I have two structs I'm working with, and they are defined nearly identical. These are defined in header files that I cannot modify.
typedef struct
{
uint32_t property1;
uint32_t property2;
} CarV1;
typedef struct
{
uint32_t property1;
uint32_t property2;
/* V2 specific properties */
uint32_t property3;
uint32_t property4;
} CarV2;
In my code, I initialize the V2 struct at the top of my file, to cover all my bases:
static const carV2 my_car = {
.property1 = value,
.property2 = value,
/* V2 specific properties */
.property3 = value,
.property4 = value
};
Later, I want to retrieve the values I have initialized and copy them into the struct to be returned from a function via void pointer. I sometimes want V2 properties of the car, and sometimes V1. How can I memcpy safely without having duplicate definitions/initializations? I'm fairly new to C, and its my understanding that this is ugly and engineers to follow me in looking at this code will not approve. What's a clean way to do this?
int get_properties(void *returned_car){
int version = get_version();
switch (version){
case V1:
{
CarV1 *car = returned_car;
memcpy(car, &my_car, sizeof(CarV1)); // is this safe? What's a better way?
}
case V2:
{
CarV2 *car = returned_car;
memcpy(car, &my_car, sizeof(CarV2));
}
}
}
Yes, it's definitely possible to do what you're asking.
You can use a base struct member to implement inheritance, like this:
typedef struct
{
uint32_t property1;
uint32_t property2;
} CarV1;
typedef struct
{
CarV1 base;
/* V2 specific properties */
uint32_t property3;
uint32_t property4;
} CarV2;
In this case, you're eliminating the duplicate definitions. Of course, on a variable of type CarV2*, you can't reference the fields of the base directly - you'll have to do a small redirection, like this:
cv2p->base.property1 = 0;
To upcast to CarV1*, do this:
CarV1* cv1p = &(cv2p->base);
c1vp->property1 = 0;
You've written memcpy(&car, &my_car, sizeof(CarV1)). This looks like a mistake, because it's copying the data of the pointer variable (that is, the address of your struct, instead of the struct itself). Since car is already a pointer (CarV1*) and I'm assuming that so is my_car, you probably wanted to do this instead:
memcpy(car, my_car, sizeof(CarV1));
If my_car is CarV2* and car is CarV1* (or vice versa), then the above code is guaranteed to work by the C standard, because the first member of a struct is always at a zero offset and, therefore, the memory layout of those two for the first sizeof(CarV1) bytes will be identical.
The compiler is not allowed to align/pad that part differently (which I assume is what you meant about optimizing), because you've explicitly declared the first part of CarV2 to be a CarV1.
Since in your case you are stuck with identically defined structs that you can't change, you may find useful that the C standard defines a macro/special form called offsetof.
To be absolutely sure about your memory layouts, I'd advise that you put a series of checks during the initialization phase of your program that verifies whether the offsetof(struct CarV1, property1) is equal to offsetof(struct CarV2, property1) etc for all common properties:
void validateAlignment(void)
{
if (offsetof(CarV1, property1) != offsetof(CarV2, property1)) exit(-1);
if (offsetof(CarV1, property2) != offsetof(CarV2, property2)) exit(-1);
// and so on
}
This will stop the program for going ahead in case the compiler has done anything creative with the padding.
It also won't slow down your program's initialization because offsetof is actually calculated at compile time. So, with all the optimizations in place, the void validateAlignment(void) function should be optimized out completely (because a static analysis would show that the exit conditions are always false).
What you wrote will almost work, except that instead of memcpy(&car, ... you should just have memcpy (car, ..., but there is no reason to use memcpy in such a case. Rather, you should just copy each of the fields in a separate statement.
car->property1 = my_car.property1
(is my_car a pointer or not? it's impossible to tell from the code fragment)
For the second case, I think you can just assign the entire struct: *car = my_car
there is no perfect solution but one way is to use a union
typedef union car_union
{
CarV1 v1;
CarV2 v2;
} Car;
that way the size will not differ when you do a memcpy - if version v1 then v2 specific parts will not be initialized.
In C and Objective-C, this is fine in practice. (In theory, the compiler must see the declaration of a union containing both structs as members).
In C++ (and Objective-C++), the language very carefully describes when this is safe and when it isn't. For example, if you start with
typedef struct {
public:
...
then the compiler is free to re-arrange where struct members are. If the struct uses no C++ features then you are safe.
