Designing an API with compile-time option to remove first parameter to most functions and use a glob
- by tomlogic
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.)
