I know its basic but I am not familiar with C and I couldn't understand the answers here in the subject .
Inside a C file I have this functions :
void uart_event_handle(app_uart_evt_t * p_event)
{
}
static void uart_init(void)
{
}
void initialize()
{
uart_init();
}
The static function uart_init() was inside some example program main.c , i am trying to put it inside another C file ( this one)
The error occurs only when I call : uart_init(); . Otherwise it will not happen.
Declaring a function static makes it invisible outside the translation unit. This is similar to declaring fields private in a class, because static "hides" the function from all other files.
This lets you define a new function with the same name in some other file without worrying about name collisions. At the same time, static makes it impossible to call the function from outside the .c file where it is defined.
Your example provides a use case for making static functions: initialize becomes part of the "public" interface of the library, while uart_init remains hidden.
Related
My understanding is that a local static variable has "program scope"; that is, once defined it lives until the program terminates. Furthermore, my understanding is that local statics are also accessible outside the current module/C-file they are defined inside of:
void doSomething() {
static int myVar = 5; // Program scope & accessible to other modules
}
My understanding is that global static variables also have program scope, but that they are only visible to the current module/C-file:
static int myVar = 5; // Program scope but "module private"
void doSomething() {
// Whatever here...
}
Isn't this a bit backwards? I would have expected it to be the other way around. So I ask: is my understanding above correct? If so, then what is the motivation for local statics to be accessible outside their current module, but not global statics?
You messed up all. Static local variable is a place to hide data from other functions but retain data for future calls of that function.
Static global variables have
static storage duration (Retain data for future call)
internal linkage (variable is restricted to single file) and
file scope (Only in the current file the variable can be referenced)
Static local variables have
static storage duration
no linkage
block scope
Nope. Local Static variables are accessible with in given module.
While Global static variables are accessible with in given C-file.
Local static variable has limited scope to the function only.As you can see, following code gives compilation error because it tries to access "prox" variable from main even though it is static.
int main(void)
{
test();
printf("The PROX is : %d", prox);
}
test()
{
static prox;
prox=prox+5;
printf("INSIDE prox[%d]",prox);
}
Is there an mechanism or trick to run a function when a program loads?
What I'm trying to achieve...
void foo(void)
{
}
register_function(foo);
but obviously register_function won't run.
so a trick in C++ is to use initialization to make a function run
something like
int throwaway = register_function(foo);
but that doesn't work in C. So I'm looking for a way around this using standard C (nothing platform / compiler specific )
If you are using GCC, you can do this with a constructor function attribute, eg:
#include <stdio.h>
void foo() __attribute__((constructor));
void foo() {
printf("Hello, world!\n");
}
int main() { return 0; }
There is no portable way to do this in C, however.
If you don't mind messing with your build system, though, you have more options. For example, you can:
#define CONSTRUCTOR_METHOD(methodname) /* null definition */
CONSTRUCTOR_METHOD(foo)
Now write a build script to search for instances of CONSTRUCTOR_METHOD, and paste a sequence of calls to them into a function in a generated .c file. Invoke the generated function at the start of main().
Standard C does not support such an operation. If you don't wish to use compiler specific features to do this, then your next best bet might be to create a global static flag that is initialized to false. Then whenever someone invokes one of your operations that require the function pointer to be registered, you check that flag. If it is false you register the function then set the flag to true. Subsequent calls then won't have to perform the registration. This is similar to the lazy instantiation used in the OO Singleton design pattern.
There is no standard way of doing this although gcc provides a constructor attribute for functions.
The usual way of ensuring some pre-setup has been done (other than a simple variable initialization to a compile time value) is to make sure that all functions requiring that pre-setup. In other words, something like:
static int initialized = 0;
static int x;
int returnX (void) {
if (!initialized) {
x = complicatedFunction();
initialized = 1;
}
return x;
}
This is best done in a separate library since it insulates you from the implementation.
I have a callback inside a C DLL (static void __stdcall) . I want another program to register it as such (by passing it the func ptr) and then call the calback inside the DLL. I have had no luck so far. However, the same callback works if its inside a regular C++ program. I am now wondering if having callbacks in a DLL is even possible. Any help will be appreciated!
