Interfacing with C

Section author: Gustavo A. Hoffmann

Ada allows us to interface with code in many languages, including C and C++. This section discusses how to interface with C.

Multi-language project

By default, when using gprbuild we only compile Ada source files. To compile C files as well, we need to modify the project file used by gprbuild. We use the Languages entry, as in the following example:

project Multilang is

   for Languages use ("ada", "c");

   for Source_Dirs use ("src");
   for Main use ("main.adb");
   for Object_Dir use "obj";

end Multilang;

Type convention

To interface with data types declared in a C application, you specify the Convention aspect on the corresponding Ada type declaration. In the following example, we interface with the C_Enum enumeration declared in a C source file:

procedure Show_C_Enum is

   type C_Enum is (A, B, C) with Convention => C;
   --  Use C convention for C_Enum
begin
   null;
end Show_C_Enum;

To interface with C's built-in types, we use the Interfaces.C package, which contains most of the type definitions we need. For example:

with Interfaces.C; use Interfaces.C;

procedure Show_C_Struct is

   type c_struct is record
      a : int;
      b : long;
      c : unsigned;
      d : double;
   end record
     with Convention => C;

begin
   null;
end Show_C_Struct;

Here, we're interfacing with a C struct (C_Struct) and using the corresponding data types in C (int, long, unsigned and double). This is the declaration in C:

struct c_struct
{
    int         a;
    long        b;
    unsigned    c;
    double      d;
};

Foreign subprograms

Calling C subprograms in Ada

We use a similar approach when interfacing with subprograms written in C. In this case, an additional aspect is required: Import. For example:

with Interfaces.C; use Interfaces.C;

procedure Show_C_Func is

   function my_func (a : int) return int
     with
       Import        => True,
       Convention    => C;
   --  Imports function 'my_func' from C.
   --  You can now call it from Ada.

begin
   null;
end Show_C_Func;

This code interfaces with the following declaration in the C header file:

int my_func (int a);

Here's the corresponding C definition:

#include "my_func.h"

int my_func (int a)
{
    return a * 2;
}

If you want, you can use a different subprogram name in the Ada code. For example, we could call the C function Get_Value:

with Interfaces.C; use Interfaces.C;
with Ada.Text_IO;  use Ada.Text_IO;

procedure Show_C_Func is

   function Get_Value (a : int) return int
     with
       Import        => True,
       Convention    => C,
       External_Name => "my_func";

   --  Imports function 'my_func' from C and
   --  rename it to 'Get_Value'

   V : int;
begin
   V := Get_Value (2);
   Put_Line ("Result is " & int'Image (V));
end Show_C_Func;

Calling Ada subprograms in C

You can also call Ada subprograms from C applications. You do this with the Export aspect. For example:

with Interfaces.C; use Interfaces.C;

package C_API is

   function My_Func (a : int) return int
     with
       Export        => True,
       Convention    => C,
       External_Name => "my_func";

end C_API;

This is the corresponding body that implements that function:

package body C_API is

   function My_Func (a : int) return int is
   begin
      return a * 2;
   end My_Func;

end C_API;

On the C side, we do the same as we would if the function were written in C: simply declare it using the extern keyword. For example:

#include <stdio.h>

extern int my_func (int a);

int main (int argc, char **argv) {

  int v = my_func(2);

  printf("Result is %d\n", v);

  return 0;
}

Foreign variables

Using C global variables in Ada

To use global variables from C code, we use the same method as subprograms: we specify the Import and Convention aspects for each variable we want to import.

Let's reuse an example from the previous section. We'll add a global variable (func_cnt) to count the number of times the function (my_func) is called:

/*% filename: test.h */

extern int func_cnt;

int my_func (int a);

The variable is declared in the C file and incremented in my_func:

#include "test.h"

int func_cnt = 0;

int my_func (int a)
{
  func_cnt++;

  return a * 2;
}

In the Ada application, we just reference the foreign variable:

with Interfaces.C; use Interfaces.C;
with Ada.Text_IO;  use Ada.Text_IO;

procedure Show_C_Func is

   function my_func (a : int) return int
     with
       Import        => True,
       Convention    => C;

   V : int;

   func_cnt : int
     with
       Import        => True,
       Convention    => C;
   --  We can access the func_cnt variable from test.c

begin
   V := my_func (1);
   V := my_func (2);
   V := my_func (3);
   Put_Line ("Result is " & int'Image (V));

   Put_Line ("Function was called " & int'Image (func_cnt) & " times");
end Show_C_Func;

As we see by running the application, the value of the counter is the number of times my_func was called.