Related
Given I'll return a large struct in a function like here:
#include <stdio.h>
// this is a large struct
struct my_struct {
int x[64];
int y[64];
int z[64];
};
struct my_struct get_my_struct_from_file(const char *filename) {
int tmp1, tmp2; // some tmp. variables
struct my_struct u;
// ... load values from filename ...
return u;
}
int main() {
struct my_struct res = get_my_struct_from_file("tmp.txt"); // <-- here
printf("x[0] = %d\n", res.x[0]);
// ... print all values ...
}
At the place marked by here, do I have to assume that this large struct is copied or is it likely that the compiler does something to avoid this?
Thank you
… do I have to assume that this large struct is copied…
No, of course you do not have to make that assumption. Nobody requires you to make that assumption, and it would be unwise to adopt the statement as an assumption rather than deriving it from known information, such as compiler documentation or inspection of the generated assembly code.
In the specific code you show, it is likely good compilers will optimize so that the structure is not copied. (Testing with Apple Clang 11 confirms it does this optimization.) But that is likely overly simplified code. If a call to get_my_struct_from_file appears in a translation unit separate from its definition, the compiler will not know what get_my_struct_from_file is accessing. If the destination object, res in this example, has had its address previously passed to some other routine in some other translation unit, then the compiler cannot know that other routine did not stash the address somewhere and that get_my_struct_from_file is not using it. So the compiler would have to treat the structure returned by get_my_struct_from_file and the structure the return value is being assigned to as separate; it could not coalesce them to avoid the copy.
To ensure the compiler does what you want, simply tell it what you want it to do. Write the code so that the function puts the results directly in the structure you want to put it in:
void get_my_struct_from_file(struct my_struct *result, const char *filename)
{
…
}
...
get_my_struct_from_file(&res, "tmp.txt");
At the place marked by here, do I have to assume that this large struct is copied or is it likely that the compiler does something to avoid this?
Semantically, the structure is copied from the function's local variable to the caller's variable. These are distinct objects, and just like objects of other types, setting one structure equal to another requires copying from the representation of one to the representation of the other.
The only way to avoid a copy would be for the compiler to treat the local variable as an alias for the caller's structure, but that would be wrong in the general case. Such aliasing can easily produce observably different behavior than would occur without.
It is possible that in some specific cases, the compiler can indeed avoid the copy, but if you want to ensure that no copying happens then you should set up the wanted aliasing explicitly:
void get_my_struct_from_file(const char *filename, struct my_struct *u) {
int tmp1, tmp2; // some tmp. variables
// ... load values from filename into *u
}
int main() {
struct my_struct res = { 0 };
get_my_struct_from_file("tmp.txt", &res);
printf("x[0] = %d\n", res.x[0]);
// ... print all values ...
}
#define LENGTH 6
typedef char data_t[LENGTH];
struct foo {
const data_t data;
...
}
...
void bar(data_t data) {
printf("%.6s\n", data);
struct foo myfoo = {*data};
printf("%.6s\n", foo.data);
}
I'm trying to have this struct which holds directly the data I'm interested in, sizeof(foo) == 6+the rest, not sizeof(foo) == sizeof(void*)+the rest. However I can't find a way to initialize a struct of type foo with a data_t. I think maybe I could remove the const modifier from the field and use memcpy but I like the extra safety and clarity.
I don't get any compile errors but when I run the code I get
123456
1??
so the copy didn't work properly I think.
This is for an arduino (or similar device) so I'm trying to keep it to very portable code.
Is it just not possible ?
EDIT: removing the const modifier on the data_t field doesn't seem to help.
It is possible to do this, for some cost >=0.
typedef struct
{
char c[LENGTH];
} data_t; // this struct is freely copyable
struct foo
{
const data_t data; // but this data member is not
int what;
};
void foo (char* x) {
data_t d; // declare freely copyable struct instance
memcpy(d.c, x, sizeof(d.c)); // memcpy it
struct foo foo = { d, 42 }; // initialise struct instance with const member
...
};
Some compilers (e.g. clang) are even able to optimise away the redundant copying (from x to d.c and then from d to foo.data ⇒ from x straight to foo.data). Others (gcc I'm looking at you) don't seem to be able to achieve this.
