How do I write a dispatcher, if my compiler's support for pointers-to-functions is broken?

Question

I am working on an embedded application where the device is controlled through a command interface. I mocked the command dispatcher in VC and had it working to my satisfaction; but when I then moved the code over to the embedded environment, I found out that the compiler has a broken implementation of pointer-to-func's.

Here's how I originally implemented the code (in VC):

/* Relevant parts of header file  */
typedef struct command {
  const char *code;
  void *set_dispatcher;
  void *get_dispatcher;
  const char *_description;
} command_t;

#define COMMAND_ENTRY(label,dispatcher,description) {(const char*)label, &set_##dispatcher, &get_##dispatcher, (const char*)description} 


/* Dispatcher data structure in the C file */
const command_t commands[] = {
  COMMAND_ENTRY("DH", Dhcp, "DHCP (0=off, 1=on)"),
  COMMAND_ENTRY("IP", Ip, "IP Address (192.168.1.205)"),
  COMMAND_ENTRY("SM", Subnet, "Subunet Mask (255.255.255.0)"),
  COMMAND_ENTRY("DR", DefaultRoute, "Default router (192.168.1.1)"),
  COMMAND_ENTRY("UN", Username, "Web username"),
  COMMAND_ENTRY("PW", Password, "Web password"),
  ...
}


/* After matching the received command string to the command "label", the command is dispatched */
if (pc->isGetter)
  return ((get_fn_t)(commands[i].get_dispatcher))(pc);
else
  return ((set_fn_t)(commands[i].set_dispatcher))(pc);
  }

Without the use of function pointers, it seems like my only hope is to use switch()/case statements to call functions. But I'd like to avoid having to manually maintain a large switch() statement.

What I was thinking of doing is moving all the COMMAND_ENTRY lines into a separate include file. Then wraps that include file with varying #define and #undefines. Something like:

/* Create enum's labels */
#define COMMAND_ENTRY(label,dispatcher,description) SET_##dispatcher, GET_##dispatcher
typedef enum command_labels = {
#include "entries.cinc"
  DUMMY_ENUM_ENTRY} command_labels_t;
#undefine COMMAND_ENTRY


/* Create command mapping table */
#define COMMAND_ENTRY(label,dispatcher,description) {(const char*)label, SET_##dispatcher, GET_##dispatcher, (const char*)description} 
const command_t commands[] = {
#include "entries.cinc"
  NULL /* dummy */ };
#undefine COMMAND_ENTRY

/*...*/

int command_dispatcher(command_labels_t dispatcher_id) {
/* Create dispatcher switch statement */
#define COMMAND_ENTRY(label,dispatcher,description) case SET_##dispatcher: return set_##dispatcher(pc); case GET_##dispatcher: return get_##dispatcher(pc);
switch(dispatcher_id) {
#include "entries.cinc"
default:
  return NOT_FOUND;
}
#undefine COMMAND_ENTRY
}

Does anyone see a better way to handle this situation? Sadly, 'get another compiler' is not a viable option. :(

--- Edit to add: Just to clarify, the particular embedded environment is broken in that the compiler is supposed to create a "function-pointer table" which is then used by the compiler to resolve calls to functions through a pointer. Unfortunately, the compiler is broken and doesn't generate a correct function-table.

So I don't have an easy way to extract the func address to invoke it.

--- Edit #2: Ah, yes, the use of void *(set|get)_dispatcher was my attempt to see if the problem was with the typedefine of the func pointers. Originally, I had

typedef int (*set_fn_t)(cmdContext_t *pCmdCtx);
typedef int (*get_fn_t)(cmdContext_t *pCmdCtx);

typedef struct command {
  const char *code;
  set_fn_t set_dispatcher;
  get_fn_t get_dispatcher;
  const char *_description;
} command_t;

Answer 1

try this syntax:

return (*((get_fn_t)commands[i].get_dispatcher))(pc);

It's been awhile since I've done C & function pointers, but I believe the original C syntax required the * when dereferencing function pointers but most compilers would let you get away without it.

Answer 2

To what extent is the pointer-to-function broken?

If the compiler allows you to get the address of a function (I'm from C++, but &getenv is what I mean), you could wrap the calling convention stuff into assembler.

