A set of function pointers grouped
into a data structure are often
referred to as a virtual function
table (VFT).
The above statement makes me feel that virtual function == function pointer,is that so?
There is no built-in support for virtual functions in C.
In C++ virtual functions are specified via a v-table. And the entries in a vtable can be implemented as function pointers.
That’s wrong because these are different levels of abstraction.
An analogy may help: saying that virtual functions and function pointers are identical is like saying that wheels and bikes are identical.
While it’s true that function pointers and virtual functions may look much the same “under the hood”, they are different things – both conceptionally (a virtual function is an overriable member method of a class while a function pointer is simply an indirection of a function) and syntactically (calling them is completely different).
They may serve the same purpose, however. In particular, both provide a means of deferring a calling decision (which function to call in this situation?) until runtime when normal call dispatching happens at compile time.
I'd say close, but not quite. A virtual function is still a function, but it's normally called via a pointer, not directly.
Yes, a virtual function table is often implemented under the hood as a table of function pointers. However, there is also other hardware to go along with the table of pointers to make the functions actually "virtual". You have to have a mechanism in place to bind a call to the correct pointer at run-time, etc. I say this because it would be wrong to think that since a virtual function is a function pointer at its most basic level that that makes any function pointer a virtual function.
Actually C++ Supports Virtual Functions , but C Does not supports VF because both are totally different concepts
Virtual function in C++ by definition is a function declared with keyword virtual (immediately or in one of the base classes). That's all.
Now, calls to virtual functions can be resolved statically or dynamically. A dynamically-resolved call is a call that resolved in accordance with dynamic type of the object used in the call. That's all.
Nothing in the above has any references to any "function pointers". However, in a typical implementation in order to implement the proper behavior of dynamic calls, a table with function pointers (pointing to virtual functions) is used. This table is what is known as "VMT", "VFT" or "vtable".
In other words, function pointer is an implementation detail typically used to provide support for dynamic calls to virtual functions.
To illustrate it further, note, for example, that even if some function is virtual, but it is never called dynamically, then there's no need to generate any "pointers" for that function. For this reason, some compilers do not generate VMTs for abstract classes, since even though these classes have virtual functions, these functions are never called dynamically.
I guessed it's from the Understanding Linux Network Internals book -- We're talking about C here, and you've got your parenthesis wrong -- it's virtual (function table), not (virtual function) table :). Virtual functions are a C++ only term.
Which doesn't mean you can't code OOP in ANSI C...
Related
I have seen some libraries for microcontrollers that do a special job like controlling an SPI device. They define extern functions with a context pointer for byte sending and pin configuration which the user must implement. Is this a good practice to do so?
Some others use function pointers inside their context but these pointer may be very expensive and some older compilers do not allow function pointers with a lot of parameters due to parameter passing restrictions (I remember this from the C51 compiler)
It depends on what you want to achieve. Using extern functions that need to be implemented by a higher layer is a very efficient way to use callbacks. Especially when they are called frequently, they have a lower performance penalty than function pointers, which need to be dereferenced. However, function pointers to callback functions are more flexible, as you can allow them to be null. Furthermore, it is possible to change them at execution time.
Do gcc's function attributes extensions have any effect when used on the type of function pointers?
The function attributes can be used when declaring the type of function pointers but at least some of them seem to have no effect.
https://stackoverflow.com/a/28740576/1128289
The gcc documentation itself does not address this issue.
Very generally, the C standard basically says that handling an object over a pointer whose type is not aligned with the type of the object itself generates undefined behaviour - There are many exceptions to this general rule, but, apparently, none of them seems to apply to your case.
That means we're moving on very unsafe grounds here, in the first place.
First, you need to distinguish function attributes into two classes:
Function attributes that actually change something in the behavior or location of the function itself like aligned or interrupt- A function that is not attributed that way will not change its inner code once you declare a pointer to it interrupt, for example (the code that is generated for the function would need to dynamically change - for example, a "return from interrupt" instruction replaced by a "return normally" one - depending on along what type of pointer it would have been called). This is obviously not possible.
Function attributes that tell the calling code something about the behaviour that can be expected from the function - like noreturn or malloc, for example. Those attributes sometimes might not modify the function code itself (you'll never know, however...), but rather tell the calling code something about assumptions it can make in order to optimise. These assumptions will affect the calling function only and thus can be triggered by tweaking a pointer (you don't even need a pointer to do that, a modified function prototype should suffice - for the language, that would actually turn out to have the same effect). If you tell the compiler it can make such assumptions and those turn out not to be true, this will, however, lead to all sorts of things go wrong. After all, C makes the programmer responsible for making sure a pointer points to the right type of thing.
There are, however, function attributes that actually restrict the amount of assumptions a calling function would be allowed to make (the most obvious would be malloc that tells calling code it returns yet untyped and uninitialized memory). Those should be relatively safe to use (I do, however, fail to come up with a use case atm)
Without very detailed knowledge on what belongs into (1) or (2) above, and what exactly a function attribute might affect in both called and calling code (which would be very difficult to achieve, because I don't recall to ever have seen that documented in detail), you thus will not be able to decide whether this pointer tweaking actually is possible and what side effects it might generate.
