Hide struct definition in static library - c

I need to provide a C static library to the client and need to be able to make a struct definition unavailable. On top of that I need to be able to execute code before the main at library initialization using a global variable.
Here's my code:
private.h
#ifndef PRIVATE_H
#define PRIVATE_H
typedef struct TEST test;
#endif
private.c (this should end up in a static library)
#include "private.h"
#include <stdio.h>
struct TEST
{
TEST()
{
printf("Execute before main and have to be unavailable to the user.\n");
}
int a; // Can be modified by the user
int b; // Can be modified by the user
int c; // Can be modified by the user
} TEST;
main.c
test t;
int main( void )
{
t.a = 0;
t.b = 0;
t.c = 0;
return 0;
}
Obviously this code doesn't work... but show what I need to do... Anybody knows how to make this work? I google quite a bit but can't find an answer, any help would be greatly appreciated.
TIA!

If you're using gcc you can use the constructor attribute,
void runs_before_main(void) __attribute__((constructor))
{
...
}
From the gcc documentation
The constructor attribute causes the
function to be called automatically
be- fore execution enters main ().
Similarly, the destructor attribute
causes the function to be called
automatically after main () has
completed or exit () has been called.
Functions with these attributes are
useful for initializing data that will
be used implicitly during the
execution of the program.
You may provide an optional integer
priority to control the order in which
constructor and destructor functions
are run. A constructor with a smaller
priority number runs before a
constructor with a larger priority
number; the opposite relationship
holds for destructors. So, if you have
a constructor that allocates a
resource and a destructor that
deallocates the same resource, both
functions typically have the same
priority. The priorities for
constructor and destructor functions
are the same as those specified for
namespace-scope C++ objects
If you want to hide a struct from users, declare the struct in a header but define it in the c file, passing around pointers. As an example:
// foo.h
typedef struct private_foo foo;
foo * create_foo(void);
void free_foo(foo * f);
// foo.c
struct private_foo {
int i;
}
foo * create_foo(void){
foo * f = malloc(sizeof(*foo));
if (f) f->i = 1;
return f;
}
...
foo->i can then not be accessed outside foo.c.

If you want the client code to be able to use "t.a = ...", then you cannot hide the struct definition. What you want is called an opaque type, that will look something like this:
public.h:
struct foo;
set_a( struct foo *, int );
struct foo * new_foo(void);
main.c:
#include <public.h>
int main( void )
{
struct foo *k;
k = new_foo();
set_a( k, 5 );
}
The structure definition is only available to the library. If you do not make the library source code available, it is possible to completely hide it from the users of the library.

There is no portable way in C to ensure your code will run before main(). What I would do is just maintain an initialised flag in your library, set to false, and then refuse to do anything until your init function has been called.
As in:
static int initialised = 0;
int init (void) {
// do something.
initialised = 1;
return ERR_OK;
}
int all_other_functions (void) {
if (!init)
return ERR_NOT_INITED;
// do something.
return ERR_OK;
}

Related

Why isn't this a valid method to create a default struct instance in C? [duplicate]

