I am trying to simulate generics in C by having some preprocessor definitions for a matrix type. Here is an excerpt of that:
#define __matrix_struct(TYPE) \
struct { \
uint32_t sz; \
TYPE **ptr; \
}
#define __matrix_t(TYPE) matrix_ ## TYPE
#define __matrix_ptr_t(TYPE) __matrix_t(TYPE) *
#define __matrix_typedef(TYPE) typedef __matrix_struct(TYPE) __matrix_t(TYPE)
#define __matrix_allocator_name(TYPE) TYPE ## _matrix_alloc
#define __matrix_allocator(TYPE) \
__matrix_ptr_t(TYPE) __matrix_allocator_name(TYPE) (uint32_t sz) { \
uint32_t i; \
__matrix_ptr_t(TYPE) m = (__matrix_ptr_t(TYPE)) malloc(sizeof(__matrix_t(TYPE))); \
m->ptr = (TYPE **) malloc(sz * sizeof(TYPE *)); \
for (i = 0; i < sz; ++i) { \
m->ptr[i] = (TYPE *) calloc(sz, sizeof(TYPE)); \
} \
return m; \
}
#define __matrix_deallocator_name(TYPE) TYPE ## _matrix_free
#define __matrix_deallocator(TYPE) \
void __matrix_deallocator_name(TYPE) (__matrix_ptr_t(TYPE) m) { \
uint32_t i; \
for (i = 0; i < m->sz; i++) { \
free(m->ptr[i]); \
} \
free(m->ptr); \
free(m); \
}
#define matrix_alloc_ptr(TYPE, SIZE) __matrix_allocator_name(TYPE) (SIZE)
#define matrix_dealloc_ptr(TYPE, PTR_NAME) __matrix_deallocator_name(TYPE) (PTR_NAME)
In another file, byte_matrix.h, I am trying to define a matrix of uint8_t values, as follows:
#include "matrix.h"
typedef uint8_t byte;
__matrix_typedef(byte);
__matrix_allocator(byte)
__matrix_deallocator(byte)
When I try to compile, I get the following errors:
CMakeFiles/tictac.dir/game/board.c.o: In function `byte_matrix_alloc':
/home/victor/dev/pc/tictac/game/../matrix/byte_matrix.h:13: multiple definition of `byte_matrix_alloc'
CMakeFiles/tictac.dir/main.c.o:/home/victor/dev/pc/tictac/game/../matrix/byte_matrix.h:13: first defined here
CMakeFiles/tictac.dir/game/board.c.o: In function `byte_matrix_free':
/home/victor/dev/pc/tictac/game/../matrix/byte_matrix.h:14: multiple definition of `byte_matrix_free'
CMakeFiles/tictac.dir/main.c.o:/home/victor/dev/pc/tictac/game/../matrix/byte_matrix.h:14: first defined here
I cannot understand why it would point to times to the same line and complain about that definition, since every header I wrote has include guards. Could you please explain this to me? Also if you know of a better approach to my problem, please let me know. Thanks.
Also I need to compile with -std=c99 if that matters in this case.
A quick fix would be to add static to your function definitions. This will create a static copy of these functions in each compilation unit which references the header. If you want the functions to be inlined every time, this is the way to go.
An alternative way to do it would be to keep function declarations in a .h file, and actual definitions in a single .c file. This approach will avoid duplication, and the compiler will not inline them (unless your linker supports link time optimization).
The reason is that you are including this header file in multiple compilation units. After the preprocessor does all the textual replacements, you end up with actual separate function definitions inside your .c files. And if you don't specify that you want them to be static, they are by default extern, which means that now the compiler doesn't know how to differentiate them if some other part of the code wants to call them.
This is what you basically do whenever you create a header file: you create a list of declarations which will be included in many compilation units, but there is always a single extern definition in a single .c file.
Another way (relative to the proposed by Groo) is to create two macros.
