Static allocation of opaque data types - c

Question

Very often malloc() is absolutely not allowed when programming for embedded systems. Most of the time I'm pretty able to deal with this, but one thing irritates me: it keeps me from using so called 'opaque types' to enable data hiding. Normally I'd do something like this:
// In file module.h
typedef struct handle_t handle_t;
handle_t *create_handle();
void operation_on_handle(handle_t *handle, int an_argument);
void another_operation_on_handle(handle_t *handle, char etcetera);
void close_handle(handle_t *handle);
// In file module.c
struct handle_t {
int foo;
void *something;
int another_implementation_detail;
};
handle_t *create_handle() {
handle_t *handle = malloc(sizeof(struct handle_t));
// other initialization
return handle;
}
There you go: create_handle() performs a malloc() to create an 'instance'. A construction often used to prevent having to malloc() is to change the prototype of create_handle() like this:
void create_handle(handle_t *handle);
And then the caller could create the handle this way:
// In file caller.c
void i_am_the_caller() {
handle_t a_handle; // Allocate a handle on the stack instead of malloc()
create_handle(&a_handle);
// ... a_handle is ready to go!
}
But unfortunately this code is obviously invalid, the size of handle_t isn't known!
I never really found a solution to solve this in a proper way. I'd very like to know if anyone has a proper way of doing this, or maybe a complete different approach to enable data hiding in C (not using static globals in the module.c of course, one must be able to create multiple instances).

Answer 1

You can use the _alloca function. I believe that it's not exactly Standard, but as far as I know, nearly all common compilers implement it. When you use it as a default argument, it allocates off the caller's stack.
// Header
typedef struct {} something;
size_t get_size();
something* create_something(void* mem);
// Usage
something* ptr = create_something(_alloca(get_size())); // or define a macro.
// Implementation
size_t get_size() {
return sizeof(real_handle_type);
}
something* create_something(void* mem) {
real_handle_type* ptr = (real_handle_type*)mem;
// Fill out real_type
return (something*)mem;
}
You could also use some kind of object pool semi-heap - if you have a maximum number of currently available objects, then you could allocate all memory for them statically, and just bit-shift for which ones are currently in use.
#define MAX_OBJECTS 32
real_type objects[MAX_OBJECTS];
unsigned int in_use; // Make sure this is large enough
something* create_something() {
for(int i = 0; i < MAX_OBJECTS; i++) {
if (!(in_use & (1 << i))) {
in_use |= (1 << i);
return &objects[i];
}
}
return NULL;
}
My bit-shifting is a little off, been a long time since I've done it, but I hope that you get the point.

Answer 2

One way would be to add something like
#define MODULE_HANDLE_SIZE (4711)
to the public module.h header. Since that creates a worrying requirement of keeping this in sync with the actual size, the line is of course best auto-generated by the build process.
The other option is of course to actually expose the structure, but document it as being opaque and forbidding access through any other means than through the defined API. This can be made more clear by doing something like:
#include "module_private.h"
typedef struct
{
handle_private_t private;
} handle_t;
Here, the actual declaration of the module's handle has been moved into a separate header, to make it less obviously visible. A type declared in that header is then simply wrapped in the desired typedef name, making sure to indicate that it is private.
Functions inside the module that take handle_t * can safely access private as a handle_private_t value, since it's the first member of the public struct.

Answer 3

Unfortunately, I think the typical way to deal with this problem is by simply having the programmer treat the object as opaque - the full structure implementation is in the header and available, it's just the responsibility of the programmer to not use the internals directly, only through the APIs defined for the object.
If this isn't good enough, a few options might be:
use C++ as a 'better C' and declare the internals of the structure as private.
run some sort of pre-processor on the headers so that the internals of the structure are declared, but with unusable names. The original header, with good names, will be available to the implementation of the APIs that manage the structure. I've never seen this technique used - it's just an idea off the top of my head that might be possible, but seems like far more trouble than it's worth.
have your code that uses opaque pointers declare the statically allocated objects as extern (ie., globals) Then have a special module that has access to the full definition of the object actually declare these objects. Since only the 'special' module has access to the full definition, the normal use of the opaque object remains opaque. However, now you have to rely on your programmers to not abuse the fact that thee objects are global. You have also increased the change of naming collisions, so that need to be managed (probably not a big problem, except that it might occur unintentionally - ouch!).
I think overall, just relying on your programmers to follow the rules for the use of these objects might be the best solution (though using a subset of C++ isn't bad either in my opinion). Depending on your programmers to follow the rules of not using the structure internals isn't perfect, but it's a workable solution that is in common use.