Also, in my opinion, there is not much of a difference between tweaking a pointer to a function and calling an external function with a (deliberately) wrong prototype. This might give you some insight on what you are actually trying to do...
Suppose that I have a function pointer which can be invoked to do some tasks. How can I store the piece of code, to which the pointer is pointing, to a file on disk so I can later load the file and have the function pointer available again?
Use case: This will be done inside a JIT compiler to prevent the future overhead of JIT-ing in the next run of the same program.
Edit: The answer to "Save and load function pointers to file" are not what I am looking for. That question is dealing with a limited number of functions to which people have suggested using indices. But in my case, the function can be anything with any content.
There is no portable way to do that. Your options are:
Associate pointers with symbolic names of your choosing, e.g. using a global mapping table, and serialize the function name. On deserialization, look up the actual pointer in the mapping.
Serialize real function names, possibly also contained with a mapping. On deserialization, use dlsym (or equivalent on non-Unix platforms, such as GetProcAddress) to get the function pointer.
I have come across the fact that function pointers can be used to implement callbacks. Is there any other usage of function pointers? Is there any other situation that function pointers proved to be useful?
How about sorting? Pass in a function pointer to compare any two elements.
How about filtering? Pass in a function pointer to decide whether an input element should be contained in the output of a filter.
How about transformations? Pass in a function pointer to convert an input element to an output element.
These are all collection-based uses, but they're very useful. (The broad equivalent of function pointers in .NET is delegates, and they're the basis of LINQ, which allows very simple querying, transformations, grouping etc.)
Anywhere you want to be able to abstract out the idea of "a single piece of behaviour", writing a generic function which doesn't need to know the details of that behaviour, a function pointer could be useful.
In addition to what Jon wrote, function pointers in C can be used to implement OO programming style (e.g. polymorphism).
Function pointers (or their typed and more advanced equivalents) are a helpful feature when implementing inversion of control related patterns. All examples mentioned are applications of IoC principle (the sorting algorithm does not control the used predicate, the call to an object method is delayed until run-time etc)
Regards,
Paul
A function pointer is used in any situation where the function to be called is determined at runtime rather than compile-time. This includes callbacks, but may also be used as a switch-case alternative for example, and to adapt the behaviour of a function by passing a function pointer that defines that behaviour - this is how the standard library qsort() function works for example, enabling it to sort any kind of object.
I have used them in particular to implement a command line parser that evaluates C expressions entered as strings at run-time, and can include function calls. This uses a symbol table to lookup the pointer to the function so it can be called on demand from the operator.
All you might ever wish to know on the subject can be found at The Function Pointer Tutorials
In the end function pointers are just one of those rarely used tools you keep in your bag. If you understand them, when the situation arises where it may provide a solution, you will hopefully recognise it.
It is the only way you can implement Higher Order Functions in C.
As others have mentioned, I've found that one of the most significant uses of function pointers (other than for callbacks) is to enable the construction of generic data structures.
Say you want to construct a hashmap with arbitrary keys and values. One way to do that is declare both void *key and void *value and then pass in two function pointers during the initialization phase: int (*hashcode)(void*) and int (*equals)(void*, void*).
This gives you the ability to build a hashmap that can take basically anything that you can write the above two functions for. In my case, the key was a fixed size character buffer and the value was a pointer to a struct.
It is also used in the following
Making jump tables(like vector tables or ISR)
making the function abstract
Developing Finite State Machines (as state, action and triggered even can easily be implemented using the function pointers, the design also seems to be easy and more readable in that)
Event Driven Framework(GUI - gtk is an example)
Other than callbacks (great for abstraction), function pointers can be used to implement polymorphism in C. This is done extensively in the Linux kernel, and common C libraries such as glibc, GTK+ and GLib.
It doesn't seem like it would be too hard to implement in assembly.
gcc also has a flag (-fnested-functions) to enable their use.
It turns out they're not actually all that easy to implement properly.
Should an internal function have access to the containing scope's variables?
If not, there's no point in nesting it; just make it static (to limit visibility to the translation unit it's in) and add a comment saying "This is a helper function used only by myfunc()".
If you want access to the containing scope's variables, though, you're basically forcing it to generate closures (the alternative is restricting what you can do with nested functions enough to make them useless).
I think GCC actually handles this by generating (at runtime) a unique thunk for every invocation of the containing function, that sets up a context pointer and then calls the nested function. This ends up being a rather Icky hack, and something that some perfectly reasonable implementations can't do (for example, on a system that forbids execution of writable memory - which a lot of modern OSs do for security reasons).
The only reasonable way to make it work in general is to force all function pointers to carry around a hidden context argument, and all functions to accept it (because in the general case you don't know when you call it whether it's a closure or an unclosed function). This is inappropriate to require in C for both technical and cultural reasons, so we're stuck with the option of either using explicit context pointers to fake a closure instead of nesting functions, or using a higher-level language that has the infrastructure needed to do it properly.
