I have to write code in C where the user has to have flexibility in choosing any existing DB, write to files, or implement their own storage mechanism. I need wrapper functions that redirect to the right functions corresponding to the storage mechanism selected at runtime or compile time. Say my storage options are FLATFILE and SQLDB and my wrapper function is insert(value). So, if I select FLATFILE as my storage, when I call the wrapper function insert(value), it should in turn call the function that writes to a file. If I choose a SQLDB, insert(value) should call the function that insert the values in the data base.
I know I can somehow use a structure of function pointers to do wrapper functions, but I have no idea how.
Does anyone know of any docs, links, examples, etc I could refer to, to understand and implement something like this? Any pointers will be appreciated. Thanks!
Thanks!
#define BACKEND_FLATFILE 0
#define BACKEND_SQLDB 1
void insert_flatfile(const t_value *v) {
...
}
void insert_sqldb(const t_value *v) {
...
}
void (*insert_functions[]) (const t_value *) = {
insert_flatfile,
insert_sqldb,
};
void insert_wrapper(t_value *v, int backend) {
insert_functions[backend](v);
}
Besides, the different functions for one backend should be stuffed into a struct and you should create an array of such structs instead of one array per wrapper function.
You can use a simple version such as:
struct backend {
int (*insert)(...);
int (*remove)(...);
...
};
static struct backend db_backend = { db_insert, db_remove, ... };
static struct backend other_backend = { other_insert, other_remove, ... };
const struct backend *get_backend(enum backend_type type)
{
switch (type)
{
case DB_BACKEND:
return &db_backend;
case DB_OTHER:
return &db_other;
...
}
}
All of the above can be hidden inside a C file, with get_backend and the enumeration being public. Then you can use it like this:
struct backend *b = get_backend(DB_BACKEND);
b->insert(...);
b->remove(...);
Many details are missing, of course (many people like using typedef, for example). This is a basic setup, you can also create wrapper functions if you don't like the b->insert(...) syntax or if you want to set the back end once and then use insert() and remove() in the code. This is also useful if you already have some code that calls insert() directly and you want to direct the call to the right back end.
If you want a more elaborate solution, have a look at http://www.cs.rit.edu/~ats/books/ooc.pdf. You don't have to implement every last detail from it, but it can give you a few ideas.
Related
I've tried multiple sources for solutions to this problem. They all either require modifying the source code, and architecture specific exploit such as writing in a jmp instruction to detour the function, or using a macro and including the c file. The first one is extremely annoying to deal with, the second is usually not possible due to page protections, and the third introduces a lot of problems with linking multiple files containing different mocks and unit test for the same source file. Is there any better method of doing this?
You can user function pointer in your nominal code. You assign them at init with nominal implemetation in your application. In your unit test you can then assign the function pointer to the mock implmentation. Function pointer is a common practice used to implement interface in C.
Here is a gist of how that could be done:
typedef struct {
void (*method) ();
} interface;
void run(itf *interface)
{
itf->method();
}
void methodImpl()
{
printf("nominal code");
}
void methodMock()
{
printf("mock code");
}
void do_run()
{
interface itf;
itf.method = methodImpl;
run(&itf);
}
void test_run()
{
interface itf;
itf.method = methodMock;
run(&itf);
}
I have a library which provides function calls to a user as below:
int* g_ID = NULL;
void processing(int p1, char p2)
{
int ID = newID();
g_ID = &ID;
callback(p1, p2);
return ID;
}
void SendResponse()
{
sendID(*g_ID);
}
The user sets up its application by registering its callback function with the signature void (f*)(int p1, char p2) and should not have knowledge about the ID used internally the library. So the user space code looks something like:
main()
{
RegisterCallback(HandleRequest);
while (inProgress())
sleep(1); /* just sleep here */
}
void (HandleRequest*)(int val1, char val2)
{
/* ... do something user specific ... */
SendResponse();
return;
}
The problem here is, that the library (handling IDs and g_ID is not thread safe) !! User's callback is invoked asynchronously by other library functions, as threads. Several threads can be executed this way in parallel. But I won't give the user visibility of library internal IDs.
I know the code snippets above are not perfect. There're just to demonstrate my intention ... SendResponse() is not yet implemented ;-).
I hope, someone can give some ideas how to "implement" SendResponse() and to keep thread safety.
You could use a threadlocal here to keep the g_ID, rather than making using a global. This will work in the scenario, as I understand it, that there may be multiple concurrent calls to process() from different threads, but that the process() method is as shown - that the SendResponse() call will only occur within the scope (runtime scope, not lexical) of the callback() method. That is true in the code shown. It could be untrue if HandleRequest did something exotic like kick off another thread an then return (but you could certainly ban that by documentation).
The other, more classic, approach is to encapsulate all the state you care about, like g_ID, into a void *, or opaque_state * or whatever, that you pass to the callback, and then methods like SendRespose() take that as an argument. If you don't like void * you can implement the opaque_state * version without exposing any details of that structure using a forward declaration.
I'm new at C, so sorry for my lack of knowledge (my C-book here is really massive :)
I would like to extend a shared library (libcustomer.so) with closed source, but public known api.
Is something like this possible?
rename libcustomer.so to liboldcustomer.so
create an extended shared library libcustomer.so (so others implicitly use the extended one)
link liboldcustomer.so into my extended libcustomer.so via -loldcustomer
forward any not extra-implemented methods directly to the old "liboldcustomer.so"
I don't think it would work that way (the name is compiled into the .so, isn't it?).
But what's the alternative?
For #4: is there a general way to do this, or do I have to write a method named like the old one and forward the call (how?)?
Because the original libcustomer.so (=liboldcustomer.so) can change from time to time, all that stuff should work dynamically.
For security reasons, our system has no LD_PRELOAD (otherwise I would take that :( ).
Think about extended validation-checks & some better NPE-handlings.
Thanks in advance for your help!
EDIT:
I'm just implementing my extension as shown in the answer, but I have one unhandled case at the moment:
How can I "proxy" the structs from the extended library?
For example I have this:
customer.h:
struct customer;
customer.c:
struct customer {
int children:1;
int age;
struct house *house_config;
};
Now, in my customer-extension.c I am writing all the public methods form customer.c, but how do I "pass-thru" the structs?
Many thanks for your time & help!
So you have OldLib with
void func1();
int func2();
... etc
The step 4 might look like creating another library with some static initialization.
Create NewLib with contents:
void your_func1();
void (*old_func1_ptr)() = NULL;
int (*old_func2_ptr)() = NULL;
void func1()
{
// in case you don't have static initializers, implement lazy loading
if(!old_func1_ptr)
{
void* lib = dlopen("OldLibFileName.so", RTLD_NOW);
old_func1_ptr = dlsym(lib, "func1");
}
old_func1_ptr();
}
int func2()
{
return old_func2_ptr();
}
// gcc extension, static initializer - will be called on .so's load
// If this is not supported, then you should call this function
// manually after loading the NewLib.so in your program.
// If the user of OldLib.so is not _your_ program,
// then implement lazy-loading in func1, func2 etc. - check function pointers for being NULL
// and do the dlopen/dlsym calls there.
__attribute__((constructor))
void static_global_init()
{
// use dlfcn.h
void* lib = dlopen("OldLibFileName.so", RTLD_NOW);
old_func1_ptr = dlsym(lib, "func1");
...
}
The static_global_init and all the func_ptr's can be autogenerated if you have some description of the old API. After the NewLib is created, you certainly can replace the OldLib.
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.