Ur C FFI Example

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The Ur C FFI makes it easy to interface with C libraries. Here is a simple example project that uses the FFI. The complete source is available in a tarball.

For a detailed reference, see the manual.


The Library

The following files live in a subdirectory lib of the base directory for our example.


The central piece of an FFI library is a .urs file declaring the interface of your C code.

val hello : unit -> string
(* Returns the string "Hello" *)

val important : string -> string
(* Adds an exclamation mark at the end of its argument *)

val counter : unit -> transaction int
(* Every time this is called, it increments a global counter and returns the old value. *)


We need to restate this interface in a way that GCC can understand.

#include <urweb.h>

uw_Basis_string uw_Lib_hello(uw_context, uw_unit);
uw_Basis_string uw_Lib_important(uw_context, uw_Basis_string);
uw_Basis_int uw_Lib_counter(uw_context, uw_unit);


#include <string.h>
#include <stdio.h>

#include <urweb.h>

uw_Basis_string uw_Lib_hello(uw_context ctx, uw_unit u) {
  return "Hello";

uw_Basis_string uw_Lib_important(uw_context ctx, uw_Basis_string s) {
  uw_Basis_string s2 = uw_malloc(ctx, strlen(s)+2);

  sprintf(s2, "%s!", s);
  return s2;

static int counter;

uw_Basis_int uw_Lib_counter(uw_context ctx, uw_unit u) {
  return counter++;


A project file ties together the whole library.

ffi lib
include lib.h
link lib.o
effectful Lib.counter


CFLAGS := -I/usr/local/include/urweb

all: lib.o

Client Application

Here is one way to use the library we just defined, based out of subdirectory app.

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val main : unit -> transaction page


library ../lib/lib

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