The problem requires implementing a ring buffer into which a producer writes and from which a consumer reads. I have done this for a data type. I want to extend this so that it will work for any primitive data type but have not been able to figure out a good way to do this. I want the program to take inputs from the command line like so "program_name data_type size_of_buffer".
I could templatize the buffer.start pointer and pass the data type around but I don't know of a way to assign the data type name to a variable. Anyone have any ideas?
struct buffer{
int * start;
int size;
}buffer;
int * producer=NULL;
int * consumer=NULL;
bool donewriting;
bool sleeping;
void *mywrite(void *);
void *myread(void *);
void *mywrite(void * ){
do{
cout<<"In Thread 1"<<endl;
static int x=0;
*producer=x;
cout<<"Write thread: wrote value "<<x<<" into buffer"<<endl;
producer++;
if(producer==buffer.start+10)
{ producer=buffer.start;
donewriting=true;
}
if(x==5)
{
cout<<"Thread 1 going to sleep"<<endl;
Sleep(2000);
}
x++;
} while(producer!=buffer.start);
}
void *myread(void *){
while(!donewriting)
{ //cout<<"In Thread 2"<<endl;
if(consumer<producer)
{ cout<<"Read thread: read value "<<*consumer<<" from buffer"<<endl;
consumer++;
}
}
}
int main()
{
buffer.size=10;
buffer.start=new int(10);
producer=buffer.start;
consumer=buffer.start;
donewriting=false;
cout<<"In main"<<endl;
pthread_t writeThread,readThread;
pthread_create(&writeThread,NULL,mywrite,NULL);
pthread_create(&readThread,NULL,myread,NULL);
pthread_join(writeThread,NULL);
pthread_join(readThread,NULL);
return 0;
}
Unfortunately, your goal is not possible given the scenario you have described. Internally, C++ programs compile templated code for each datatype used with the template. In other words, if you have a templated function x() and you call
x<int>();
x<float>();
the compiler will produce two copies of the templated function: one that uses the int datatype and another that uses the float datatype. Before you call the function, no code is produced at all. Therefore, you cannot build the program and pass arbitrary data types to it.
Of course, if the command-line call is only intended as a test for the code, and you'll use it in a library in future, you're ok - but you'll still have to specify the type you're using in your code.
The current way you're doing this, using void * is probably the best way.
It might be better to specify the size in bytes of the type you wish to use, rather than the specific type itself. As long as your final producer and consumer share knowledge of the type in question, you'll be fine.
Related
Given the following:
class ReadWrite {
public:
int Read(size_t address);
void Write(size_t address, int val);
private:
std::map<size_t, int> db;
}
In read function when accessing an address which no previous write was made to I want to either throw exception designating such error or allow that and return 0, in other words I would like to either use std::map<size_t, int>::operator[]() or std::map<size_t, int>::at(), depending on some bool value which user can set. So I add the following:
class ReadWrite {
public:
int Read(size_t add) { if (allow) return db[add]; return db.at(add);}
void Write(size_t add, int val) { db[add] = val; }
void Allow() { allow = true; }
private:
bool allow = false;
std::map<size_t, int> db;
}
The problem with that is:
Usually, the program will have one call of allow or none at the beginning of the program and then afterwards many accesses. So, performance wise, this code is bad because it every-time performs the check if (allow) where usually it's either always true or always false.
So how would you solve such problem?
Edit:
While the described use case (one or none Allow() at first) of this class is very likely it's not definite and so I must allow user call Allow() dynamically.
Another Edit:
Solutions which use function pointer: What about the performance overhead incurred by using function pointer which is not able to make inline by the compiler? If we use std::function instead will that solve the issue?
Usually, the program will have one call of allow or none at the
beginning of the program and then afterwards many accesses. So,
performance wise, this code is bad because it every-time performs the
check if (allow) where usually it's either always true or always
false. So how would you solve such problem?
I won't, The CPU will.
the Branch Prediction will figure out that the answer is most likely to be same for some long time so it will able to optimize the branch in the hardware level very much. it will still incur some overhead, but very negligible.
If you really need to optimize your program, I think your better use std::unordered_map instead of std::map, or move to some faster map implementation, like google::dense_hash_map. the branch is insignificant compared to map-lookup.