I'd like to quote something from the BDFL (Guido van Rossum):
This is because nested function definitions don't have access to the
local variables of the surrounding block -- only to the globals of the
containing module. This is done so that lookup of globals doesn't
have to walk a chain of dictionaries -- as in C, there are just two
nested scopes: locals and globals (and beyond this, built-ins).
Therefore, nested functions have only a limited use. This was a
deliberate decision, based upon experience with languages allowing
arbitraries nesting such as Pascal and both Algols -- code with too
many nested scopes is about as readable as code with too many GOTOs.
Emphasis is mine.
I believe he was referring to nested scope in Python (and as David points out in the comments, this was from 1993, and Python does support fully nested functions now) -- but I think the statement still applies.
The other part of it could have been closures.
If you have a function like this C-like code:
(*int()) foo() {
int x = 5;
int bar() {
x = x + 1;
return x;
}
return &bar;
}
If you use bar in a callback of some sort, what happens with x? This is well-defined in many newer, higher-level languages, but AFAIK there's no well-defined way to track that x in C -- does bar return 6 every time, or do successive calls to bar return incrementing values? That could have potentially added a whole new layer of complication to C's relatively simple definition.
See C FAQ 20.24 and the GCC manual for potential problems:
If you try to call the nested function
through its address after the
containing function has exited, all
hell will break loose. If you try to
call it after a containing scope level
has exited, and if it refers to some
of the variables that are no longer in
scope, you may be lucky, but it's not
wise to take the risk. If, however,
the nested function does not refer to
anything that has gone out of scope,
you should be safe.
This is not really more severe than some other problematic parts of the C standard, so I'd say the reasons are mostly historical (C99 isn't really that different from K&R C feature-wise).
There are some cases where nested functions with lexical scope might be useful (consider a recursive inner function which doesn't need extra stack space for the variables in the outer scope without the need for a static variable), but hopefully you can trust the compiler to correctly inline such functions, ie a solution with a seperate function will just be more verbose.
Nested functions are a very delicate thing. Will you make them closures? If not, then they have no advantage to regular functions, since they can't access any local variables. If they do, then what do you do to stack-allocated variables? You have to put them somewhere else so that if you call the nested function later, the variable is still there. This means they'll take memory, so you have to allocate room for them on the heap. With no GC, this means that the programmer is now in charge of cleaning up the functions. Etc... C# does this, but they have a GC, and it's a considerably newer language than C.
It also wouldn't be too hard to add members functions to structs but they are not in the standard either.
Features are not added to C standard based on soley whether or not they are easy to implement. It's a combination of many other factors including the point in time in which the standard was written and what was common / practical then.
One more reason: it is not at all clear that nested functions are valuable. Twenty-odd years ago I used to do large scale programming and maintenance in (VAX) Pascal. We had lots of old code that made heavy use of nested functions. At first, I thought this was way cool (compared to K&R C, which I had been working in before) and started doing it myself. After awhile, I decided it was a disaster, and stopped.
The problem was that a function could have a great many variables in scope, counting the variables of all the functions in which it was nested. (Some old code had ten levels of nesting; five was quite common, and until I changed my mind I coded a few of the latter myself.) Variables in the nesting stack could have the same names, so that "inner" function local variables could mask variables of the same name in more "outer" functions. A local variable of a function, that in C-like languages is totally private to it, could be modified by a call to a nested function. The set of possible combinations of this jazz was near infinite, and a nightmare to comprehend when reading code.
So, I started calling this programming construct "semi-global variables" instead of "nested functions", and telling other people working on the code that the only thing worse than a global variable was a semi-global variable, and please do not create any more. I would have banned it from the language, if I could. Sadly, there was no such option for the compiler...
ANSI C has been established for 20 years. Perhaps between 1983 and 1989 the committee may have discussed it in the light of the state of compiler technology at the time but if they did their reasoning is lost in dim and distant past.
I disagree with Dave Vandervies.
Defining a nested function is much better coding style than defining it in global scope, making it static and adding a comment saying "This is a helper function used only by myfunc()".
What if you needed a helper function for this helper function? Would you add a comment "This is a helper function for the first helper function used only by myfunc"? Where do you take the names from needed for all those functions without polluting the namespace completely?
How confusing can code be written?
But of course, there is the problem with how to deal with closuring, i.e. returning a pointer to a function that has access to variables defined in the function from which it is returned.
Either you don't allow references to local variables of the containing function in the contained one, and the nesting is just a scoping feature without much use, or you do. If you do, it is not a so simple feature: you have to be able to call a nested function from another one while accessing the correct data, and you also have to take into account recursive calls. That's not impossible -- techniques are well known for that and where well mastered when C was designed (Algol 60 had already the feature). But it complicates the run-time organization and the compiler and prevent a simple mapping to assembly language (a function pointer must carry on information about that; well there are alternatives such as the one gcc use). It was out of scope for the system implementation language C was designed to be.