# \#ffi **URL:** https://discuss.ocaml.org/tag/ffi/24.md [Latest](https://discuss.ocaml.org/latest.md) · [Categories](https://discuss.ocaml.org/categories.md) · [Tags](https://discuss.ocaml.org/tags.md) --- ## [Writing C bindings in 2024](https://discuss.ocaml.org/t/writing-c-bindings-in-2024/15368)
**Author:** [@giltho](https://discuss.ocaml.org/u/giltho)\ **Replies:** 13\ **Last updated:** [May 6, 2026, 10:02pm UTC](https://discuss.ocaml.org/t/writing-c-bindings-in-2024/15368 "2026-05-06T22:02:18Z")
Hello, This is just a quick post to ask what the current “standard” way of writing C bindings is. By hand? Using Ctypes? I thought it was Ctypes, but the documentation is rather incomplete (or I could not find a more c… --- ## [OCaml 5 FFI, use of \`Int\_val\` inside \`caml\_enter\_blocking\_section()\`](https://discuss.ocaml.org/t/ocaml-5-ffi-use-of-int-val-inside-caml-enter-blocking-section/17872)
**Author:** [@lindig](https://discuss.ocaml.org/u/lindig)\ **Replies:** 8\ **Last updated:** [March 8, 2026, 3:37pm UTC](https://discuss.ocaml.org/t/ocaml-5-ffi-use-of-int-val-inside-caml-enter-blocking-section/17872 "2026-03-08T15:37:54Z")
Pull requests for the compiler suggest that Int\_val should not be used inside caml\_enter\_blocking\_section(), and in particular not in a parameter position of an FFI call like close\_unix.c- caml\_enter\_blocking\_section()… --- ## [Calling Ocaml C FFI using Ctypes pointers](https://discuss.ocaml.org/t/calling-ocaml-c-ffi-using-ctypes-pointers/17336)
**Author:** [@orbitz](https://discuss.ocaml.org/u/orbitz)\ **Replies:** 6\ **Last updated:** [September 30, 2025, 12:00pm UTC](https://discuss.ocaml.org/t/calling-ocaml-c-ffi-using-ctypes-pointers/17336 "2025-09-30T12:00:55Z")
My question: Is there a well-understood way to mix Ctypes.Foreign data structures with Ocaml C Stubs FFI? I am running into some corruption issues because I think I am doing this translation incorrectly. Background: I… --- ## [Cygwin linking error for ocaml called from C example](https://discuss.ocaml.org/t/cygwin-linking-error-for-ocaml-called-from-c-example/16974)
**Author:** [@frusak](https://discuss.ocaml.org/u/frusak)\ **Replies:** 2\ **Last updated:** [July 15, 2025, 3:11pm UTC](https://discuss.ocaml.org/t/cygwin-linking-error-for-ocaml-called-from-c-example/16974 "2025-07-15T15:11:48Z")
Hello and may your day be nice. TLDR; could not compile (link) example from ocaml manual for embedding ocaml into c program. (no ctypes) My setup: opam with default cygwin configuration and ocaml 5.3 on Windows 10. So… --- ## [Learning Ctypes](https://discuss.ocaml.org/t/learning-ctypes/16889)
**Author:** [@ComanderP](https://discuss.ocaml.org/u/ComanderP)\ **Replies:** 2\ **Last updated:** [June 30, 2025, 1:50pm UTC](https://discuss.ocaml.org/t/learning-ctypes/16889 "2025-06-30T13:50:09Z")
Hi everyone, First of all, sorry if this is a dumb question, this is my 1st time using ctypes. I’m trying to use it to interact with a C struct that contains a char\* field. I wrote a small function to test it out by wri… --- ## [C pointers on OCaml side and checks for null](https://discuss.ocaml.org/t/c-pointers-on-ocaml-side-and-checks-for-null/16078)
**Author:** [@vrotaru](https://discuss.ocaml.org/u/vrotaru)\ **Replies:** 1\ **Last updated:** [February 2, 2025, 6:52pm UTC](https://discuss.ocaml.org/t/c-pointers-on-ocaml-side-and-checks-for-null/16078 "2025-02-02T18:52:15Z")
There seem to be 4 ways of treating C pointers as OCaml values. Wrap them in a block with the Abstract\_tag Wrap them in a block with the Custom\_tag Copy them to a nativeint Mask the last bit so that OCaml GC treat this… --- ## [Confusion with CAMLParam in C bindings](https://discuss.ocaml.org/t/confusion-with-camlparam-in-c-bindings/16064)