__matrix_allocator_declare with just prototype of function -- for h-file(s)
__matrix_allocator_define with function body -- for one (selected by you) c-file
This way requires to handle two macros and to not forget add function-body macro in some file, but (and it is more important for embedded applications on small microcontrollers) it guarantees that only one function instance will consume memory.
Related
I am unsure about where to write the declaration and the call of a macro that replaces the code with a function. I do not really know if I should write the macro to the .h or .c file.
Before reading some stuff on the best ways to create libraries, I was just putting all the code in a header file and including it on my main, i.e.
#ifndef LIB
#define LIB
#define def_func(type) \
type func(type x) \
{ \
// Do something
}
func(int)
#endif
Some other functions use these defined functions so I had to call the macro to the .h file.
Firstly, I think that a few small edits are needed in the code from the question:
#ifndef LIB
#define LIB
#define def_func(type) \
type func(type x) \
{ \
/* Do something */ \
}
def_func(int)
#endif
The question does not detail what objective is being achieved, but I would assume that the goal is to create something that behaves like a template in C++, where the code is defined in one place and instances are created for different types by using the macro def_func.
One thing to be aware of is if you are using def_func more than once in your project, then you are going to have linker errors due to the same global symbol being used in multiple places, even if def_func is used in separate files. This could be avoided by making the function static if def_func is used multiple times but never more than once in the same file. Although, this would restrict the function from being called from multiple files.
Global symbol redefinition could also be avoided by adding another argument to the #define as follows:
#define def_func(func, type) \
type func(type x) \
{ \
/* Do something */ \
}
This would allow the function identifier to be specified uniquely. For example:
def_func(func_int, int)
would expand to:
int func_int(int x)
{
/* Do something */
}
This way a unique identifier would be created for each instance.
The #define should be placed in the header file if you intend to use the macro from multiple C source files. The macro should be used within C source files only, since using this in the header file would instantiate an object with the same global symbol in each case the header file is included. Although, this would be allowable if the functions are defined as static.
Lastly, if you plan to call the function created from multiple locations, you will need a macro that can be used in a header file associated with the module defining the function to prototype the function. For example:
#define def_func_proto(func, type) \
type func(type x)
So to sum it up, your library .h file would contain:
#define def_func(func, type) \
type func(type x) \
{ \
/* Do something */ \
}
#define def_func_proto(func, type) \
type func(type x)
Then using the integer case as an example, the C source file may include:
def_func(func_int, int)
Which would expand to (note that the actual expansion will not have line breaks):
int func_int(int x)
{
/* Do something */
}
In this case, the header file may contain:
def_func_proto(func_int, int);
Which would expand to:
int func_int(int x);
Finally, I would note that am not certain that this is a good programming practice, in general. You will want to be very cautious in implementing this in your program.
I am using X macros to generate functions setting GPIOs to 0 or 1 (I generate around 60 functions to set around 30 GPIOs). Here is an example (I have just written this example, so the syntax may be wrong):
/* X(pin_name, pin_nb) */
#define CPLD_GPIOs \
X(Pin0, 0) \
X(Pin1, 1) \
X(Pin2, 2) \
X(Pin3, 3)
I generate the functions to access to these GPIOs:
#define X(pin_name, pin_nb) \
static void SetOn_GPIO##pin_name (void) { \
SetOn_GPIOpins(pin_nb);\
}
CPLD_GPIOs
#undef X
The same process exists for SetOff_GPIOXXX functions.
Is there a way I can access the function generated above by the compiler as SetOn_GPIOPin2 in an other part of the program without directly writing the function name? (In order to keep the code as global as possible)
At the end of preprocessing, we should only have SetOn_GPIOPin2(); (and not every X-macro entries) generated from X-Macro.
Before pre-processing:
void foo ()
{
/* some code */
/*
* Macro to generate the desired function.