If you pass around pointers to data_t rather than straight char pointers, you won't need this additional memcpy step. OTOH in order to access the char array inside foo you need another level of member access (.data.c instead of just .data; this has no runtime cost though).
It's impossible to do it in a standard compliant way.
Due to its being const, const char data[6]; must be initialized to be usable, and it may only be initialized statically (static objects with no initializer get automatically zeroed), with a string literal, or with a brace-enclosed initializer list. You cannot initialize it with a pointer or another array.
If I were you, I would get rid of the const, document that .data shouldn't be changed post-initialization, and then use memcpy to initialize it.
(const on struct members doesn't work very well in my opinion. It effectively prevents you from being able to have initializer functions, and while C++ gets around the problem a little bit by having special language support for its constructor functions, the problem still remains if the const members are arrays).
I am currently working on an embedded system and I have a component on a board which appears two times. I would like to have one .c and one .h file for the component.
I have the following code:
typedef struct {
uint32_t pin_reset;
uint32_t pin_drdy;
uint32_t pin_start;
volatile avr32_spi_t *spi_module;
uint8_t cs_id;
} ads1248_options_t;
Those are all hardware settings. I create two instances of this struct (one for each part).
Now I need to keep an array of values in the background. E.g. I can read values from that device every second and I want to keep the last 100 values. I would like this data to be non-accessible from the "outside" of my component (only through special functions in my component).
I am unsure on how to proceed here. Do I really need to make the array part of my struct? What I thought of would be to do the following:
int32_t *adc_values; // <-- Add this to struct
int32_t *adc_value_buffer = malloc(sizeof(int32_t) * 100); // <-- Call in initialize function, this will never be freed on purpose
Yet, I will then be able to access my int32_t pointer from everywhere in my code (also from outside my component) which I do not like.
Is this the only way to do it? Do you know of a better way?
Thanks.
For the specific case of writing hardware drivers for a microcontroller, which this appears to be, please consider doing like this.
Otherwise, use opaque/incomplete type. You'd be surprised to learn how shockingly few C programmers there are who know how to actually implement 100% private encapsulation of custom types. This is why there's some persistent myth about C lacking the OO feature known as private encapsulation. This myth originates from lack of C knowledge and nothing else.
This is how it goes:
ads1248.h
typedef struct ads1248_options_t ads1248_options_t; // incomplete/opaque type
ads1248_options_t* ads1248_init (parameters); // a "constructor"
void ads1248_destroy (ads1248_options_t* ads); // a "destructor"
ads1248.c
#include "ads1248.h"
struct ads1248_options_t {
uint32_t pin_reset;
uint32_t pin_drdy;
uint32_t pin_start;
volatile avr32_spi_t *spi_module;
uint8_t cs_id;
};
ads1248_options_t* ads1248_init (parameters)
{
ads1248_options_t* ads = malloc(sizeof(ads1248_options_t));
// do things with ads based on parameters
return ads;
}
void ads1248_destroy (ads1248_options_t* ads)
{
free(ads);
}
main.c
#include "ads1248.h"
int main()
{
ads1248_options_t* ads = ads1248_init(parameters);
...
ads1248_destroy(ads);
}
Now the code in main cannot access any of the struct members, all members are 100% private. It can only create a pointer to a struct object, not an instance of it. Works exactly like abstract base classes in C++, if you are familiar with that. The only difference is that you'll have to call the init/destroy functions manually, rather than using true constructors/destructors.
It's common that structures in C are defined completely in the header, although they're totally opaque (FILE, for example), or only have some of their fields specified in the documentation.
C lacks private to prevent accidental access, but I consider this a minor problem: If a field isn't mentioned in the spec, why should someone try to access it? Have you ever accidentally accessed a member of a FILE? (It's probably better not to do things like having a published member foo and a non-published fooo which can easily be accessed by a small typo.) Some use conventions like giving them "unusual" names, for example, having a trailing underscore on private members.
Another way is the PIMPL idiom: Forward-declare the structure as an incomplete type and provide the complete declaration in the implementation file only. This may complicate debugging, and may have performance penalties due to less possibilities for inlining and an additional indirection, though this may be solvable with link-time optimization. A combination of both is also possible, declaring the public fields in the header along with a pointer to an incomplete structure type holding the private fields.
I would like this data to be non-accessible from the "outside" of my
component (only through special functions in my component).
