even when its layout matches MoonBit's exactly. MoonBit passes every non-scalar argument by reference, so a C function expecting struct { int x; int y; } in registers instead receives the low words of a pointer to a box and reads garbage. There is no spelling of the call that fixes it from MoonBit's side, so moonbind leaves the function out and names the problem. Structs reached through a pointer are unaffected and are bound normally.

    #Usage

    Three complete scenarios live in examples/, each one runnable with moon run --target native:

    #1. A real library: SQLite

    examples/sqlite3 is testdata/sqlite/sqlite3.h (SQLite is in the public domain) run through moonbind and checked in. examples/sqlite3-demo is a program written against it that creates a table, inserts rows through a prepared statement, and reads them back.

    $ moon run examples/sqlite3-demo --target native
craters by size:
  Tycho — 5 craters
  Copernicus — 3 craters
  Kepler — 2 craters
3 rows

    Nothing in main.mbt mentions a pointer size, an offset or a C type. The sqlite3 * handle, the sqlite3_stmt * resets and the SQLITE_* status codes all came out of the header.

    #2. A header small enough to read

    examples/greet is a forty-line header with a hand-written C implementation, generated in one command:

    moonbind examples/greet/greet.h --out examples/greet --report

    (The generated moon.pkg names only the stub that moonbind itself writes; this example's own greet.c is added to its native-stub list by hand, which is what any project using the bindings would do for the library it is binding.)

    It is the case that shows the awkward parts side by side: an opaque type that is only ever named, a const char * going in as Bytes, a string coming back as a VoidPtr that the generated copy_cstr copies into MoonBit memory, an int that arrives as Int32, and a greet_point_sum that takes a struct by value and is therefore reported and left out:

    $ moonbind examples/greet/greet.h --out examples/greet --report
left out: greet_point_sum — parameter 0 passes greet_point by value, and MoonBit passes records by reference
left out: greet_ctx — the definition is not in this translation unit

    The second line is not a failure: greet_ctx is opaque on purpose, and what the generator is reporting is that it could not mirror the definition — so it emits an opaque handle instead. Run against that header:

    $ moon run examples/greet-demo --target native
library says: greet 1.0
moonbind greets Tycho
greet_hello returned 1
moonbind greets Kepler
greet_hello returned 2
count: 2
null count: -1

    #3. Inspecting a header without binding it

    --verify prints the layout table and exits; nothing is written. This is what you run first when a binding misbehaves, and what the CI job compares against clang.

    $ moonbind testdata/layout/packed.h --verify --target aarch64
struct P1  0:0-25 a  3:2-8 b [sizeof=5, align=1]
struct P2  0:0-25 a  3:2-8 b [sizeof=6, align=2]
union P9  0 c  0:0-19 a [sizeof=4, align=4]

    #Building

    #Install the Mooncakes library

    The published module is icyaaaww/moonbind@0.1.0:

    moon add icyaaaww/moonbind@0.1.0

    Import the stage you need, for example "icyaaaww/moonbind/layout" or "icyaaaww/moonbind/cparse", in your package's moon.pkg. Use the wasm-gc target for the generator and its filesystem-dependent stages. The module's root package has no generator API; the APIs live in the stage packages above.

    For the CLI, clone https://github.com/icyaaaww/moonbind and run:

    moon run cmd/main --target wasm-gc -- --help
moon run cmd/main --target wasm-gc -- testdata/layout/packed.h --verify

    The abbreviated moonbind commands in this README refer to that CLI entry point; adding the library does not install a global moonbind executable.

    #Build from source

    MoonBit 0.1.20261016 or newer.

    moon update
moon check --target wasm-gc
moon build --target wasm-gc
moon test  --target wasm-gc

    The tool itself builds for wasm-gc. The bindings it generates are extern "C" declarations, so their packages declare supported-targets = [ "native" ] and the demos are run with --target native.

    #Roadmap

    #License

    Apache-2.0. See LICENSE.

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