If you want to decrease the time-cost, you have to increase the memory-cost. Accepting that, you can do this with a function pointer. Below is my answer:
class ReadWrite {
public:
void Write(size_t add, int val) { db[add] = val; }
// when allowed, make the function pointer point to read2
void Allow() { Read = &ReadWrite::read2;}
//function pointer that points to read1 by default
int (ReadWrite::*Read)(size_t) = &ReadWrite::read1;
private:
int read1(size_t add){return db.at(add);}
int read2(size_t add) {return db[add];}
std::map<size_t, int> db;
};
The function pointer can be called as the other member functions. As an example:
ReadWrite rwObject;
//some code here
//...
rwObject.Read(5); //use of function pointer
//
Note that non-static data member initialization is available with c++11, so the int (ReadWrite::*Read)(size_t) = &ReadWrite::read1; may not compile with older versions. In that case, you have to explicitly declare one constructor, where the initialization of the function pointer can be done.
You can use a pointer to function.
class ReadWrite {
public:
void Write(size_t add, int val) { db[add] = val; }
int Read(size_t add) { (this->*Rfunc)(add); }
void Allow() { Rfunc = &ReadWrite::Read2; }
private:
std::map<size_t, int> db;
int Read1(size_t add) { return db.at(add); }
int Read2(size_t add) { return db[add]; }
int (ReadWrite::*Rfunc)(size_t) = &ReadWrite::Read1;
}
If you want runtime dynamic behaviour you'll have to pay for it at runtime (at the point you want your logic to behave dynamically).
You want different behaviour at the point where you call Read depending on a runtime condition and you'll have to check that condition.
No matter whether your overhad is a function pointer call or a branch, you'll find a jump or call to different places in your program depending on allow at the point Read is called by the client code.
Note: Profile and fix real bottlenecks - not suspected ones. (You'll learn more if you profile by either having your suspicion confirmed or by finding out why your assumption about the performance was wrong.)
I'm using a C library where the user is supposed to register a int (*functionPtr)(int,int); callback.
Then, some file is processed, and during the file processing the user receives callback with the previously given function, up to thousands for a single file processing.
My question is, how to manage state for this kind of worflow?
For instance I'd like to count how many times the callback was called. The only way I'm thinking of doing this is :
int c = 0; //global variable
int callback(int i, int j) {
++c;
}
Which is basically not very pretty and forces me to manage global variables.
Is there a way to do this effeciently in C++ ? In my case I do not have access to C++11
The standard way is to support a context pointer:
void myAPI(void (*f)(void *, int, int), void *context) {
...
// Call the callback passing the provided context
f(context, x, y);
...
}
The users can then recover custom data from the context if needed... for example
void inc_counter(void *context, int x, int y) {
*((int *)context) += 1;
}
void foo() {
int count = 0;
myAPI(inc_counter, &count);
}
In other words the idea is to add an opaque void * parameter to the callback interface and also a void * value to pass in that parameter when invoking the function.
This doesn't add any coupling but works around the problem that C has no closures.
I have a this function to read 1d arrays from an unformatted fortran file:
template <typename T>
void Read1DArray(T* arr)
{
unsigned pre, post;
file.read((char*)&pre, PREPOST_DATA);
for(unsigned n = 0; n < (pre/sizeof(T)); n++)
file.read((char*)&arr[n], sizeof(T));
file.read((char*)&post, PREPOST_DATA);
if(pre!=post)
std::cout << "Failed read fortran 1d array."<< std::endl;
}
I call this like so:
float* new_array = new float[sizeof_fortran_array];
Read1DArray(new_array);
Assume Read1DArray is part of a class, which contains an ifstream named 'file', and sizeof_fortran_array is already known. (And for those not quite so familiar with fortran unformatted writes, the 'pre' data indicates how long the array is in bytes, and the 'post' data is the same)
My issue is that I have a scenario where I may want to call this function with either a float* or a double*, but this will not be known until runtime.
Currently what I do is simply have a flag for which data type to read, and when reading the array I duplicate the code something like this, where datatype is a string set at runtime:
if(datatype=="float")
Read1DArray(my_float_ptr);
else
Read1DArray(my_double_ptr);
Can someone suggest a method of rewriting this so that I dont have to duplicate the function call with the two types? These are the only two types it would be necessary to call it with, but I have to call it a fair few times and I would rather not have this duplication all over the place.