* For e.g: SetOn_GPIOPin2();
*/
/* some code */
}
After pre-processing:
void foo ()
{
/* some code */
/* Function resulting of the pre-processing */
SetOn_GPIOPin2();
/* some code */
}
From comments on the question, your objective appears to be to protect against the case in which your X macro is modified to produce differently-named functions. If that's so, then I think you're making unnecessary work for yourself: whether those names are changed is under your (and any other project developers') control, and a name change such as you are concerned about will not go unnoticed very long. So don't change them.
But if you're determined to go this route then no, there is no way to make the preprocessor extract the function names generated by your macros from their larger replacement text. Instead, you would need to inject them, via the same macro, into both the X macros and your other code. Like this, perhaps:
/* X(pin_name, pin_nb) */
#define CPLD_GPIOs(gen) \
X(Pin0, 0, gen) \
X(Pin1, 1, gen) \
X(Pin2, 2, gen) \
X(Pin3, 3, gen)
// Generates the wanted function names:
#define GPIO_ON(pin_name, pin_nb) SetOn_GPIO##pin_name
#define X(pin_name, pin_nb, gen) \
static void gen(pin_name, pin_nb) (void) { \
SetOn_GPIOpins(pin_nb);\
}
CPLD_GPIOs
#undef X
// ...
void some_function(void) {
GPIO_ON(pin_name, pin_nb)();
}
But note well that although this technique might have other applications, such as to generating multiple sets of functions with the same set of X macros, it just kicks the can down the road with respect to the specific objective you described. You can rely on the name-generator macro to produce the same names for function declarations and function calls, but you still have the problem that the X macro can be modified to generate function declarations with different names.
Using our trusty Boost.Preprocessor rocket launcher, this is preatty easy:
#include <boost/preprocessor/tuple/rem.hpp>
#include <boost/preprocessor/control/if.hpp>
#include <boost/preprocessor/comparison/equal.hpp>
#define X_SELECT_PIN(pin_nb, selected_pin_nb, ...) \
BOOST_PP_TUPLE_REM_CTOR(BOOST_PP_IF( \
BOOST_PP_EQUAL(pin_nb, selected_pin_nb), \
(__VA_ARGS__), \
() \
))
// Usage
#define X(pin_name, pin_nb)\
X_SELECT_PIN(pin_nb, 2, SetOn_GPIO##pin_name (void);)
CPLD_GPIOs
#undef X
This uses BOOST_PP_IF to expand your pattern only for the selected pin. The added parentheses and BOOST_PP_TUPLE_REM_CTOR are there to protect the macro from expansions containing commas.
I am trying to write some reusable generic type-safe code in C, using macros, similar to how klib works:
#define Fifo_define(TYPE) \
\
typedef struct { \
TYPE *head; \
TYPE *tail; \
size_t capacity; \
} Fifo_##TYPE, *pFifo_##TYPE; \
\
inline Fifo_##TYPE * Fifo_##TYPE##_init(size_t capacity) { \
Fifo_##TYPE * fifo = calloc(1, sizeof(Fifo_##TYPE)); \
TYPE * data = calloc(capacity, sizeof(TYPE)); \
fifo->head = data; \
fifo->tail = data; \
fifo->capacity = capacity; \
}
// define macros
#define Fifo(TYPE) Fifo_##TYPE
#define Fifo_init(TYPE, capacity) Fifo_##TYPE_init(capacity)
And then I just use it with any type parameter:
Fifo_define(int32_t);
...
Fifo(int32_t) *myFifo = Fifo_init(int32_t, 100);
However, writing this is rather convoluted and error prone, with no IDE editor support (IntelliSense), so I wondered if there are any tricks which might allow me to (perhaps) add a few defines and then include the file, without having to end each line with \?
Something like:
// no idea how to do this, just checking if similar concept is possible
#define FIFO_TYPE int
#define FIFO_NAME Fifo_int
#include <generic-fifo.h>
#undef FIFO_NAME
#undef FIFO_TYPE
And I would somehow get all the right structs and functions. The problem is that there is a lot of parameter concatenation in these macros, so I am not sure if this can be done in a simpler manner than the first snippet?