**Author:** [@giltho](https://discuss.ocaml.org/u/giltho)\ **Replies:** 15\ **Last updated:** [February 2, 2025, 12:16pm UTC](https://discuss.ocaml.org/t/confusion-with-camlparam-in-c-bindings/16064 "2025-02-02T12:16:02Z")
Hi, I’m a bit confused by the official OCaml documentation regarding C interfaces. Rule 1 of “living in harmony with the GC” is: A function that has parameters or local variables of type value must begin with a call … --- ## [Handling of Ctypes char ptr](https://discuss.ocaml.org/t/handling-of-ctypes-char-ptr/16074)
**Author:** [@mhitza](https://discuss.ocaml.org/u/mhitza)\ **Replies:** 3\ **Last updated:** [February 1, 2025, 5:05pm UTC](https://discuss.ocaml.org/t/handling-of-ctypes-char-ptr/16074 "2025-02-01T17:05:43Z")
I’m trying to write C library bindings, and could use a few pointers and guidance as LLMs I’ve tried hallucinate a lot on this topic. What I’m struggling with at the moment is reading a single character at a time from a… --- ## [Static CStubs for higher-order functions](https://discuss.ocaml.org/t/static-cstubs-for-higher-order-functions/15989)
**Author:** [@giltho](https://discuss.ocaml.org/u/giltho)\ **Replies:** 13\ **Last updated:** [January 30, 2025, 9:57pm UTC](https://discuss.ocaml.org/t/static-cstubs-for-higher-order-functions/15989 "2025-01-30T21:57:34Z")
Hello, I’m trying to write bindings for the clang parser and I’m stuck trying to pass a visitor function created from OCaml. The function stub created receives a static\_funptr and I’d like to be able to pass an ocaml fu… --- ## [Calling ocaml from C has semi-random, silent crashes](https://discuss.ocaml.org/t/calling-ocaml-from-c-has-semi-random-silent-crashes/15986)
**Author:** [@Josef\_Thorne](https://discuss.ocaml.org/u/Josef_Thorne)\ **Replies:** 8\ **Last updated:** [January 20, 2025, 3:46pm UTC](https://discuss.ocaml.org/t/calling-ocaml-from-c-has-semi-random-silent-crashes/15986 "2025-01-20T15:46:51Z")
I’m encountering some strange problems with interfacing C, Ocaml, and Python. The overall goal is to use some parsing functions written in ocaml in Python Django, bridging by way of C. If I startup my django site and … --- ## [Toy Autograd Engine in OCaml with Apple Accelerate Backend](https://discuss.ocaml.org/t/toy-autograd-engine-in-ocaml-with-apple-accelerate-backend/15239)
**Author:** [@jewelltaylor](https://discuss.ocaml.org/u/jewelltaylor)\ **Replies:** 2\ **Last updated:** [September 4, 2024, 11:28am UTC](https://discuss.ocaml.org/t/toy-autograd-engine-in-ocaml-with-apple-accelerate-backend/15239 "2024-09-04T11:28:26Z")
I have been venturing to learn a new language and I landed on OCaml after hearing a few interesting talks from Jane Street. I just made public a toy autograd engine in OCaml with an Apple Accelerate backend if anyone is … --- ## [\[BLOG\] Fighting Mutation with Mutation in Living](https://discuss.ocaml.org/t/blog-fighting-mutation-with-mutation-in-living/15003)
**Author:** [@Fizzixnerd](https://discuss.ocaml.org/u/Fizzixnerd)\ **Replies:** 0\ **Last updated:** [July 21, 2024, 2:43pm UTC](https://discuss.ocaml.org/t/blog-fighting-mutation-with-mutation-in-living/15003 "2024-07-21T14:43:19Z")
Hi there, New blog post about fixing the mistakes in the living library. Please take a look if you’re interested in interfacing ocaml with external resources. In particular, I think the library comes good with its g… --- ## [\[BLOG\] A Tour of the Living Library -- A Safer FFI](https://discuss.ocaml.org/t/blog-a-tour-of-the-living-library-a-safer-ffi/14981)
