Designing an API with compile-time option to remove first parameter to most functions and use a global

Question

I'm trying to design a portable API in ANSI C89/ISO C90 to access a wireless networking device on a serial interface. The library will have multiple network layers, and various versions need to run on embedded devices as small as an 8-bit micro with 32K of code and 2K of data, on up to embedded devices with a megabyte or more of code and data.

In most cases, the target processor will have a single network interface and I'll want to use a single global structure with all state information for that device. I don't want to pass a pointer to that structure through the network layers.

In a few cases (e.g., device with more resources that needs to live on two networks) I will interface to multiple devices, each with their own global state, and will need to pass a pointer to that state (or an index to a state array) through the layers.

I came up with two possible solutions, but neither one is particularly pretty. Keep in mind that the full driver will potentially be 20,000 lines or more, cover multiple files, and contain hundreds of functions.

The first solution requires a macro that discards the first parameter for every function that needs to access the global state:

// network.h
   typedef struct dev_t {
      int var;
      long othervar;
      char name[20];
   } dev_t;

   #ifdef IF_MULTI
      #define foo_function( x, a, b, c)      _foo_function( x, a, b, c)
      #define bar_function( x)               _bar_function( x)
   #else
      extern dev_t DEV;
      #define IFACE (&DEV)
      #define foo_function( x, a, b, c)      _foo_function( a, b, c)
      #define bar_function( x)               _bar_function( )
   #endif

   int bar_function( dev_t *IFACE);
   int foo_function( dev_t *IFACE, int a, long b, char *c);

// network.c
       #ifndef IF_MULTI
          dev_t DEV;
       #endif
   int bar_function( dev_t *IFACE)
   {
      memset( IFACE, 0, sizeof *IFACE);

      return 0;
   }

   int foo_function( dev_t *IFACE, int a, long b, char *c)
   {
      bar_function( IFACE);
      IFACE->var = a;
      IFACE->othervar = b;
      strcpy( IFACE->name, c);

      return 0;
   }

The second solution defines macros to use in the function declarations:

// network.h
   typedef struct dev_t {
      int var;
      long othervar;
      char name[20];
   } dev_t;

   #ifdef IF_MULTI
      #define DEV_PARAM_ONLY        dev_t *IFACE
      #define DEV_PARAM             DEV_PARAM_ONLY,
   #else
      extern dev_t DEV;
      #define IFACE (&DEV)
      #define DEV_PARAM_ONLY        void
      #define DEV_PARAM
   #endif

   int bar_function( DEV_PARAM_ONLY);
   // I don't like the missing comma between DEV_PARAM and arg2...
   int foo_function( DEV_PARAM int a, long b, char *c);

// network.c
       #ifndef IF_MULTI
          dev_t DEV;
       #endif
   int bar_function( DEV_PARAM_ONLY)
   {
      memset( IFACE, 0, sizeof *IFACE);

      return 0;
   }

   int foo_function( DEV_PARAM int a, long b, char *c)
   {
      bar_function( IFACE);
      IFACE->var = a;
      IFACE->othervar = b;
      strcpy( IFACE->name, c);

      return 0;
   }

The C code to access either method remains the same:

// multi.c - example of multiple interfaces
   #define IF_MULTI
   #include "network.h"
   dev_t if0, if1;

   int main()
   {
      foo_function( &if0, -1, 3.1415926, "public");
      foo_function( &if1, 42, 3.1415926, "private");

      return 0;
   }

// single.c - example of a single interface
   #include "network.h"
   int main()
   {
      foo_function( 11, 1.0, "network");

      return 0;
   }

Is there a cleaner method that I haven't figured out? I lean toward the second since it should be easier to maintain, and it's clearer that there's some macro magic in the parameters to the function. Also, the first method requires prefixing the function names with "_" when I want to use them as function pointers.

I really do want to remove the parameter in the "single interface" case to eliminate unnecessary code to push the parameter onto the stack, and to allow the function to access the first "real" parameter in a register instead of loading it from the stack. And, if at all possible, I don't want to have to maintain two separate codebases.

Thoughts? Ideas? Examples of something similar in existing code?

(Note that using C++ isn't an option, since some of the planned targets don't have a C++ compiler available.)

Answer 1

I like your second solution. I just prefer declaring every function twice rather than have that PARAM macro in the public header. I much prefer to put macro hijinks in the hidden C file.

// common header
#ifdef IF_MULTI
    int foo_func1(dev_t* if, int a);
    int foo_func2(dev_t* if, int a, int b);
    int foo_func3(dev_t* if);
#else
    int foo_func1(int a);
    int foo_func2(int a, int b);
    int foo_func3();
#endif

// your C file
#ifdef IF_MULTI
    #define IF_PARM dev_t* if,
    #define GET_IF() (if)
#else
    dev_t global_if;
    #define IF_PARM
    #define GET_IF() (&global_if)
#endif

int foo_func1(IF_PARM int a)
{
    GET_IF()->x = a;
    return GET_IF()->status;
}
int foo_func2(IF_PARM int a, int b)
int foo_func3(IF_PARM);

Answer 2

Here's a solution that won't work if you have threads (or switch interfaces on re-entrance or something like that), but it is a clean interface, and it might work for you.

You could have your single instance functions using a global DEV, and have your multi interface functions set this global and call their single instance counterparts.

For example:

dev_t *DEV;

int foo_function(int x, int y)
{
    /* DEV->whatever; */
    return DEV->status;
}

int foo_function_multi(dev_t *IFACE, int x, int y)
{
    DEV = IFACE;
    return foo_function(x, y);
}

Another option is to use variadic args, and pass and fetch an extra arg (which contains the interface to use) #ifdef MULTI, but that's horrible because you lose your type safety, and would prevent passing the arg in a register which you possibly care quite a bit about on your platform. Also, all functions with variadic args must have at least one named argument, and your question is all about avoiding arguments! But anyway:

#ifndef MULTI
dev_t *DEV;
#endif
int foo(int x, int y, ...)
{
#ifdef MULTI
    va_list args;
    va_start(args, y);
    dev_t *DEV = va_arg(args, (dev_t*));
    va_end(args);
#endif
    /* DEV->whatever */
    return DEV->status;
}

// call from single
int quux()
{
    int status = foo(23, 17);
}

// call from multi
int quux()
{
    int status = foo(23, 17, &if0);
}

Personally I prefer your first solution :-)