Not really recommended in this case, but you can do something like what you want to achieve with X-macros:
#define SUPPORTED_TYPES \
X(int) \
X(double) \
X(char)
#define X(TYPE) \
typedef struct { \
TYPE *head; \
TYPE *tail; \
size_t capacity; \
} Fifo_##TYPE, *pFifo_##TYPE;
SUPPORTED_TYPES
#undef X
#define X(TYPE) \
inline Fifo_##TYPE * Fifo_##TYPE##_init(size_t capacity) \
{ \
Fifo_##TYPE * fifo = calloc(1, sizeof(Fifo_##TYPE)); \
TYPE * data = calloc(capacity, sizeof(TYPE)); \
fifo->head = data; \
fifo->tail = data; \
fifo->capacity = capacity; \
}
SUPPORTED_TYPES
#undef X
But this didn't really improve the situation all that much. It got rid of the need for a single, ugly Fifo_define macro, so you can split up the code in several sections. But the macro mess remains.
I would recommend some completely different approach. Two suggestions:
Handle the type-generic things in the classic C way, in run-time. Use callbacks. Keep track of the used type with an enum, if needed.
C11 _Generic allows all kinds of type safety tricks and can be used to phase out such messy macros. Example that implements "functors". The macro itself is kept minimal and the different implementations for various types is typed out. (That's usually what you end up doing anyway, when you do type-generic programming.)
If you are using complex macros, consider using m4 instead of the C pre-processor. m4 is similar to the C pre-processor but is much more powerful and can do things like have multiple lines without a line continuation character.
Using code generators like m4 is called meta-programming.
Using m4 in C can be accomplished by treating it as a pre-pre-processor like this:
% grep -v '#include' file1 file2 | m4 > outfile
% m4 file1 file2 | cc
Since m4 works in a similar way to the C pre-processor at the basic level, it will generally convert any ordinary C macros correctly in addition to supporting its own advanced features.
Please,
could you tell me what does the code below do?
...code...
#include file.h
...code...
I was used to put includes a the beggining of each file. I have never seen this before and also wasn't able to find anything in the internet.
#include is a pre-processor directive that takes the file given as the argument and dumps its contents in the current file. Typically, this is used to include definitions of commons functions from header files, but there's no necessity to use it in that way.
It's usage is not limited to the starting of the file, but note that the variables, macros or functions declared in this header will not be usable before the include statement even if on the same file.
As everyone tells you #include can be used every where (provided it is on its own logicial line). And there are cases where you want to #include several times the same file. Read first the X macro wikipage, and the C preprocessor wikipage.
And I have a concrete example in my MELT monitor (related to MELT ...).
I have a predef-monimelt.h (generated) file containing lines like
MOM_PREDEFINED_NAMED( name, id,hash) e.g.