**Author:** [@Fizzixnerd](https://discuss.ocaml.org/u/Fizzixnerd)\ **Replies:** 1\ **Last updated:** [July 17, 2024, 3:50pm UTC](https://discuss.ocaml.org/t/blog-a-tour-of-the-living-library-a-safer-ffi/14981 "2024-07-17T15:50:43Z")
Hi there, I’ve written a new blog post on the living library I announced a few days ago. Please give it a read if you’re interested in safe use of Ctypes, or otherwise need lifetime management in OCaml. Would love to … --- ## [\[BLOG\] A (Possibly) Safer Interface to the Ctypes FFI](https://discuss.ocaml.org/t/blog-a-possibly-safer-interface-to-the-ctypes-ffi/14954)
**Author:** [@Fizzixnerd](https://discuss.ocaml.org/u/Fizzixnerd)\ **Replies:** 10\ **Last updated:** [July 17, 2024, 3:09pm UTC](https://discuss.ocaml.org/t/blog-a-possibly-safer-interface-to-the-ctypes-ffi/14954 "2024-07-17T15:09:04Z")
Hi there, another blog post. This time I discuss ideas for a new interface that helps localize the possibilities of errors when working with a Ctypes-style FFI. Comment below if you like/hate it please! Matt --- ## [\[ANN\] living 0.1.0](https://discuss.ocaml.org/t/ann-living-0-1-0/14964)
**Author:** [@Fizzixnerd](https://discuss.ocaml.org/u/Fizzixnerd)\ **Replies:** 0\ **Last updated:** [July 13, 2024, 1:48pm UTC](https://discuss.ocaml.org/t/ann-living-0-1-0/14964 "2024-07-13T13:48:42Z")
I’m pleased to announce the first pre-opam version of the living library, currently available only on GitHub for testing. I have some basic tests and a README explaining what it’s for, but basically, it prevents mistake… --- ## [\[BLOG\] OCaml FFI Sharp Edges and How to Avoid Them](https://discuss.ocaml.org/t/blog-ocaml-ffi-sharp-edges-and-how-to-avoid-them/14935)
**Author:** [@Fizzixnerd](https://discuss.ocaml.org/u/Fizzixnerd)\ **Replies:** 7\ **Last updated:** [July 10, 2024, 6:31pm UTC](https://discuss.ocaml.org/t/blog-ocaml-ffi-sharp-edges-and-how-to-avoid-them/14935 "2024-07-10T18:31:28Z")
Wrote another blog post about my adventures in Godotcaml. Check it out if you’re interested in some details of memory management with a Ctypes FFI. Would love to hear input to some of the questions asked in the post, t… --- ## [How to prevent a closure that passed to a C function as a callback from being garbage collected prematurely?](https://discuss.ocaml.org/t/how-to-prevent-a-closure-that-passed-to-a-c-function-as-a-callback-from-being-garbage-collected-prematurely/14901)
**Author:** [@ben131](https://discuss.ocaml.org/u/ben131)\ **Replies:** 8\ **Last updated:** [July 4, 2024, 3:56pm UTC](https://discuss.ocaml.org/t/how-to-prevent-a-closure-that-passed-to-a-c-function-as-a-callback-from-being-garbage-collected-prematurely/14901 "2024-07-04T15:56:23Z")
I need to call an async C function that returns a future type(something like a promise type in Lwt or Eio). There is a future\_set\_callback function on the C side, and it receives a function pointer and will call this fun… --- ## [20+ ways to build an executable with foreign libs](https://discuss.ocaml.org/t/20-ways-to-build-an-executable-with-foreign-libs/14227)
**Author:** [@mobileink](https://discuss.ocaml.org/u/mobileink)\ **Replies:** 0\ **Last updated:** [March 1, 2024, 6:29pm UTC](https://discuss.ocaml.org/t/20-ways-to-build-an-executable-with-foreign-libs/14227 "2024-03-01T18:29:50Z")
Foreign libs can be linked into OCaml executables in a variety of ways. The pertinent info is distributed across several topics in the manual. So I put together a suite of 20-some MWEs demonstrating the options: emitt… --- ## [Packaging foreign archives with bindings using ctypes](https://discuss.ocaml.org/t/packaging-foreign-archives-with-bindings-using-ctypes/13685)