We can use the External_Name aspect to give a different name for the variable in the Ada application in the same we do for subprograms.

Using Ada variables in C

You can also use variables declared in Ada files in C applications. In the same way as we did for subprograms, you do this with the Export aspect.

Let's reuse a past example and add a counter, as in the previous example, but this time have the counter incremented in Ada code:

with Interfaces.C; use Interfaces.C;

package C_API is

   func_cnt : int := 0
     with
       Export     => True,
       Convention => C;

   function My_Func (a : int) return int
     with
       Export        => True,
       Convention    => C,
       External_Name => "my_func";

end C_API;

The variable is then increment in My_Func:

--% filename: c_api.adb
package body C_API is

   function My_Func (a : int) return int is
   begin
      func_cnt := func_cnt + 1;
      return a * 2;
   end My_Func;

end C_API;

In the C application, we just need to declare the variable and use it:

#include <stdio.h>

extern int my_func (int a);

extern int func_cnt;

int main (int argc, char **argv) {

  int v;

  v = my_func(1);
  v = my_func(2);
  v = my_func(3);

  printf("Result is %d\n", v);

  printf("Function was called %d times\n", func_cnt);

  return 0;
}

Again, by running the application, we see that the value from the counter is the number of times that my_func was called.

Generating bindings

In the examples above, we manually added aspects to our Ada code to correspond to the C source-code we're interfacing with. This is called creating a binding. We can automate this process by using the Ada spec dump compiler option: -fdump-ada-spec. We illustrate this by revisiting our previous example.

This was our C header file:

extern int func_cnt;

int my_func (int a);

To create Ada bindings, we'll call the compiler like this:

gcc -c -fdump-ada-spec -C ./test.h

The result is an Ada spec file called test_h.ads:

pragma Ada_2005;
pragma Style_Checks (Off);

with Interfaces.C; use Interfaces.C;

package test_h is

   func_cnt : aliased int;  -- ./test.h:3
   pragma Import (C, func_cnt, "func_cnt");

   function my_func (arg1 : int) return int;  -- ./test.h:5
   pragma Import (C, my_func, "my_func");

end test_h;

Now we simply refer to this test_h package in our Ada application:

with Interfaces.C; use Interfaces.C;
with Ada.Text_IO;  use Ada.Text_IO;
with test_h;       use test_h;

procedure Show_C_Func is
   V : int;
begin
   V := my_func (1);
   V := my_func (2);
   V := my_func (3);
   Put_Line ("Result is " & int'Image (V));

   Put_Line ("Function was called " & int'Image (func_cnt) & " times");
end Show_C_Func;

You can specify the name of the parent unit for the bindings you're creating as the operand to fdump-ada-spec:

gcc -c -fdump-ada-spec -fada-spec-parent=Ext_C_Code -C ./test.h

This creates the file ext_c_code-test_h.ads:

package Ext_C_Code.test_h is -- automatic generated bindings... end Ext_C_Code.test_h;

Adapting bindings

The compiler does the best it can when creating bindings for a C header file. However, sometimes it has to guess about the translation and the generated bindings don't always match our expectations. For example, this can happen when creating bindings for functions that have pointers as arguments. In this case, the compiler may use System.Address as the type of one or more pointers. Although this approach works fine (as we'll see later), this is usually not how a human would interpret the C header file. The following example illustrates this issue.

Let's start with this C header file:

/*% filename: test.h */

struct test;

struct test * test_create(void);

void test_destroy(struct test *t);

void test_reset(struct test *t);

void test_set_name(struct test *t, char *name);

void test_set_address(struct test *t, char *address);

void test_display(const struct test *t);

And the corresponding C implementation:

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

#include "test.h"

struct test {
  char name[80];
  char address[120];
};

static size_t
strlcpy(char *dst, const char *src, size_t dstsize)
{
  size_t len = strlen(src);
  if (dstsize) {
    size_t bl = (len < dstsize-1 ? len : dstsize-1);
    ((char*)memcpy(dst, src, bl))[bl] = 0;
  }
  return len;
}

struct test * test_create(void)
{
  return malloc (sizeof (struct test));
}

void test_destroy(struct test *t)
{
  if (t != NULL) {
    free(t);
  }
}

void test_reset(struct test *t)
{
  t->name[0]    = '\0';
  t->address[0] = '\0';
}

void test_set_name(struct test *t, char *name)
{
  strlcpy(t->name, name, sizeof(t->name));
}

void test_set_address(struct test *t, char *address)
{
  strlcpy(t->address, address, sizeof(t->address));
}

void test_display(const struct test *t)
{
  printf("Name:    %s\n", t->name);
  printf("Address: %s\n", t->address);
}