You can do it in this way (a big malloc including the data):
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
typedef struct {
uint32_t pin_reset;
uint32_t pin_drdy;
uint32_t pin_start;
volatile avr32_spi_t *spi_module;
uint8_t cs_id;
} ads1248_options_t;
void fn(ads1248_options_t *x)
{
int32_t *values = (int32_t *)(x + 1);
/* values are not accesible via a member of the struct */
values[0] = 10;
printf("%d\n", values[0]);
}
int main(void)
{
ads1248_options_t *x = malloc(sizeof(*x) + (sizeof(int32_t) * 100));
fn(x);
free(x);
return 0;
}
You could make a portion of your structure private like this.
object.h
struct object_public {
uint32_t public_item1;
uint32_t public_item2;
};
object.c
struct object {
struct object_public public;
uint32_t private_item1;
uint32_t *private_ptr;
}
A pointer to an object can be cast to a pointer to object_public because object_public is the first item in struct object. So the code outside of object.c will reference the object through a pointer to object_public. While the code within object.c references the object through a pointer to object. Only the code within object.c will know about the private members.
The program should not define or allocate an instance object_public because that instance won't have the private stuff appended to it.
The technique of including a struct as the first item in another struct is really a way for implementing single inheritance in C. I don't recall ever using it like this for encapsulation. But I thought I would throw the idea out there.
You can:
Make your whole ads1248_options_t an opaque type (as already discussed in other answers)
Make just the adc_values member an opaque type, like:
// in the header(.h)
typedef struct adc_values adc_values_t;
// in the code (.c)
struct adc_values {
int32_t *values;
};
Have a static array of array of values "parallel" to your ads1248_options_t and provide functions to access them. Like:
// in the header (.h)
int32_t get_adc_value(int id, int value_idx);
// in the code (.c)
static int32_t values[MAX_ADS][MAX_VALUES];
// or
static int32_t *values[MAX_ADS]; // malloc()-ate members somewhere
int32_t get_adc_value(int id, int value_idx) {
return values[id][value_idx]
}
If the user doesn't know the index to use, keep an index (id) in your ads1248_options_t.
Instead of a static array, you may provide some other way of allocating the value arrays "in parallel", but, again, need a way to identify which array belongs to which ADC, where its id is the simplest solution.
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).
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.
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.
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.
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.
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 /*, ...*/);
}
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.
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.
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.
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
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.
I'm having a very big struct in an existing program. This struct includes a great number of bitfields.
I wish to save a part of it (say, 10 fields out of 150).
An example code I would use to save the subclass is:
typedef struct {int a;int b;char c} bigstruct;
typedef struct {int a;char c;} smallstruct;
void substruct(smallstruct *s,bigstruct *b) {
s->a = b->a;
s->c = b->c;
}
int save_struct(bigstruct *bs) {
smallstruct s;
substruct(&s,bs);
save_struct(s);
}
I also wish that selecting which part of it wouldn't be too much hassle, since I wish to change it every now and then. The naive approach I presented before is very fragile and unmaintainable. When scaling up to 20 different fields, you have to change fields both in the smallstruct, and in the substruct function.
I thought of two better approaches. Unfortunately both requires me to use some external CIL like tool to parse my structs.
The first approach is automatically generating the substruct function. I'll just set the struct of smallstruct, and have a program that would parse it and generate the substruct function according to the fields in smallstruct.
The second approach is building (with C parser) a meta-information about bigstruct, and then write a library that would allow me to access a specific field in the struct. It would be like ad-hoc implementation of Java's class reflection.
For example, assuming no struct-alignment, for struct
struct st {
int a;
char c1:5;
char c2:3;
long d;
}
I'll generate the following meta information:
int field2distance[] = {0,sizeof(int),sizeof(int),sizeof(int)+sizeof(char)}
int field2size[] = {sizeof(int),1,1,sizeof(long)}
int field2bitmask[] = {0,0x1F,0xE0,0};
char *fieldNames[] = {"a","c1","c2","d"};
I'll get the ith field with this function:
long getFieldData(void *strct,int i) {
int distance = field2distance[i];
int size = field2size[i];
int bitmask = field2bitmask[i];
void *ptr = ((char *)strct + distance);
long result;
switch (size) {
case 1: //char
result = *(char*)ptr;
break;
case 2: //short
result = *(short*)ptr;
...