Thanks
EDIT:
In response to the suggestion to wrap it in a call_any_of function, this wouldnt be enough because at times I do things like this:
if(datatype=="float")
{
Read1DArray(my_float_ptr);
Do_stuff(my_float_ptr);
}
else
{
Read1DArray(my_double_ptr);
Do_stuff(my_double_ptr);
}
// More stuff happening in between
if(datatype=="float")
{
Read1DArray(my_float_ptr);
Do_different_stuff(my_float_ptr);
}
else
{
Read1DArray(my_double_ptr);
Do_different_stuff(my_double_ptr);
}
If you think about the title you will realize that there is a contradiction in that the template instantiation is performed at compile time but you want to dispatch based on information available only at runtime. At runtime you cannot instantiate a template, so that is impossible.
The approach you have taken is actually the right one: instantiate both options at compile time, and decide which one to use at runtime with the available information. That being said you might want to think your design.
I imagine that not only reading but also processing will be different based on that runtime value, so you might want to bind all the processing in a (possibly template) function for each one of the types and move the if further up the call hierarchy.
Another approach to avoid having to dispatch based on type to different instantiations of the template would be to loose some of the type safety and implement a single function that takes a void* to the allocated memory and a size argument with the size of the type in the array. Note that this will be more fragile, and it does not solve the overall problem of having to act on the different arrays after the data is read, so I would not suggest following this path.
Because you don't know which code path to take until runtime, you'll need to set up some kind of dynamic dispatch. Your current solution does this using an if-else which must be copied and pasted everywhere it is used.
An improvement would be to generate a function that performs the dispatch. One way to achieve this is by wrapping each code path in a member function template, and using an array of member function pointers that point to specialisations of that member function template. [Note: This is functionally equivalent to dynamic dispatch using virtual functions.]
class MyClass
{
public:
template <typename T>
T* AllocateAndRead1DArray(int sizeof_fortran_array)
{
T* ptr = new T[sizeof_fortran_array];
Read1DArray(ptr);
return ptr;
}
template <typename T>
void Read1DArrayAndDoStuff(int sizeof_fortran_array)
{
Do_stuff(AllocateAndRead1DArray<T>(sizeof_fortran_array));
}
template <typename T>
void Read1DArrayAndDoOtherStuff(int sizeof_fortran_array)
{
Do_different_stuff(AllocateAndRead1DArray<T>(sizeof_fortran_array));
}
// map a datatype to a member function that takes an integer parameter
typedef std::pair<std::string, void(MyClass::*)(int)> Action;
static const int DATATYPE_COUNT = 2;
// find the action to perform for the given datatype
void Dispatch(const Action* actions, const std::string& datatype, int size)
{
for(const Action* i = actions; i != actions + DATATYPE_COUNT; ++i)
{
if((*i).first == datatype)
{
// perform the action for the given size
return (this->*(*i).second)(size);
}
}
}
};
// map each datatype to an instantiation of Read1DArrayAndDoStuff
MyClass::Action ReadArrayAndDoStuffMap[MyClass::DATATYPE_COUNT] = {
MyClass::Action("float", &MyClass::Read1DArrayAndDoStuff<float>),
MyClass::Action("double", &MyClass::Read1DArrayAndDoStuff<double>),
};
// map each datatype to an instantiation of Read1DArrayAndDoOtherStuff
MyClass::Action ReadArrayAndDoOtherStuffMap[MyClass::DATATYPE_COUNT] = {
MyClass::Action("float", &MyClass::Read1DArrayAndDoOtherStuff<float>),
MyClass::Action("double", &MyClass::Read1DArrayAndDoOtherStuff<double>),
};
int main()
{
MyClass object;
// call MyClass::Read1DArrayAndDoStuff<float>(33)
object.Dispatch(ReadArrayAndDoStuffMap, "float", 33);
// call MyClass::Read1DArrayAndDoOtherStuff<double>(542)
object.Dispatch(ReadArrayAndDoOtherStuffMap, "double", 542);
}
If performance is important, and the possible set of types is known at compile time, there are a few further optimisations that could be performed:
Change the string to an enumeration that represents all the possible data types and index the array of actions by that enumeration.
Give the Dispatch function template parameters that allow it to generate a switch statement to call the appropriate function.