This question already has an answer here:
Why can't I assign values to global variables outside a function in C?
(1 answer)
Closed 1 year ago.
So suppose I have the following two files:
// main.h
struct Player {
int health;
int stamina;
};
// main.c
#include "main.h"
#include <stdio.h>
int main() {
struct Player newPlayer;
newPlayer.health = 100;
newPlayer.stamina = 100;
return 0;
}
This works fine.
Now suppose I have these two files:
// main.h
struct Player {
int health;
int stamina;
};
struct Player defaultPlayer;
defaultPlayer.health = 100; // error here
defaultPlayer.stamina = 100; // error here
// main.c
#include "main.h"
#include <stdio.h>
int main() {
struct Player newPlayer = defaultPlayer;
return 0;
}
The error in question is:
Unknown type name 'defaultPlayer'
[clang: unknown_typename]
My question is: Why can't I define a default struct in the .h file?
You cannot assign values to members at the file scope because only declarations and definitions are allowed on the scope level.
While defaultPlayer.health = 100 is an expression.
To set default values put it into struct initializer:
struct Player defaultPlayer = {
.health = 100,
.stamina = 100,
};
Moreover it may be a good idea to make it const.
You can make it an extern symbol. The defaultPlayer would only be declared in main.h and defined main.c.
// main.h
extern const struct Player defaultPlayer;
//main.c
const struct Player defaultPlayer = {
.health = 100,
.stamina = 100,
};
Cons: the compiler cannot inline values of members for translation units other than main.c.
Or use a static object in the header.
// main.h
static const struct Player defaultPlayer = {
.health = 100,
.stamina = 100,
};
Cons: compiler may complain about unused variables. What can be silenced by adding a dummy function:
static inline void dummy(void) { (void)&defaultPlayer; }
The third option is to make it a macro that expands to the compound literal.
#define defaultPlayer (const struct Player){ .health = 100, .stamina = 100 }
IMO, works the best.
A naive try:
A header defines the interface the object will offer to other objects. Therefore it should not contain any implementation detail.
Imagine you have the following files:
/* pretty_print.h */
// header guard
void pretty_print(char* str);
/* pretty_print.c */
#include <stdio.h>
#include "pretty_print.h"
void pretty_print(char* str) {
printf("%s", str);
}
#include "pretty_print.h"
int main() {
pretty_print("Hey there");
}
There will be two objects generated: main.o and pretty_print.o
None of which is yet executable. Neither does main.o contain any function pretty_print.
Linking both together will provide main.o the necessary pretty_print function (as well as do some more stuff).
Now assume there is some sort of data, let's say int x in the pretty_print.h .
As main.o included the header file, there will be such an x. However, pretty_print.o will have an x as well, as it includes pretty_print.h. So there will be two x and the compiler can't know which one to choose.
But I can declare the struct, I just can't change it's values
Yes you can, but you shouldn't declare instances in headers.
To answer, why you can't change it's values, remember there is a main function. When is this "global code" in your execution path? When is it intended to be run?
Please just fix...
You can always use struct initialization in the following way:
struct foo_t {
int x;
};
struct foo_t foo = { .x = 42 };
If you run in any further errors I assume you have to use the extern keyword with a pointer to a player-struct and initalize it with a default player in main

Structured module interface for C

I want to improve my C code style a bit and as well encapsulate my code a bit more. Furthermore, the interface to my modules should be clear and easy to handle.
In my project I have several modules containing the different functions.
modul_1.c
static int modul_1_func_2(void);
void modul_1_func_1(void) { ... }
int modul_1_func_2(void) { ... }
void modul_1_func_3(int) { ... }
modul_1.h
void modul_1_func_1(void);
void modul_1_func_3(int);
There are local and global module-function.
With a struct, it should be possible to call the functions from the main like
modul_1.modul_1_func_1();
In know that I have to use function pointers within the structure, but I don't know how to initialize them.
I don't want a a dynamic pointer where I have to change the address each time before I use it. I want to have several pointers within the structure — one for each global function.
I already tried this without success:
typedef void (*func_1_temp)(void);
typedef void (*func_3_temp)(int);
struct {
func_1_temp func_1;
func_3_temp func_3;
} HMI = {
func_1 = &modul_1_func_1(),
func_3 = &modul_1_func_3()
};
Does anyone know what I have to do that my dreams come true?
I hope to get a clear interface with this approach.
For other ideas I am thankful as well.
The name of a function is, at least in this context, directly usable as a
pointer to the function.
mod.h
void modul_1_func_1(void);
void modul_1_func_3(int);
granicus% cat mod.c
static int modul_1_func_2(void);
mod.c
void modul_1_func_1(void) { return; }
int modul_1_func_2(void) { return 1; }
void modul_1_func_3(int x) { modul_1_func_2(); return; }
granicus% cat main.c
#include "mod.h"
typedef void (*func_1_temp)(void);
typedef void (*func_3_temp)(int);
main.c
int main(void) {
struct {
func_1_temp func_1;
func_3_temp func_3;
} HMI = {
modul_1_func_1,
modul_1_func_3,
};
HMI.func_1();
return 0;
}
Compiling on my system with gcc -Wall main.c mod.c gives no errors or warnings.