}
if (bitmask == 0) return result;
return (result & bitmask) >> num_of_trailing_zeros(bitmask);
}
Both methods requires extra work, but once the parser is in your makefile - changing the substruct is a breeze.
However I'd rather do that without any external dependencies.
Does anyone have any better idea? Where my ideas any good, is there some availible implementation of my ideas on the internet?
From your description, it looks like you have access to and can modify your original structure. I suggest you refactor your substructure into a complete type (as you did in your example), and then make that structure a field on your big structure, encapsulating all of those fields in the original structure into the smaller structure.
Expanding on your small example:
typedef struct
{
int a;
char c;
} smallstruct;
typedef struct
{
int b;
smallstruct mysub;
} bigstruct;
Accessing the smallstruct info would be done like so:
/* stack-based allocation */
bigstruct mybig;
mybig.mysub.a = 1;
mybig.mysub.c = '1';
mybig.b = 2;
/* heap-based allocation */
bigstruct * mybig = (bigstruct *)malloc(sizeof(bigstruct));
mybig->mysub.a = 1;
mybig->mysub.c = '1';
mybig->b = 2;
But you could also pass around pointers to the small struct:
void dosomething(smallstruct * small)
{
small->a = 3;
small->c = '3';
}
/* stack based */
dosomething(&(mybig.mysub));
/* heap based */
dosomething(&((*mybig).mysub));
Benefits:
No Macros
No external dependencies
No memory-order casting hacks
Cleaner, easier-to-read and use code.
If changing the order of the fields isn't out of the question, you can rearrange the bigstruct fields in such a way that the smallstruct fields are together, and then its simply a matter of casting from one to another (possibly adding an offset).
Something like:
typedef struct {int a;char c;int b;} bigstruct;
typedef struct {int a;char c;} smallstruct;
int save_struct(bigstruct *bs) {
save_struct((smallstruct *)bs);
}
Macros are your friend.
One solution would be to move the big struct out into its own include file and then have a macro party.
Instead of defining the structure normally, come up with a selection of macros, such as BEGIN_STRUCTURE, END_STRUCTURE, NORMAL_FIELD, SUBSET_FIELD
You can then include the file a few times, redefining those structures for each pass. The first one will turn the defines into a normal structure, with both types of field being output as normal. The second would define NORMAL_FIELD has nothing and would create your subset. The third would create the appropriate code to copy the subset fields over.
You'll end up with a single definition of the structure, that lets you control which fields are in the subset and automatically creates suitable code for you.
Just to help you in getting your metadata, you can refer to the offsetof() macro, which also has the benefit of taking care of any padding you may have
I suggest to take this approach:
Curse the guy who wrote the big structure. Get a voodoo doll and have some fun.
Mark each field of the big structure that you need somehow (macro or comment or whatever)
Write a small tool which reads the header file and extracts the marked fields. If you use comments, you can give each field a priority or something to sort them.
Write a new header file for the substructure (using a fixed header and footer).
Write a new C file which contains a function createSubStruct which takes a pointer to the big struct and returns a pointer to the substruct
In the function, loop over the fields collected and emit ss.field = bs.field (i.e. copy the fields one by one).
Add the small tool to your makefile and add the new header and C source file to your build
I suggest to use gawk, or any scripting language you're comfortable with, as the tool; that should take half an hour to build.
[EDIT] If you really want to try reflection (which I suggest against; it'll be a whole lot of work do get that working in C), then the offsetof() macro is your friend. This macro returns the offset of a field in a structure (which is most often not the sum of the sizes of the fields before it). See this article.
[EDIT2] Don't write your own parser. To get your own parser right will take months; I know since I've written lots of parsers in my life. Instead mark the parts of the original header file which need to be copied and then rely on the one parser which you know works: The one of your C compiler. Here are a couple of ideas how to make this work:
struct big_struct {
/**BEGIN_COPY*/
int i;
int j : 3;
int k : 2;
char * str;
/**END_COPY*/
...
struct x y; /**COPY_STRUCT*/
}
Just have your tool copy anything between /**BEGIN_COPY*/ and /**END_COPY*/.
Use special comments like /**COPY_STRUCT*/ to instruct your tool to generate a memcpy() instead of an assignment, etc.
This can be written and debugged in a few hours. It would take as long to set up a parser for C without any functionality; that is you'd just have something which can read valid C but you'd still have to write the part of the parser which understands C, and the part which does something useful with the data.