As said, I'm a C++ssie, but something in the way of

; function call
push [arg1]
push [arg2]
call [command+8] ; at the 4th location, the setter is stored
ret

If even that is broken, you could define an array of extern void* pointers which you define, again, in assembly.

Answer 3

You should try changing your struct command so the function pointers have the actual type:

typedef struct command {
  const char *code;
  set_fn_t set_dispatcher;
  get_fn_t get_dispatcher;
  const char *_description;
} command_t;

Unfortunately, function pointers are not guaranteed to be able to convert to/from void pointers (that applies only to pointers to objects).

What's the embedded environment?

---

Given the information posted in the updates to the question, I see that it's really a bugged compiler.

I think that your proposed solution seems pretty reasonable - it's probably similar to what I would have come up with.

Answer 4

A function pointer isn't actually required to fit in a void*. You could check to make sure that the value you're calling is actually the address of the function. If not, use a function pointer type in the struct: either get_fn_t, or IIRC void(*)(void) is guaranteed to be compatible with any function pointer type.

Edit: OK, assuming that calling by value can't be made to work, I can't think of a neater way to do what you need than auto-generating the switch statement. You could maybe use an off-the-shelf ASP-style preprocessor mode for ruby/python/perl/php/whatever prior to the C preprocessor. Something like this:

switch(dispatcher_id) {
<% for c in commands %>
    case SET_<% c.dispatcher %>: return set_<% c.dispatcher %>(pc); 
    case GET_<% c.dispatcher %>: return get_<% c.dispatcher %>(pc);
<% end %>
default:
    return NOT_FOUND;
}

might be a bit more readable than the macro/include trick, but introducing a new tool and setting up the makefiles is probably not worth it for such a small amount of code. And the line numbers in the debug info won't relate to the file you think of as the source file unless you do extra work in your preprocessor to specify them.

Answer 5

Can you get the vendor to fix the compiler?

Answer 6

Do you have access to the link map? If so, maybe you can hack your way around the wonky function-pointer table:

unsigned long addr_get_dhcp = 0x1111111;
unsigned long addr_set_dhcp = 0x2222222; //make these unique numbers.

/* Relevant parts of header file  */
typedef struct command {
  const char *code;
  unsigned long set_dispatcher;
  unsigned long get_dispatcher;
  const char *_description;
} command_t;

#define COMMAND_ENTRY(label,dispatcher,description) {(const char*)label, 
    addr_set_##dispatcher, addr_get_##dispatcher, (const char*)description}

Now compile, grab the relevant addresses from the link map, replace the constants, and recompile. Nothing should move, so the map ought to stay the same. (Making the original constants unique should prevent the compiler from collapsing identical values into one storage location. You may need a long long, depending on the architecture)

If the concept works, you could probably add a post-link step running a script to do the replacement automagically. Of course, this is just a theory, it may fail miserably.

Answer 7

Maybe, you need to look into the structure again:

typedef struct command {
  const char *code;
  void *set_dispatcher; //IMO, it does not look like a function pointer...
  void *get_dispatcher; //more like a pointer to void
  const char *_description;
} command_t;

Let say your dispatchers have the following similar function definition:

//a function pointer type definition
typedef int (*genericDispatcher)(int data);

Assume that the dispatchers are like below:

int set_DhcpDispatcher(int data) { return data; }
int get_DhcpDispatcher(int data) { return 2*data; }

So, the revised structure will be:

typedef struct command {
  const char *code;
  genericDispatcher set_dispatcher; 
  genericDispatcher get_dispatcher; 
  const char *_description;
} command_t;

Your macro will be:

#define COMMAND_ENTRY(label,dispatcher,description) \
{   (const char*)label, \
    set_##dispatcher##Dispatcher, \
    get_##dispatcher##Dispatcher, \
    (const char*)description }

Then, you can set your array as usual:

int main(int argc, char **argv)
{
    int value1 = 0, value2 = 0;

    const command_t commands[] = {
      COMMAND_ENTRY("DH", Dhcp, "DHCP (0=off, 1=on)")
    };

    value1 = commands[0].set_dispatcher(1);
    value2 = commands[0].get_dispatcher(2);

    printf("value1 = %d, value2 = %d", value1, value2);

    return 0;
}

Correct me, if I am wrong somewhere... ;)