Thanks.
Adding some code:
C# app:
[DllImport("DLLfilename.dll")]
public static extern void DLL_SetCallback(CallbackDelegate pfn);
public delegate void CallbackDelegate();
//setDelegate() is called in init() of the C# app
public void setDelegate()
{
CallbackDelegate CallbackDelegateInstance = new CallbackDelegate(callback);
DLL_SetCallback(CallbackDelegateInstance);
}
public void callback()
{
//This is the function which will be called by the DLL
MessageBox.Show("Called from the DLL..");
}
C DLL:
//is linked to externalLibrary.lib
#include "externalLibrary.h"
typedef void (__stdcall CallbackFunc)(void);
CallbackFunc* func; //global in DLL
//Exported
extern "C" __declspec(dllexport) void DLL_SetCallback(CallbackFunc* funcptr)
{
//setting the function pointer
func = funcptr;
return;
}
//Exported
extern "C" __declspec(dllexport) void RegisterEventHandler(Target, Stream,&ProcessEvent , NULL)
{
//ProcessEvent is func to be caled by 3rd party callback
//Call third-party function to register &ProcessEvent func-ptr (succeeds)
...
return;
}
//This is the function which never gets called from the 3rd party callback
//But gets called when all the code in the DLL is moved to a standard C program.
void __stdcall ProcessEvent (params..)
{
//Do some work..
func(); //Call the C# callback now
return;
}
Your question is a little confusing. Are you saying that you have a non-exported function within a DLL, which you wish to take the address of and pass to some external code, which will call it back? That's perfectly reasonable to do. If it's not working, here are the things to look at.
1) Make sure that the calling convention is correct on the definition of the function, the type of the function pointer within your DLL, and the type of the function pointer as declared in the external code.
2) Within your DLL, attempt to call the callback through the same function pointer that you're passing to the external code.
3) If (1) is correct, and (2) works, then fire up your debugger, put a breakpoint on the line where you're trying to call the callback from external code, and then drop into disassembly. Step through the call and see where it ends up.
I am writing a large C program for embedded use. Every module in this program has an init() function (like a constructor) to set up its static variables.
The problem is that I have to remember to call all of these init functions from main(). I also have to remember to put them back if I have commented them out for some reason.
Is there anything clever I do to make sure that all of these functions are getting called? Something along the lines of putting a macro in each init function that, when you call a check_inited() function later, sends a warning to STDOUT if not all the functions are called.
I could increment a counter, but I'd have to maintain the correct number of init functions somewhere and that is also prone to error.
Thoughts?
The following is the solution I decided on, with input from several people in this thread
My goal is to make sure that all my init functions are actually being called. I want to do
this without maintaining lists or counts of modules across several files. I can't call
them automatically as Nick D suggested because they need to be called in a certain order.
To accomplish this, a macro included in every module uses the gcc constructor attribute to
add the init function name to a global list.
Another macro included in the body of the init function updates the global list to make a
note that the function was actually called.
Finally, a check function is called in main() after all of the inits are done.
Notes:
I chose to copy the strings into an array. This not strictly necessary because the
function names passed will always be static strings in normal usage. If memory was short
you could just store a pointer to the string that was passed in.
My reusable library of utility functions is called "nx_lib". Thus all the 'nxl' designations.
This isn't the most efficient code in the world but it's only called a boot time so that
doesn't matter for me.
There are two lines of code that need to be added to each module. If either is omitted,
the check function will let you know.
you might be able to make the constructor function static, which would avoid the need to give it a name that is unique across the project.
this code is only lightly tested and it's really late so please check carefully before trusting it.
Thank you to:
pierr who introduced me to the constructor attribute.
Nick D for demonstrating the ## preprocessor trick and giving me the framework.
tod frye for a clever linker-based approach that will work with many compilers.