MOM_PREDEFINED_NAMED(GET,_9dsak0qcy0v_1c5z9th7x3i,1573018885)
MOM_PREDEFINED_NAMED(HEAD,_47fatww79x6_vh8ap22c0ch,3922245622)
MOM_PREDEFINED_NAMED(web_handler,_7sav6zery1v_24sa6jwwu6c,2339220870)
#undef MOM_PREDEFINED_NAMED
My monimelt.h file (a real header file) define external pointers and an enum, so has notably:
// declare the predefined
#define MOM_PREDEFINED_NAMED(Name,Id,H) extern momitem_t* mom_named__##Name;
#include "predef-monimelt.h"
/// declare the hash of the predefined as an enum
#define MOM_PREDEFINED_NAMED(Name,Id,H) mom_hashname__##Name = H,
enum {
#include "predef-monimelt.h"
};
My main.c file contains notably a routine :
// if this routine is compiled, we are sure that all predefined hashes
// are unique
const momitem_t *
mom_predefined_item_of_hashcode (momhash_t h) {
switch (h) {
#define MOM_PREDEFINED_NAMED(Nam,Id,Hash) case Hash: return mom_named__##Nam;
#include "predef-monimelt.h"
default:
return NULL;
}
}
but my items.c includes the predef-monimelt.h file twice (to create the predefined items at initialization, and to define their variables):
void mom_create_predefined_items (void) {
int nbnamed = 0;
#define MOM_PREDEFINED_NAMED(Nam,Id,H) do { \
mom_named__##Nam = mom_make_item_of_identcstr(#Id); \
mom_named__##Nam->i_space = momspa_predefined; \
mom_register_item_named_cstr (mom_named__##Nam, #Nam); \
nbnamed ++; \
} while(0);
#include "predef-monimelt.h"
} // end of mom_create_predefined_items
// declare the predefined
#define MOM_PREDEFINED_NAMED(Nam,Id,H) momitem_t* mom_named__##Nam;
#include "predef-monimelt.h"
FWIW, the MELT monitor is GPLv3+ licensed software
I have a C program in which I need to create a whole family of functions which have the same signatures and bodies, and differ only in their types. What I would like to do is define a macro which generates all of those functions for me, as otherwise I will spend a long time copying and modifying the original functions. As an example, one of the functions I need to generate looks like this:
int copy_key__sint_(void *key, void **args, int argc, void **out {
if ((*out = malloc(sizeof(int))) {
return 1;
}
**((_int_ **) out) = *((_int_ *) key);
return 0;
}
The idea is that I could call a macro, GENERATE_FUNCTIONS("int", "sint") or something like this, and have it generate this function. The italicized parts are what need to be plugged in.
Is this possible?
I don't understand the example function that you are giving very well, but using macros for the task is relatively easy. Just you wouldn't give strings to the macro as arguments but tokens:
#define DECLARE_MY_COPY_FUNCTION(TYPE, SUFFIX) \
int copy_function_ ## SUFFIX(unsigned count, TYPE* arg)
#define DEFINE_MY_COPY_FUNCTION(TYPE, SUFFIX) \
int copy_function_ ## SUFFIX(unsigned count, TYPE* arg) { \
/* do something with TYPE */ \
return whatever; \
}
You may then use this to declare the functions in a header file
DECLARE_MY_COPY_FUNCTION(unsigned, toto);
DECLARE_MY_COPY_FUNCTION(double, hui);
and define them in a .c file:
DEFINE_MY_COPY_FUNCTION(unsigned, toto);
DEFINE_MY_COPY_FUNCTION(double, hui);
In this version as stated here you might get warnings on superfluous `;'. But you can get rid of them by adding dummy declarations in the macros like this
#define DEFINE_MY_COPY_FUNCTION(TYPE, SUFFIX) \
int copy_function_ ## SUFFIX(unsigned count, TYPE* arg) { \
/* do something with TYPE */ \
return whatever; \
} \
enum { dummy_enum_for_copy_function_ ## SUFFIX }
Try something like this (I just tested the compilation, but not the result in an executed program):
#include "memory.h"
#define COPY_KEY(type, name) \
type name(void *key, void **args, int argc, void **out) { \
if (*out = malloc(sizeof(type))) { \
return 1; \
} \
**((type **) out) = *((type *) key); \
return 0; \
} \
COPY_KEY(int, copy_key_sint)
For more on the subject of generic programming in C, read this blog wich contains a few examples and also this book which contains interesting solutions to the problem for basic data structures and algorithm.
That should work. To create copy_key_sint, use copy_key_ ## sint.
If you can't get this to work with CPP, then write a small C program which generates a C source file.
Wouldn't a macro which just takes sizeof(*key) and calls a single function that uses memcpy be a lot cleaner (less preprocessor abuse and code bloat) than making a new function for each type just so it can do a native assignment rather than memcpy?
My view is that the whole problem is your attempt to apply C++ thinking to C. C has memcpy for a very good reason.