For example, this can be inlined by the compiler to produce code that is (generally) more optimal than both the above example and the original if-else version in your question.
class MyClass
{
public:
enum DataType
{
DATATYPE_FLOAT,
DATATYPE_DOUBLE,
DATATYPE_COUNT
};
static MyClass::DataType getDataType(const std::string& datatype)
{
if(datatype == "float")
{
return MyClass::DATATYPE_FLOAT;
}
return MyClass::DATATYPE_DOUBLE;
}
// find the action to perform for the given datatype
template<typename Actions>
void Dispatch(const std::string& datatype, int size)
{
switch(getDataType(datatype))
{
case DATATYPE_FLOAT: return Actions::FloatAction::apply(*this, size);
case DATATYPE_DOUBLE: return Actions::DoubleAction::apply(*this, size);
}
}
};
template<void(MyClass::*member)(int)>
struct Action
{
static void apply(MyClass& object, int size)
{
(object.*member)(size);
}
};
struct ReadArrayAndDoStuff
{
typedef Action<&MyClass::Read1DArrayAndDoStuff<float>> FloatAction;
typedef Action<&MyClass::Read1DArrayAndDoStuff<double>> DoubleAction;
};
struct ReadArrayAndDoOtherStuff
{
typedef Action<&MyClass::Read1DArrayAndDoOtherStuff<float>> FloatAction;
typedef Action<&MyClass::Read1DArrayAndDoOtherStuff<double>> DoubleAction;
};
int main()
{
MyClass object;
// call MyClass::Read1DArrayAndDoStuff<float>(33)
object.Dispatch<ReadArrayAndDoStuff>("float", 33);
// call MyClass::Read1DArrayAndDoOtherStuff<double>(542)
object.Dispatch<ReadArrayAndDoOtherStuff>("double", 542);
}
I am creating scripting language that first parse the code
and then copy functions (To execute the code) to one buffer\memory as the parsed code.
There is a way to copy function's binary code to buffer and then execute the whole buffer?
I need to execute all the functions at once to get better performance.
To understand my question to best I want to do something like this:
#include <vector>
using namespace std;
class RuntimeFunction; //The buffer to my runtime function
enum ByteCodeType {
Return,
None
};
class ByteCode {
ByteCodeType type;
}
void ReturnRuntime() {
return;
}
RuntimeFunction GetExecutableData(vector<ByteCode> function) {
RuntimeFunction runtimeFunction=RuntimeFunction(sizeof(int)); //Returns int
for (int i = 0 ; i < function.size() ; i++ ) {
#define CurrentByteCode function[i]
if (CurrentByteCode.Type==Return) {
runtimeFunction.Append(&ReturnRuntime);
} //etc.
#undef
}
return runtimeFunction;
}
void* CallFunc(RuntimeFunction runtimeFunction,vector<void*> custom_parameters) {
for (int i=custom_parameters-1;i>=0;--i) { //Invert parameters loop
__asm {
push custom_parameters[i]
}
}
__asm {
call runtimeFunction.pHandle
}
}
There are a number of ways of doing this, depending on how deep you want to get into generating code at runtime, but one relatively simple way of doing it is with threaded code and a threaded code interpreter.
Basically, threaded code consists of an array of function pointers, and the interpreter goes through the array calling each pointed at function. The tricky part is that you generally have each function return the address of array element containing a pointer to the next function to call, which allows you to implement things like branches and calls without any effort in the interpreter
Usually you use something like:
typedef void *(*tc_func_t)(void *, runtime_state_t *);
void *interp(tc_func_t **entry, runtime_state_t *state) {
tc_func_t *pc = *entry;
while (pc) pc = (*pc)(pc+1, state);
return entry+1;
}
That's the entire interpreter. runtime_state_t is some kind of data structure containing some runtime state (usually one or more stacks). You call it by creating an array of tc_func_t function pointers and filling them in with function pointers (and possibly data), ending with a null pointer, and then call interp with the address of a variable containing the start of the array. So you might have something like:
void *add(tc_func_t *pc, runtime_state_t *state) {
int v1 = state->data.pop();
int v2 = state->data.pop();
state->data.push(v1 + v2);
return pc; }
void *push_int(tc_func_t *pc, runtime_state_t *state) {
state->data.push((int)*pc);
return pc+1; }
void *print(tc_func_t *pc, runtime_state_t *state) {
cout << state->data.pop();
return pc; }
tc_func_t program[] = {
(tc_func_t)push_int,
(tc_func_t)2,
(tc_func_t)push_int,
(tc_func_t)2,
(tc_func_t)add,
(tc_func_t)print,
0
};
void run_prgram() {
runtime_state_t state;
tc_func_t *entry = program;
interp(&entry, &state);
}
Calling run_program runs the little program that adds 2+2 and prints the result.
Now you may be confused by the slightly odd calling setup for interp, with an extra level of indirection on the entry argument. That's so that you can use interp itself as a function in a threaded code array, followed by a pointer to another array, and it will do a threaded code call.
edit
The biggest problem with threaded code like this is related to performance -- the threaded coded interpreter is extremely unfriendly to branch predictors, so performance is pretty much locked at one threaded instruction call per branch misprediction recovery time.