**Author:** [@mgree](https://discuss.ocaml.org/u/mgree)\ **Replies:** 5\ **Last updated:** [December 21, 2023, 2:34pm UTC](https://discuss.ocaml.org/t/packaging-foreign-archives-with-bindings-using-ctypes/13685 "2023-12-21T14:34:50Z")
I’m working on libdash, which is offers OCaml and Python bindings to the dash POSIX shell parser. dune is being uncooperative around packaging the library up properly (i.e., with both bindings and code). My work is in th… --- ## [\[ANN\] ctypes 0.21.1, now with dune support](https://discuss.ocaml.org/t/ann-ctypes-0-21-1-now-with-dune-support/12675)
**Author:** [@yallop](https://discuss.ocaml.org/u/yallop)\ **Replies:** 0\ **Last updated:** [July 24, 2023, 12:39pm UTC](https://discuss.ocaml.org/t/ann-ctypes-0-21-1-now-with-dune-support/12675 "2023-07-24T12:39:49Z")
I’m happy to announce a new release of ctypes, 0.21.1, available for installation via OPAM. ctypes is a library for binding to C libraries using pure OCaml, with the primary aim of make writing C extensions as straightf… --- ## [Using ocaml-ctypes with WebAssembly?](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279)
**Author:** [@tpetr](https://discuss.ocaml.org/u/tpetr)\ **Replies:** 10\ **Last updated:** [June 5, 2023, 2:14pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279 "2023-06-05T14:14:15Z")
Hi there, I’ve been using js\_of\_ocaml to make semgrep run from inside a web browser, and am hitting an interesting issue with one of our dependencies. We use ocaml-yaml for YAML parsing, which uses ocaml-ctypes under th… --- ## [Critique my use of FFI](https://discuss.ocaml.org/t/critique-my-use-of-ffi/12193)
**Author:** [@hyphenrf](https://discuss.ocaml.org/u/hyphenrf)\ **Replies:** 10\ **Last updated:** [May 24, 2023, 10:42pm UTC](https://discuss.ocaml.org/t/critique-my-use-of-ffi/12193 "2023-05-24T22:42:38Z")
I’ve read this post just now and it made me wonder how much better/worse we stand next to Go in terms of FFI calls and finalisers. I’ve ported the provided benchmark code to OCaml and did some measurements. I’ll try my b… --- ## [\[ANN\] carray.0.0.1](https://discuss.ocaml.org/t/ann-carray-0-0-1/9938)
**Author:** [@dannywillems](https://discuss.ocaml.org/u/dannywillems)\ **Replies:** 16\ **Last updated:** [June 16, 2022, 12:10pm UTC](https://discuss.ocaml.org/t/ann-carray-0-0-1/9938 "2022-06-16T12:10:19Z")
Hi everyone, I’m glad to announce the first (experimental) release of ocaml-carray, a library mocking the Array interface to work with contiguous C array. Disclaimer: this first version is experimental and must be used… --- ## [Using thread\_local memory in a C++ thread invoked from Multicore-OCaml](https://discuss.ocaml.org/t/using-thread-local-memory-in-a-c-thread-invoked-from-multicore-ocaml/9552)
**Author:** [@hemendra](https://discuss.ocaml.org/u/hemendra)\ **Replies:** 5\ **Last updated:** [March 30, 2022, 4:37pm UTC](https://discuss.ocaml.org/t/using-thread-local-memory-in-a-c-thread-invoked-from-multicore-ocaml/9552 "2022-03-30T16:37:54Z")
Hi folks, Is there a way to use thread\_local memory in a C++ thread invoked from Multicore-OCaml ? thread.ml external spawnThread\_O : unit -\> unit = "spawnThread\_C" let main () = let t1 = Domain.spawn (fun \_ -\> … --- ## [Determining if the runtime system is currently acquired from C](https://discuss.ocaml.org/t/determining-if-the-runtime-system-is-currently-acquired-from-c/6934)