Inlined Setter and Getter functions in C

In C++ I can have a getter function declared inline in a header file:
class Cpp_Example
{
public:
unsigned int get_value(void)
{ return value;}
private:
unsigned int value;
};
By including this header file, client methods and functions can use the getter function to access a private variable.
I'm looking to model this concept in the C language:
hello.h:
#ifndef HELLO_H
#define HELLO_H
#include <stdio.h>
inline void Print_Hello(void)
{
extern const char hello_text[32];
puts(hello_text);
}
inline void Print_Value(void)
{
extern unsigned int value;
printf("Value is: %d\n", value);
}
#endif // HELLO_H
hello.c:
const char hello_text[32] = "Hello World!\n";
static unsigned int value = 5U;
main.c:
#include <stdio.h>
#include <stdlib.h>
#include "hello.h"
int main(void)
{
Print_Hello();
Print_Value();
// puts(hello_text);
return EXIT_SUCCESS;
}
I get a linker error from gcc:
$ gcc -o main.exe main.c hello.c
/tmp/cc41ZB8H.o:main.c:(.rdata$.refptr.value[.refptr.value]+0x0): undefined reference to `value'
collect2: error: ld returned 1 exit status
Is there a way to have an inline function (in a header file) access a static variable in another translation unit?
Or is there a way to implement an inlined getter function?
I'm using IAR Embedded Workbench, ARM7TDMI processor on an embedded platform.
The gcc compiler is used to testing concepts on the PC.
Edit 1: Background
I'm looking to optimize getter calls that are inside a critical section. The objective is to reduce the time spent in the critical section.
Edit 2: No Globals
The Coding Guidelines our shop uses states no global variables.
Also, this system is an RTOS running MicroCOSII.
First of all, the same way you have private variables in C++, you probably mean to have private variables for a struct rather than global. With that assumption, here's one model you can use:
/* some_type.h */
struct some_type
{
int public_data;
void *privates;
};
struct some_type_privates
{
char hello[32];
int value;
};
inline const char *get_hello(struct some_type *t)
{
struct some_type_privates *p = t->privates;
return p->hello;
}
inline int get_value(struct some_type *t)
{
struct some_type_privates *p = t->privates;
return p->value;
}
/* similarly for setters */
The same way that your private variables and their getters and setters are in the header file, you can do it in C, too.
On the side, I'd like to recommend not to try coding C++ in C. While C++ likes to complicate things a lot to prevent the idiot from breaking something, C on the other hand trusts the programmer has some degree of intelligence. Whether these assumptions are justified are not the matter of discussion. But what I mean to say is that the spirit of C is not to hide a variable so that the programmer doesn't mistakenly access it.
That said, this is how you would normally make a struct in C:
struct some_type
{
int public_data;
char hello[32]; /* read only */
/* internal */
int value;
};
(with enough documentation of course) which tells any programmer that she shouldn't write over hello but can freely read it (what you were trying to achieve by an inline getter). It also tells that value is private so the programmer shouldn't read or write it.
You can see this in many POSIX functions that take or return a struct. Some that don't need to control the access let you freely modify the struct, such as stat. Some that do need to check the input have setters, such as pthread_attr_*.
You need to remove the static keyword. static definitions are local to the compilation unit.
As Shabbas wrote, it doesn't really work that way in C.
The keyword inline implies static, even if the compilers doesn't actually inline it. If it is such a short function, it will probably inline it. But the point is, if it would not be static, it could not even consider inlineing it, as the function would need to be visible externally, it would need an address, which an inlined function doesn't have.
Since it is local in your compilation unit, it can only work on stuff known inside that compilation unit. Thus you need to say something about that value variable, much like you do need to mention it in the C++ header as well, only in C there is no such thing as private .
You can not have Inlineing and data hiding in the same case, neither in C, nor in C++.
Assuming you mean for global, statically-allocated variables you can do this:
In Example.h:
#ifndef Example
#define Example
extern int getValue();
#endif
In Example.c
#include "Example.h"
static int value;
inline int getValue() {
return value;
}
// All the functions in Example.c have read/write access
In UsesValueExample.c
#include "Example.h"
// All the functions in UsesValueExample.c have read-only access
void printValue() {
printf("value = %d", getValue());
}
If you want to get fancy and force all code to access through a getter and setter, e.g. if the variable is volatile and you want to heavily encourage all the methods to use a local cache of the variable to avoid the overhead of accessing the volatile, then:
In VolatileExample.h:
#ifndef VolatileExample
#define VolatileExample
extern int getValue();
#endif
In VolatileExample.c
#include "VolatileExample.h"
void setValue(); // Forward declaration to give write access
// All the functions in VolatileExample.c have read/write access via getters and setters
void addToValuesAndIncrementValue(int const values[], int const numValues) {
int value = getValue(); // Cache a local copy for fast access
// Do stuff with value
for (int i = 0; i < numValues; i++) {
values[i] += value;
}
value++;
// Write the cache out if it has changed
setValue(value);
}
// Put the definitions after the other functions so that direct access is denied
static volatile int value;
inline int getValue() {
return value;
}
inline void setValue(int const newValue) {
value = newValue;
}
In UsesVolatileValueExample.c
#include "VolatileExample.h"
// All the functions in UsesVolatileValueExample.c have read-only access
void printValue() {
printf("value = %d", getValue());
}
Here is a pattern I've been using to hide global variables.
Inside some header file, such as module_prefix.h, you declare the following:
typedef int value_t; // Type of the variable
static inline value_t get_name(void) __attribute__((always_inline));
static inline void set_name(value_t) __attribute__((always_inline));
static inline value_t get_name(void) {
extern value_t module_prefix_name;
return module_prefix_name;
}
static inline void set_name(value_t new_value) {
extern value_t module_prefix_name;
module_prefix_name = new_value;
}
/* Note that module_prefix_name is *no longer* in scope here. */
Then of course you have to define module_prefix_name in some compilation unit, without the static keyword, as discussed above, e.g. in module_prefix.c you have the following:
#include "module_prefix.h"
value_t module_prefix_name = MODULE_PREFIX_NAME_INIT_VALUE;
This is essentially the same pattern that Thomas Matthews tried to use, drilling down to the essence and making sure that the compiler inlines the functions always and does not unnecessarily generate explicit function bodies. Note the use of module_prefix as poor man's name spaces.

