Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. You use it the same way you would use the original type, for instance. As you can see, you could just replace the typedefed name with its definition given above. However, the syntax is appropriate, since functions - unlike other simpler types - may have a return value and parameters, thus the sometimes lengthy and complex declaration of a pointer to function.
The readability may start to be really tricky with pointers to functions arrays, and some other even more indirect flavors. Why is typedef used? To ease the reading of the code - especially for pointers to functions, or structure names.
The syntax looks odd in the pointer to function declaration That syntax is not obvious to read, at least when beginning. Using a typedef declaration instead eases the reading. Is a function pointer created to store the memory address of a function? Yes, a function pointer stores the address of a function. No, this simply tells the compiler that the FunctionFunc type will be a function pointer, it doesn't define one, like this:. With the typedef it instead defines FunctionFunc as a name for that type.
In the Backus-Naur form from there, you can see that typedef has the type storage-class-specifier. In the type declaration-specifiers you can see that you can mix many specifier types, the order of which does not matter. Soto understand the typedef on the exhaustive use you need to consult some backus-naur form that defines the syntax there are many correct grammars for ANSI C, not only that of ISO.
When you use typedef to define an alias for a function type you need to put the alias in the same place where you put the identifier of the function. In your case you define the type FunctionFunc as an alias for a pointer to function whose type checking is disabled at call and returning nothing. Learn more. Typedef function pointer?
Ask Question. Asked 9 years, 4 months ago. Active 10 months ago. Viewed k times.Till now, we have studied that the address assigned to a pointer should be of the same type as specified in the pointer declaration. For example, if we declare the int pointer, then this int pointer cannot point to the float variable or some other type of variable, i. To overcome this problem, we use a pointer to void.
void pointer in C
A pointer to void means a generic pointer that can point to any data type. We can assign the address of any data type to the void pointer, and a void pointer can be assigned to any type of the pointer without performing any explicit typecasting. In the above declaration, the void is the type of the pointer, and 'ptr' is the name of the pointer. The size of the void pointer in C is the same as the size of the pointer of character type. According to C perception, the representation of a pointer to void is the same as the pointer of character type.
The size of the pointer will vary depending on the platform that you are using. As we already know that the void pointer cannot be dereferenced, so the above code will give the compile-time error because we are printing the value of the variable pointed by the pointer 'ptr' directly. In the above code, we typecast the void pointer to the integer pointer by using the statement given below:.
Then, we print the value of the variable which is pointed by the void pointer 'ptr' by using the statement given below:. We cannot apply the arithmetic operations on void pointers in C directly. We need to apply the proper typecasting so that we can perform the arithmetic operations on the void pointers. The above code shows the compile-time error that " invalid use of void expression " as we cannot apply the arithmetic operations on void pointer directly, i.
The above code runs successfully as we applied the proper casting to the void pointer, i. We use void pointers because of its reusability. Void pointers can store the object of any type, and we can retrieve the object of any type by using the indirection operator with proper typecasting. JavaTpoint offers too many high quality services. Mail us on hr javatpoint. Please mail your requirement at hr javatpoint. Duration: 1 week to 2 week.
Command Line Arguments.Function Pointers in C
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Javatpoint Services JavaTpoint offers too many high quality services. Some important points related to void pointer are: Dereferencing a void pointer in C The void pointer in C cannot be dereferenced directly. Let's see the below example.
Output Now, we rewrite the above code to remove the error. Let's rewrite the above code to remove the error.The void pointeralso known as the generic pointer, is a special type of pointer that can be pointed at objects of any data type! However, because the void pointer does not know what type of object it is pointing to, it cannot be dereferenced directly! Rather, the void pointer must first be explicitly cast to another pointer type before it is dereferenced. Ultimately, that is up to you to keep track of.
Although some compilers allow deleting a void pointer that points to dynamically allocated memory, doing so should be avoided, as it can result in undefined behavior. It is not possible to do pointer arithmetic on a void pointer. This is because pointer arithmetic requires the pointer to know what size object it is pointing to, so it can increment or decrement the pointer appropriately.
Note that there is no such thing as a void reference. In general, it is a good idea to avoid using void pointers unless absolutely necessary, as they effectively allow you to avoid type checking.