Answer 4

One solution if to create a static pool of struct handle_t objects, and provide then as neceessary. There are many ways to achieve that, but a simple illustrative example follows:
// In file module.c
struct handle_t
{
int foo;
void* something;
int another_implementation_detail;
int in_use ;
} ;
static struct handle_t handle_pool[MAX_HANDLES] ;
handle_t* create_handle()
{
int h ;
handle_t* handle = 0 ;
for( h = 0; handle == 0 && h < MAX_HANDLES; h++ )
{
if( handle_pool[h].in_use == 0 )
{
handle = &handle_pool[h] ;
}
}
// other initialization
return handle;
}
void release_handle( handle_t* handle )
{
handle->in_use = 0 ;
}
There are faster faster ways of finding an unused handle, you could for example keep a static index that increments each time a handle is allocated and 'wraps-around' when it reaches MAX_HANDLES; this would be faster for the typical situation where several handles are allocated before releasing any one. For a small number of handles however, this brute-force search is probably adequate.
Of course the handle itself need no longer be a pointer but could be a simple index into the hidden pool. This would enhance data hiding and protection of the pool from external access.
So the header would have:
typedef int handle_t ;
and the code would change as follows:
// In file module.c
struct handle_s
{
int foo;
void* something;
int another_implementation_detail;
int in_use ;
} ;
static struct handle_s handle_pool[MAX_HANDLES] ;
handle_t create_handle()
{
int h ;
handle_t handle = -1 ;
for( h = 0; handle != -1 && h < MAX_HANDLES; h++ )
{
if( handle_pool[h].in_use == 0 )
{
handle = h ;
}
}
// other initialization
return handle;
}
void release_handle( handle_t handle )
{
handle_pool[handle].in_use = 0 ;
}
Because the handle returned is no longer a pointer to the internal data, and inquisitive or malicious user cannnot gain access to it through the handle.
Note that you may need to add some thread-safety mechanisms if you are getting handles in multiple threads.

Answer 5

I faced a similar problem in implementing a data structure in which the header of the data structure, which is opaque, holds all the various data that needs to be carried over from operation to operation.
Since re-initialization might cause a memory leak, I wanted to make sure that data structure implementation itself never actually overwrite a point to heap allocated memory.
What I did is the following:
/**
* In order to allow the client to place the data structure header on the
* stack we need data structure header size. [1/4]
**/
#define CT_HEADER_SIZE ( (sizeof(void*) * 2) \
+ (sizeof(int) * 2) \
+ (sizeof(unsigned long) * 1) \
)
/**
* After the size has been produced, a type which is a size *alias* of the
* header can be created. [2/4]
**/
struct header { char h_sz[CT_HEADER_SIZE]; };
typedef struct header data_structure_header;
/* In all the public interfaces the size alias is used. [3/4] */
bool ds_init_new(data_structure_header *ds /* , ...*/);
In the implementation file:
struct imp_header {
void *ptr1,
*ptr2;
int i,
max;
unsigned long total;
};
/* implementation proper */
static bool imp_init_new(struct imp_header *head /* , ...*/)
{
return false;
}
/* public interface */
bool ds_init_new(data_structure_header *ds /* , ...*/)
{
int i;
/* only accept a zero init'ed header */
for(i = 0; i < CT_HEADER_SIZE; ++i) {
if(ds->h_sz[i] != 0) {
return false;
}
}
/* just in case we forgot something */
assert(sizeof(data_structure_header) == sizeof(struct imp_header));
/* Explicit conversion is used from the public interface to the
* implementation proper. [4/4]
*/
return imp_init_new( (struct imp_header *)ds /* , ...*/);
}
client side:
int foo()
{
data_structure_header ds = { 0 };
ds_init_new(&ds /*, ...*/);
}