Next, we'll create our bindings:

gcc -c -fdump-ada-spec -C ./test.h

This creates the following specification in test_h.ads:

pragma Ada_2005;
pragma Style_Checks (Off);

with Interfaces.C; use Interfaces.C;
with System;
with Interfaces.C.Strings;

package test_h is

   --  skipped empty struct test

   function test_create return System.Address;  -- ./test.h:5
   pragma Import (C, test_create, "test_create");

   procedure test_destroy (arg1 : System.Address);  -- ./test.h:7
   pragma Import (C, test_destroy, "test_destroy");

   procedure test_reset (arg1 : System.Address);  -- ./test.h:9
   pragma Import (C, test_reset, "test_reset");

   procedure test_set_name (arg1 : System.Address; arg2 : Interfaces.C.Strings.chars_ptr);  -- ./test.h:11
   pragma Import (C, test_set_name, "test_set_name");

   procedure test_set_address (arg1 : System.Address; arg2 : Interfaces.C.Strings.chars_ptr);  -- ./test.h:13
   pragma Import (C, test_set_address, "test_set_address");

   procedure test_display (arg1 : System.Address);  -- ./test.h:15
   pragma Import (C, test_display, "test_display");

end test_h;

As we can see, the binding generator completely ignores the declaration struct test and all references to the test struct are replaced by addresses (System.Address). Nevertheless, these bindings are good enough to allow us to create a test application in Ada:

with Interfaces.C;         use Interfaces.C;
with Interfaces.C.Strings; use Interfaces.C.Strings;
with Ada.Text_IO;          use Ada.Text_IO;
with test_h;               use test_h;

with System;

procedure Show_Automatic_C_Struct_Bindings is

   Name    : constant chars_ptr := New_String ("John Doe");
   Address : constant chars_ptr := New_String ("Small Town");

   T : System.Address := test_create;

begin
   test_reset (T);
   test_set_name (T, Name);
   test_set_address (T, Address);

   test_display (T);
   test_destroy (T);
end Show_Automatic_C_Struct_Bindings;

We can successfully bind our C code with Ada using the automatically-generated bindings, but they aren't ideal. Instead, we would prefer Ada bindings that match our (human) interpretation of the C header file. This requires manual analysis of the header file. The good news is that we can use the automatic generated bindings as a starting point and adapt them to our needs. For example, we can:

1. Define a Test type based on System.Address and use it in all relevant functions.
2. Remove the test_ prefix in all operations on the Test type.

This is the resulting specification:

with Interfaces.C; use Interfaces.C;
with System;
with Interfaces.C.Strings;

package adapted_test_h is

   type Test is new System.Address;

   function Create return Test;
   pragma Import (C, Create, "test_create");

   procedure Destroy (T : Test);
   pragma Import (C, Destroy, "test_destroy");

   procedure Reset (T : Test);
   pragma Import (C, Reset, "test_reset");

   procedure Set_Name (T    : Test;
                       Name : Interfaces.C.Strings.chars_ptr);  -- ./test.h:11
   pragma Import (C, Set_Name, "test_set_name");

   procedure Set_Address (T       : Test;
                          Address : Interfaces.C.Strings.chars_ptr);
   pragma Import (C, Set_Address, "test_set_address");

   procedure Display (T : Test);  -- ./test.h:15
   pragma Import (C, Display, "test_display");

end adapted_test_h;

And this is the corresponding Ada body:

with Interfaces.C;         use Interfaces.C;
with Interfaces.C.Strings; use Interfaces.C.Strings;
with Ada.Text_IO;          use  Ada.Text_IO;
with adapted_test_h;       use  adapted_test_h;

with System;

procedure Show_Adapted_C_Struct_Bindings is

   Name    : constant chars_ptr := New_String ("John Doe");
   Address : constant chars_ptr := New_String ("Small Town");

   T : Test := Create;

begin
   Reset (T);
   Set_Name (T, Name);
   Set_Address (T, Address);

   Display (T);
   Destroy (T);
end Show_Adapted_C_Struct_Bindings;

Now we can use the Test type and its operations in a clean, readable way.