C: initialising global constants with external linkage

I want to statically initialise a struct that will be visible from all files which include some header.
The actual intention is to run lists of functions specified at compile time, which is why I want to initialise statically. I'd like to put the lists where they belong, in the file in which they are declared or defined.
Contrived example:
general.h:
struct Widget { int (*a)(); };
foo.h:
int oof1(void);
int oof2(void);
foo.c:
#include "foo.h"
int oof1(void) { return 1; }
int oof2(void) { return 2; }
struct Widget foo_widgets[] = { {oof1}, {oof2} };
bar.c:
#include "foo.h"
#include "baz.h"
struct Widget *foo_widgets;
struct Widget *baz_widgets;
struct WidgetsContainer {
struct Widget *widget_list;
} wlists[] =
{
{ foo_widgets },
{ baz_widgets }
};
void usage(void) { ... ; process (wlists[i].widget_list); ... }
This obviously doesn't work because "the initialiser element is not constant" - This is because when the compiler is translating bar.c, it thinks it doesn't know the location of foo_widgets (or bar_widgets).
But since bar.c #includes foo.h anyway, it's always compiled alongside foo.c:
gcc foo.c baz.c bar.c
So I'm hoping there's a way of reflecting this in the source code.
I can't declare foo_widgets in foo.h, because then I wouldn't be able to initialise it without defining it multiple times (as foo.h is included in more than one file).
Inelegant workaround
foo.h:
...
Widget *get_foos(void) { return foo_widgets; }
bar.c:
...
struct Widget_lists {
struct Widget (*widget_list)();
} wlist[] =
{
{ get_foos },
{ get_bazes }
};
void usage(void) { ...; process(wlist[i].widget_list()); ... }
Is there a better way?
You want to have a global variable that you can reach anywhere ... To do so you have to declare it as an external variable in the appropriate header.
Here, it should be done as follows :
In foo.h :
/* ... */
int oof1(void);
int oof2(void);
extern int (*foo_widget)(void)[2];
In foo.c :
int (*foo_widget)(void)[2] = {{oof1}, {oof2}};
By doing so, when including "foo.h", the foo_widget variable will be known (and expected to be defined somewhere else - in foo.c here - ).
To be more precise ... Any piece of code that needs to make use of foo_widget must have the line extern int (*foo_widget)(void)[2]; somewhere, be it in an included header (the smarter way) or just a line at the beginning of the .c file.
Of course, if you cannot know in advance the number of widgets you can have, you are likely to need a dynamic data structure such as a linked list or a tree (ordered and balanced if possible ;) ) to store them. The extern variable is likely to be a regular pointer dynamically allocated when needed. But the definition line is still needed, so in this case you might have something like struct my_struct *pointer = NULL; in the appropriate source file.
Note : I took the freedom to replace your struct Widget with a common function pointer to get the initialization simpler.