Answer 6

To expand on some old discussion in comments here, you can do this by providing an allocator function as part of the constructor call.
Given some opaque type typedef struct opaque opaque;, then
Define a function type for an allocator function typedef void* alloc_t (size_t bytes);. In this case I used the same signature as malloc/alloca for compatibility purposes.
The constructor implementation would look something like this:
struct opaque
{
int foo; // some private member
};
opaque* opaque_construct (alloc_t* alloc, int some_value)
{
opaque* obj = alloc(sizeof *obj);
if(obj == NULL) { return NULL; }
// initialize members
obj->foo = some_value;
return obj;
}
That is, the allocator gets provided the size of the opaque object from inside the constructor, where it is known.
For static storage allocation like done in embedded systems, we can create a simple static memory pool class like this:
#define MAX_SIZE 100
static uint8_t mempool [MAX_SIZE];
static size_t mempool_size=0;
void* static_alloc (size_t size)
{
uint8_t* result;
if(mempool_size + size > MAX_SIZE)
{
return NULL;
}
result = &mempool[mempool_size];
mempool_size += size;
return result;
}
(This might be allocated in .bss or in your own custom section, whatever is preferred.)
Now the caller can decide how each object is allocated and all objects in for example a resource-constrained microcontroller can share the same memory pool. Usage:
opaque* obj1 = opaque_construct(malloc, 123);
opaque* obj2 = opaque_construct(static_alloc, 123);
opaque* obj3 = opaque_construct(alloca, 123); // if supported
This is useful for the purpose of saving memory. In case you have multiple drivers in a microcontroller application and each makes sense to hide behind a HAL, they can now share the same memory pool without the driver implementer having to speculate how many instances of each opaque type that will be needed.
Say for example that we have generic HAL for hardware peripherals to UART, SPI and CAN. Rather than each implementation of the driver providing its own memory pool, they can all share a centralized section. Normally I would otherwise solve that by having a constant such as UART_MEMPOOL_SIZE 5 exposed in uart.h so that the user may change it after how many UART objects they need (like the the number of present UART hardware peripherals on some MCU, or the number of CAN bus message objects required for some CAN implementation etc etc). Using #define constants is an unfortunate design since we typically don't want application programmers to mess around with provided standardized HAL headers.

Answer 7

I'm a little confused why you say you can't use malloc(). Obviously on an embedded system you have limited memory and the usual solution is to have your own memory manager which mallocs a large memory pool and then allocates chunks of this out as needed. I've seen various different implementations of this idea in my time.
To answer your question though, why don't you simply statically allocate a fixed size array of them in module.c add an "in-use" flag, and then have create_handle() simply return the pointer to the first free element.
As an extension to this idea, the "handle" could then be an integer index rather than the actual pointer which avoids any chance of the user trying to abuse it by casting it to their own definition of the object.

Answer 8

The least grim solution I've seen to this has been to provide an opaque struct for the caller's use, which is large enough, plus maybe a bit, along with a mention of the types used in the real struct, to ensure that the opaque struct will be aligned well enough compared to the real one:
struct Thing {
union {
char data[16];
uint32_t b;
uint8_t a;
} opaque;
};
typedef struct Thing Thing;
Then functions take a pointer to one of those:
void InitThing(Thing *thing);
void DoThingy(Thing *thing,float whatever);
Internally, not exposed as part of the API, there is a struct that has the true internals:
struct RealThing {
uint32_t private1,private2,private3;
uint8_t private4;
};
typedef struct RealThing RealThing;
(This one just has uint32_t' anduint8_t' -- that's the reason for the appearance of these two types in the union above.)
Plus probably a compile-time assert to make sure that RealThing's size doesn't exceed that of Thing:
typedef char CheckRealThingSize[sizeof(RealThing)<=sizeof(Thing)?1:-1];
Then each function in the library does a cast on its argument when it's going to use it:
void InitThing(Thing *thing) {
RealThing *t=(RealThing *)thing;
/* stuff with *t */
}
With this in place, the caller can create objects of the right size on the stack, and call functions against them, the struct is still opaque, and there's some checking that the opaque version is large enough.
One potential issue is that fields could be inserted into the real struct that mean it requires an alignment that the opaque struct doesn't, and this won't necessarily trip the size check. Many such changes will change the struct's size, so they'll get caught, but not all. I'm not sure of any solution to this.
Alternatively, if you have a special public-facing header(s) that the library never includes itself, then you can probably (subject to testing against the compilers you support...) just write your public prototypes with one type and your internal ones with the other. It would still be a good idea to structure the headers so that the library sees the public-facing Thing struct somehow, though, so that its size can be checked.

Answer 9

It is simple, simply put the structs in a privateTypes.h header file. It will not be opaque anymore, still, it will be private to the programmer, since it is inside a private file.
An example here:
Hiding members in a C struct

Answer 10

This is an old question, but since it's also biting me, I wanted to provide here a possible answer (which I'm using).
So here is an example :
// file.h
typedef struct { size_t space[3]; } publicType;
int doSomething(publicType* object);
// file.c
typedef struct { unsigned var1; int var2; size_t var3; } privateType;
int doSomething(publicType* object)
{
privateType* obPtr = (privateType*) object;
(...)
}
Advantages :
publicType can be allocated on stack.
Note that correct underlying type must be selected in order to ensure proper alignment (i.e. don't use char).
Note also that sizeof(publicType) >= sizeof(privateType).
I suggest a static assert to make sure this condition is always checked.
As a final note, if you believe your structure may evolve later on, don't hesitate to make the public type a bit bigger, to keep room for future expansions without breaking ABI.
Disadvantage :
The casting from public to private type can trigger strict aliasing warnings.
I discovered later on that this method has similarities with struct sockaddr within BSD socket, which meets basically the same problem with strict aliasing warnings.