Everyone else for helping out and sharing useful tidbits.
nx_lib_public.h
This is the relevant fragment of my library header file
#define NX_FUNC_RUN_CHECK_NAME_SIZE 20
typedef struct _nxl_function_element{
char func[NX_FUNC_RUN_CHECK_NAME_SIZE];
BOOL called;
} nxl_function_element;
void nxl_func_run_check_add(char *func_name);
BOOL nxl_func_run_check(void);
void nxl_func_run_check_hit(char *func_name);
#define NXL_FUNC_RUN_CHECK_ADD(function_name) \
void cons_ ## function_name() __attribute__((constructor)); \
void cons_ ## function_name() { nxl_func_run_check_add(#function_name); }
nxl_func_run_check.c
This is the libary code that is called to add function names and check them later.
#define MAX_CHECKED_FUNCTIONS 100
static nxl_function_element m_functions[MAX_CHECKED_FUNCTIONS];
static int m_func_cnt = 0;
// call automatically before main runs to register a function name.
void nxl_func_run_check_add(char *func_name)
{
// fail and complain if no more room.
if (m_func_cnt >= MAX_CHECKED_FUNCTIONS) {
print ("nxl_func_run_check_add failed, out of space\r\n");
return;
}
strncpy (m_functions[m_func_cnt].func, func_name,
NX_FUNC_RUN_CHECK_NAME_SIZE);
m_functions[m_func_cnt].func[NX_FUNC_RUN_CHECK_NAME_SIZE-1] = 0;
m_functions[m_func_cnt++].called = FALSE;
}
// call from inside the init function
void nxl_func_run_check_hit(char *func_name)
{
int i;
for (i=0; i< m_func_cnt; i++) {
if (! strncmp(m_functions[i].func, func_name,
NX_FUNC_RUN_CHECK_NAME_SIZE)) {
m_functions[i].called = TRUE;
return;
}
}
print("nxl_func_run_check_hit(): error, unregistered function was hit\r\n");
}
// checks that all registered functions were called
BOOL nxl_func_run_check(void) {
int i;
BOOL success=TRUE;
for (i=0; i< m_func_cnt; i++) {
if (m_functions[i].called == FALSE) {
success = FALSE;
xil_printf("nxl_func_run_check error: %s() not called\r\n",
m_functions[i].func);
}
}
return success;
}
solo.c
This is an example of a module that needs initialization
#include "nx_lib_public.h"
NXL_FUNC_RUN_CHECK_ADD(solo_init)
void solo_init(void)
{
nxl_func_run_check_hit((char *) __func__);
/* do module initialization here */
}
You can use gcc's extension __attribute__((constructor)) if gcc is ok for your project.
#include <stdio.h>
void func1() __attribute__((constructor));
void func2() __attribute__((constructor));
void func1()
{
printf("%s\n",__func__);
}
void func2()
{
printf("%s\n",__func__);
}
int main()
{
printf("main\n");
return 0;
}
//the output
func2
func1
main
I don't know how ugly the following looks but I post it anyway :-)
(The basic idea is to register function pointers, like what atexit function does.
Of course atexit implementation is different)
In the main module we can have something like this:
typedef int (*function_t)(void);
static function_t vfunctions[100]; // we can store max 100 function pointers
static int vcnt = 0; // count the registered function pointers
int add2init(function_t f)
{
// todo: error checks
vfunctions[vcnt++] = f;
return 0;
}
...
int main(void) {
...
// iterate vfunctions[] and call the functions
...
}
... and in some other module:
typedef int (*function_t)(void);
extern int add2init(function_t f);
#define M_add2init(function_name) static int int_ ## function_name = add2init(function_name)
int foo(void)
{
printf("foo\n");
return 0;
}
M_add2init(foo); // <--- register foo function
Why not write a post processing script to do the checking for you. Then run that script as part of your build process... Or better yet, make it one of your tests. You are writing tests, right? :)
For example, if each of your modules has a header file, modX.c. And if the signature of your init() function is "void init()"...
Have your script grep through all your .h files, and create a list of module names that need to be init()ed. Then have the script check that init() is indeed called on each module in main().