Can I re-define a function or check if it exists?

I have a question about (re-)defining functions. My goal is to have a script where I can choose to define a function or not.
Like this:
void func(){}
int main(){
if (func)func();
}
AND without the function, just:
int main(){
if (func)func();
}
Anybody an idea?
You can do this in GCC using its weak function attribute extension:
void func() __attribute__((weak)); // weak declaration must always be present
int main() {
if (func) func();
// ...
}
// optional definition:
void func() { ... }
This works even if func() is defined in another .c file or a library.
Something like this, I think. Haven't used function pointers much, so I may have gotten the syntax slightly wrong.
void func()
{
#define FUNC_PRESENT
// code
}
void (*funcptr)();
#ifdef FUNC_PRESENT
funcptr = func;
#else
funcptr = NULL;
#endif
int main()
{
if (funcptr)
funcptr();
}
Use function pointers, set them dynamically based on runtime conditions, and check for null pointers or wrap them in methods that do that check for you.
Only option in C I can think of.
In C++ you could combine templates and DLLs to dynamically define at runtime.
Really the only way that you can "choose to define a function or not" is with C preprocessor directives. For example:
#ifdef some_name
void func() {
do_whatever();
}
#else
//the else part is optional
#endif
To set these "variables" you use #define some_name
The trouble is, all of this needs to be done at compile time (before that, actually) so it can't be done with an if statement like in your example. If you want an if statement to control your program flow, just use it and don't bother with trying to rename functions or using function pointers or something.
Introduction
I guess that you are trying to do this:
Two modules, a.o and b.o
b.o contains a definition for void foo()
a.o calls void foo() only if b.o is also linked into the final executable.
This could be useful for a "plugin" system.
Variation 1
You can simulate it using function pointers. I don't know enough C to write this in proper C code, but pseudocode looks like this:
a.h
extern collectionOfFuncPtrs_t list;
int addFuncPtr();
a.c
#include "a.h"
collectionOfFuncPtrs_t list;
int addFuncPtr(FuncPtr p) {
- add func ptr to list
- return 0
}
int main() {
- loop through list of function pointers
- call functions through them
}
b.c
#include "a.h"
void bar() { /* ... */ }
static int dummy = addFuncPtr(&bar);
c.c
#include "a.h"
void ayb() { /* ... */ }
static int dummy = addFuncPtr(&ayb);
Conclusion
Now, you can link in b.o and/or c.o as you wish, and int main() will only call bar() and/or ayb() if they exist.
Variation 2
Experiment with variations on this theme if it looks like it may be useful to you. In particular, if you have only a specific number of conditionally-defined functions, you could use a bunch of individual function pointers rather than some list:
a.h
extern fptr_t bar_ptr, ayb_ptr;
a.c
#include "a.h"
int main() {
if (bar_ptr)
bar_ptr();
if (ayb_ptr)
ayb_ptr();
}
b.c
#include "a.h"
void bar() { /* ... */ }
fptr_t bar_ptr = &bar;
b_dummy.c
#include "a.h"
fptr_t bar_ptr = 0;
c.c
#include "a.h"
void ayb() { /* ... */ }
fptr_t ayb_ptr = &ayb;
c_dummy.c
#include "a.h"
fptr_t ayb_ptr = 0;
Conclusion
Now either link b.o or b_dummy.o; and either link c.o or c_dummy.o.
I hope you get the general idea, anyway...!
Bootnote
This is a lot easier in C++ where you can write a module registration system very easily with std::maps and constructors.
In C? Only by using the preprocessor as stated in other answers.
C isn't a dynamic language like, say, Python.
The right way to do what I think you're asking about in C is to use function pointers. You can take the address of a function, assign it to a variable, test it for nil, etc. However, plain old C isn't a very dynamic language; you might be better off using a different language.
if you don't mind compiler specific extension, you can use __if_exists:
#include <iostream>
using namespace std;
// uncomment the following, and it'll still work
void maybeFunc(){ cout << "running maybe" << endl; }
int main(){
cout << "hi!" << endl;
__if_exists(maybeFunc)
cout << "maybe exists!" << endl;
maybeFunc();
}
}
this works in msvc by default, and in clang if you use the -fms-extensions flag.

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