If you want more performance, you pretty much have to go to full-on runtime code generation. LLVM provides a good, machine independent interface to doing that, along with pretty good optimizers for common platforms that will produce pretty good code at runtime.
I would like to do something like:
for(int i=0;i<10;i++)
addresses[i] = & function(){ callSomeFunction(i) };
Basically, having an array of addresses of functions with behaviours related to a list of numbers.
If it's possible with external classes like Boost.Lambda is ok.
Edit: after some discussion I've come to conclusion that I wasn't explicit enough. Please read Creating function pointers to functions created at runtime
What I really really want to do in the end is:
class X
{
void action();
}
X* objects;
for(int i=0;i<0xFFFF;i++)
addresses[i] = & function(){ objects[i]->action() };
void someFunctionUnknownAtCompileTime()
{
}
void anotherFunctionUnknowAtCompileTime()
{
}
patch someFunctionUnknownAtCompileTime() with assembly to jump to function at addresses[0]
patch anotherFunctionUnknownAtCompileTime() with assembly to jump to function at addresses[1]
sth, I don't think your method will work because of them not being real functions but my bad in not explaining exactly what I want to do.
If I understand you correctly, you're trying to fill a buffer with machine code generated at runtime and get a function pointer to that code so that you can call it.
It is possible, but challenging. You can use reinterpret_cast<> to turn a data pointer into a function pointer, but you'll need to make sure that the memory you allocated for your buffer is flagged as executable by the operating system. That will involve a system call (LocalAlloc() on Windows iirc, can't remember on Unix) rather than a "plain vanilla" malloc/new call.
Assuming you've got an executable block of memory, you'll have to make sure that your machine code respects the calling convention indicated by the function pointer you create. That means pushing/popping the appropriate registers at the beginning of the function, etc.
But, once you've done that, you should be able to use your function pointer just like any other function.
It might be worth looking at an open source JVM (or Mono) to see how they do it. This is the essence of JIT compilation.
Here is an example I just hacked together:
int func1( int op )
{
printf( "func1 %d\n", op );
return 0;
}
int func2( int op )
{
printf( "func2 %d\n", op );
return 0;
}
typedef int (*fp)(int);
int main( int argc, char* argv[] )
{
fp funcs[2] = { func1, func2 };
int i;
for ( i = 0; i < 2; i++ )
{
(*funcs[i])(i);
}
}
The easiest way should be to create a bunch of boost::function objects:
#include <boost/bind.hpp>
#include <boost/function.hpp>
// ...
std::vector< boost::function<void ()> > functors;
for (int i=0; i<10; i++)
functors.push_back(boost::bind(callSomeFunction, i));
// call one of them:
functors[3]();
Note that the elements of the vector are not "real functions" but objects with an overloaded operator(). Usually this shouldn't be a disadvantage and actually be easier to handle than real function pointers.
You can do that simply by defining those functions by some arbitrary names in the global scope beforehand.
This is basically what is said above but modifying your code would look something like this:
std::vector<int (*) (int)> addresses;
for(int i=0;i<10;i++) {
addresses[i] = &myFunction;
}
I'm not horribly clear by what you mean when you say functions created at run time... I don't think you can create a function at run time, but you can assign what function pointers are put into your array/vector at run time. Keep in mind using this method all of your functions need to have the same signature (same return type and parameters).
You can't invoke a member function by itself without the this pointer. All instances of a class have the function stored in one location in memory. When you call p->Function() the value of p is stored somewhere (can't remember if its a register or stack) and that value is used as base offset to calculate locations of the member variables.
So this means you have to store the function pointer and the pointer to the object if you want to invoke a function on it. The general form for this would be something like this:
class MyClass {
void DoStuf();
};
//on the left hand side is a declaration of a member function in the class MyClass taking no parameters and returning void.
//on the right hand side we initialize the function pointer to DoStuff
void (MyClass::*pVoid)() = &MyClass::DoStuff;
MyClass* pMyClass = new MyClass();
//Here we have a pointer to MyClass and we call a function pointed to by pVoid.
pMyClass->pVoid();
As i understand the question, you are trying to create functions at runtime (just as we can do in Ruby). If that is the intention, i'm afraid that it is not possible in compiled languages like C++.
Note: If my understanding of question is not correct, please do not downvote :)