If your single module represents "class" entity and has instance constructor, you can use following construction:
static inline void init(void) { ... }
static int initialized = 0;
#define INIT if (__predict_false(!initialized)) { init(); initialized = 1; }
struct Foo *
foo_create(void)
{
INIT;
...
}
where "__predict_false" is your compiler's branch prediction hint. When first object is created, module is auto-initialized (for once).
Splint (and probably other Lint variants) can give a warning about functions that are defined but not called.
It's interesting that most compilers will warn you about unused variables, but not unused functions.
Larger running time is not a problem
You can conceivably implement a kind of "state-machine" for each module, wherein the actions of a function depend on the state the module is in. This state can be set to BEFORE_INIT or INITIALIZED.
For example, let's say we have module A with functions foo and bar.
The actual logic of the functions (i.e., what they actually do) would be declared like so:
void foo_logic();
void bar_logic();
Or whatever the signature is.
Then, the actual functions of the module (i.e., the actual function declared foo()) will perform a run-time check of the condition of the module, and decide what to do:
void foo() {
if (module_state == BEFORE_INIT) {
handle_not_initialized_error();
}
foo_logic();
}
This logic is repeated for all functions.
A few things to note:
This will obviously incur a huge penalty performance-wise, so is
probably not a good idea (I posted
anyway because you said runtime is
not a problem).
This is not a real state-machine, since there are only two states which are checked using a basic if, without some kind of smart general logic.
This kind of "design-pattern" works great when you're using separate threads/tasks, and the functions you're calling are actually called using some kind of IPC.
A state machine can be nicely implemented in C++, might be worth reading up on it. The same kind of idea can conceivably be coded in C with arrays of function pointers, but it's almost certainly not worth your time.
you can do something along these lines with a linker section. whenever you define an init function, place a pointer to it in a linker section just for init function pointers. then you can at least find out how many init functions have been compiled.
and if it does not matter what order the init functions are called, and the all have the same prototype, you can just call them all in a loop from main.
the exact details elude my memory, but it works soemthing like this::
in the module file...
//this is the syntax in GCC..(or would be if the underscores came through in this text editor)
initFuncPtr thisInit __attribute((section(.myinits)))__= &moduleInit;
void moduleInit(void)
{
// so init here
}
this places a pointer to the module init function in the .myinits section, but leaves the code in the .code section. so the .myinits section is nothing but pointers. you can think of this as a variable length array that module files can add to.
then you can access the section start and end address from the main. and go from there.
if the init functions all have the same protoytpe, you can just iterate over this section, calling them all.
this, in effect, is creating your own static constructor system in C.
if you are doing a large project and your linker is not at least this fully featured, you may have a problem...
Can I put up an answer to my question?
My idea was to have each function add it's name to a global list of functions, like Nick D's solution.
Then I would run through the symbol table produced by -gstab, and look for any functions named init_* that had not been called.
This is an embedded app so I have the elf image handy in flash memory.
However I don't like this idea because it means I always have to include debugging info in the binary.
What's the best way to create a singleton in C? A concurrent solution would be nice.
I am aware that C isn't the first language you would use for a singleton.
First, C is not suitable for OO programming. You'd be fighting all the way if you do. Secondly, singletons are just static variables with some encapsulation. So you can use a static global variable. However, global variables typically have far too many ills associated with them. You could otherwise use a function local static variable, like this:
int *SingletonInt() {
static int instance = 42;
return &instance;
}
or a smarter macro:
#define SINGLETON(t, inst, init) t* Singleton_##t() { \
static t inst = init; \
return &inst; \
}
#include <stdio.h>
/* actual definition */
SINGLETON(float, finst, 4.2);
int main() {
printf("%f\n", *(Singleton_float()));
return 0;
}
And finally, remember, that singletons are mostly abused. It is difficult to get them right, especially under multi-threaded environments...
You don't need to. C already has global variables, so you don't need a work-around to simulate them.
It's the same as the C++ version pretty much. Just have a function that returns an instance pointer. It can be a static variable inside the function. Wrap the function body with a critical section or pthread mutex, depending on platform.