**Author:** [@zbaylin](https://discuss.ocaml.org/u/zbaylin)\ **Replies:** 0\ **Last updated:** [December 7, 2020, 9:28pm UTC](https://discuss.ocaml.org/t/determining-if-the-runtime-system-is-currently-acquired-from-c/6934 "2020-12-07T21:28:05Z")
Is it possible to determine if the runtime lock is currently acquired from C? I currently have some code which is sometimes called with the lock acquired and sometimes not. If the lock is already acquired, calling caml\_a… --- ## [Writing a wrapper stub to something dynamic, where static linking would produce huge executables?](https://discuss.ocaml.org/t/writing-a-wrapper-stub-to-something-dynamic-where-static-linking-would-produce-huge-executables/6726)
**Author:** [@hyphenrf](https://discuss.ocaml.org/u/hyphenrf)\ **Replies:** 10\ **Last updated:** [November 11, 2020, 5:58pm UTC](https://discuss.ocaml.org/t/writing-a-wrapper-stub-to-something-dynamic-where-static-linking-would-produce-huge-executables/6726 "2020-11-11T17:58:56Z")
What I have: dune opam opencv (headers in /usr/include, and shared objects) a cxx stub main project in ocaml What I want: to build the project with dune The problem: I can’t seem to find in dune docs the way to ge… --- ## [Strange error when calling external c functions](https://discuss.ocaml.org/t/strange-error-when-calling-external-c-functions/5777)
**Author:** [@thautwarm](https://discuss.ocaml.org/u/thautwarm)\ **Replies:** 2\ **Last updated:** [May 16, 2020, 1:29am UTC](https://discuss.ocaml.org/t/strange-error-when-calling-external-c-functions/5777 "2020-05-16T01:29:49Z")
I’m trying to call Python’s C APIs in OCaml efficiently. external pyinit : unit -\> unit = "Py\_Initialize" external errprint : unit -\> unit = "PyErr\_Print" external compile : string -\> string -\> int -\> pyo = "Py\_CompileS… --- ## [Casting a Ctype pointer to an abstract type](https://discuss.ocaml.org/t/casting-a-ctype-pointer-to-an-abstract-type/5491)
**Author:** [@ifazk](https://discuss.ocaml.org/u/ifazk)\ **Replies:** 2\ **Last updated:** [April 13, 2020, 9:53pm UTC](https://discuss.ocaml.org/t/casting-a-ctype-pointer-to-an-abstract-type/5491 "2020-04-13T21:53:10Z")
I have two libraries, one which uses Ctypes and another which does FFI directly and I would like to pass values between them. module Ct = struct type t = unit ptr let foo : unit -\> t = ... end module Fi = struct … --- ## [OPAM/packaging questions around OCaml bindings to custom C library](https://discuss.ocaml.org/t/opam-packaging-questions-around-ocaml-bindings-to-custom-c-library/5120)
**Author:** [@mgree](https://discuss.ocaml.org/u/mgree)\ **Replies:** 15\ **Last updated:** [February 17, 2020, 11:58pm UTC](https://discuss.ocaml.org/t/opam-packaging-questions-around-ocaml-bindings-to-custom-c-library/5120 "2020-02-17T23:58:03Z")
I have an OCaml/C library that I’m trying to build and package on OPAM. I’m completely lost. For reference, the library is libdash, but I’ll try to keep things abstract. The library has three parts: A C library in src … --- ## [OCaml type for custom block](https://discuss.ocaml.org/t/ocaml-type-for-custom-block/3892)
**Author:** [@unlm](https://discuss.ocaml.org/u/unlm)\ **Replies:** 1\ **Last updated:** [June 7, 2019, 7:08am UTC](https://discuss.ocaml.org/t/ocaml-type-for-custom-block/3892 "2019-06-07T07:08:58Z")
What type should I use for custom blocks (created in C code) in OCaml? Is it safe to declare abstract type and then use it for custom blocks? Is it safe to declare extensible variant type with some constructors and… [Next page](https://discuss.ocaml.org/tag/ffi/24.md?match_all_tags=true&page=1&tags%5B%5D=ffi)