#include <stdlib.h>
struct A
{
int a;
int b;
};
struct A* getObject()
{
static struct A *instance = NULL;
// do lock here
if(instance == NULL)
{
instance = malloc(sizeof(*instance));
instance->a = 1;
instance->b = 2;
}
// do unlock
return instance;
};
Note that you'd need a function to free up the singleton too. Especially if it grabs any system resources that aren't automatically released on process exit.
EDIT: My answer presumes the singleton you are creating is somewhat complex and has a multi-step creation process. If it's just static data, go with a global like others have suggested.
A singleton in C will be very weird . . . I've never seen an example of "object oriented C" that looked particularly elegant. If possible, consider using C++. C++ allows you to pick and choose which features you want to use, and many people just use it as a "better C".
Below is a pretty typical pattern for lock-free one-time initialization. The InterlockCompareExchangePtr atomically swaps in the new value if the previous is null. This protects if multiple threads try to create the singleton at the same time, only one will win. The others will delete their newly created object.
MyObj* g_singleton; // MyObj is some struct.
MyObj* GetMyObj()
{
MyObj* singleton;
if (g_singleton == NULL)
{
singleton = CreateNewObj();
// Only swap if the existing value is null. If not on Windows,
// use whatever compare and swap your platform provides.
if (InterlockCompareExchangePtr(&g_singleton, singleton, NULL) != NULL)
{
DeleteObj(singleton);
}
}
return g_singleton;
}
DoSomethingWithSingleton(GetMyObj());
Here's another perspective: every file in a C program is effectively a singleton class that is auto instantiated at runtime and cannot be subclassed.
Global static variables are your private class members.
Global non static are public (just declare them using extern in some header file).
Static functions are private methods
Non-static functions are the public ones.
Give everything a proper prefix and now you can use my_singleton_method() in lieu of my_singleton.method().
If your singleton is complex you can write a generate_singleton() method to initialize it before use, but then you need to make sure all the other public methods check if it was called and error out if not.
I think this solution might be the simplest and best for most use cases...
In this example, I am creating a single instance global dispatch queue, which you'd definitely do, say, if you were tracking dispatch source events from multiple objects; in that case, every object listening to the queue for events could be notified when a new task is added to the queue. Once the global queue is set (via queue_ref()), it can be referenced with the queue variable in any file in which the header file is included (examples are provided below).
In one of my implementations, I called queue_ref() in AppDelegate.m (main.c would work, too). That way, queue will be initialized before any other calling object attempts to access it. In the remaining objects, I simply called queue. Returning a value from a variable is much faster than calling a function, and then checking the value of the variable before returning it.
In GlobalQueue.h:
#ifndef GlobalQueue_h
#define GlobalQueue_h
#include <stdio.h>
#include <dispatch/dispatch.h>
extern dispatch_queue_t queue;
extern dispatch_queue_t queue_ref(void);
#endif /* GlobalQueue_h */
In GlobalQueue.c:
#include "GlobalQueue.h"
dispatch_queue_t queue;
dispatch_queue_t queue_ref(void) {
if (!queue) {
queue = dispatch_queue_create_with_target("GlobalDispatchQueue", DISPATCH_QUEUE_SERIAL, dispatch_get_main_queue());
}
return queue;
}
To use:
#include "GlobalQueue.h" in any Objective-C or C implementation source file.
Call queue_ref() to use the dispatch queue. Once queue_ref() has been called, the queue can be used via the queue variable in all source files
Examples:
Calling queue_ref():
dispatch_queue_t serial_queue_with_queue_target = dispatch_queue_create_with_target("serial_queue_with_queue_target", DISPATCH_QUEUE_SERIAL, **queue_ref()**);
Calling queue:
dispatch_queue_t serial_queue_with_queue_target = dispatch_queue_create_with_target("serial_queue_with_queue_target", DISPATCH_QUEUE_SERIAL, **queue**));]
Just do
void * getSingleTon() {
static Class object = (Class *)malloc( sizeof( Class ) );
return &object;
}
which works in a concurrent environment too.