For example, the following would be valid:. Many other places where void pointers would once be used to handle multiple data types are now better done using templates, which also offer strong type checking. However, very occasionally, you may still find a reasonable use for the void pointer.
If it does, can anyone explain why it does work?? We don't want to read the value from the pointer at that point.
It's your responsibility to keep track of the type and not treat the data as something it's not. I find it hard to understand the casting part, any references for learning that would be greatly appreciated. Make sure you understood lesson S. After that, it's quite simple.
They're useful mainly when you want a generic way to point at something but will disambiguate what that is via some other mechanism. They were used more in the C world, as a way to do a primitive form of polymorphism. Could you give examples to prove this statement?
Ritchie book "The C" till now. Typedef is use to create new data type name. It is pretty much a similar situation with dereferencing a void pointer.
You can't do it directly. But you can do it somehow. I understand the purpose of the example code here. But in 'real life' isn't better to have three explicit functions like printIntprintFloat and printCstring? You wouldn't write code like this in practice at all, as there are safer alternatives eg. You're right, it'd be better to call the correct function right away if the type is known. You'll see code like this in cases where the type is unknown at compile-time.
Yes, it is. Your question indicates that you are misreading the function's return type. There is a big difference between:. Yes, this function is returning a pointer to allocated memory of a specified size. It's different in malloc in the sense that it's guaranteed to return a pointer.
On failure it will exit the application. In Java there is no pointer arithmetic. It think this is what you are asking about. For example, imagine that malloc returns a pointer of type int.
You would possibly receive the pointer, which is basically a pointer to an array. Then you would index it like regular arrays. The problem is that C doesn't have functions overloading. So, we have to find a way to write a generic malloc. Otherwise, you would end up with a different malloc for every type. The solution, is to send the required number of bytes that you need.
Then, you can index it however you like. This gives a greater flexibility and one for all solution. So, the whole idea is that it doesn't matter how we index the memory we got from malloc. All what we have to do is to specify the type of the newly created array to index it properly.
Learn more. Asked 10 years, 8 months ago. Active 5 years, 8 months ago. Viewed 10k times. Active Oldest Votes. Amber Amber k 66 66 gold badges silver badges bronze badges. The reliability of that conversion is implementation dependent. William Pursell William Pursell k 41 41 gold badges silver badges bronze badges.
JaredPar JaredPar k gold badges silver badges bronze badges. You sure about that? I see, you mean "it's different from malloc " referring to this emalloc example - thought you were referring to malloc itself. Earwicker: malloc can return NULL, but emalloc cannot. If it can't get memory, the program exits and emalloc does not return.In an unsafe context, a type may be a pointer type, a value type, or a reference type. A pointer type declaration takes one of the following forms:.
Only an unmanaged type can be a referent type. Pointer types do not inherit from object and no conversions exist between pointer types and object. Also, boxing and unboxing do not support pointers. However, you can convert between different pointer types and between pointer types and integral types.
For example:. A pointer cannot point to a reference or to a struct that contains references, because an object reference can be garbage collected even if a pointer is pointing to it. The garbage collector does not keep track of whether an object is being pointed to by any pointer types. The following are examples of pointer type declarations:. For example, consider the following declaration:. There are several examples of pointers in the topics fixed Statement and Pointer Conversions.
The following example uses the unsafe keyword and the fixed statement, and shows how to increment an interior pointer. You can paste this code into the Main function of a console application to run it.
These examples must be compiled with the -unsafe compiler option set. However, you can use a cast to convert a void pointer to any other pointer type, and vice versa.
6.13 — Void pointers
A pointer can be null. Applying the indirection operator to a null pointer causes an implementation-defined behavior. Passing pointers between methods can cause undefined behavior.
Consider a method that returns a pointer to a local variable through an inoutor ref parameter or as the function result. If the pointer was set in a fixed block, the variable to which it points may no longer be fixed. The following table lists the operators and statements that can operate on pointers in an unsafe context:. For more information about pointer related operators, see Pointer related operators.
For more information, see the Pointer types section of the C language specification. You may also leave feedback directly on GitHub. Skip to main content. Exit focus mode. WriteLine "" ; Console. C language specification For more information, see the Pointer types section of the C language specification.
Software Engineering Stack Exchange is a question and answer site for professionals, academics, and students working within the systems development life cycle. It only takes a minute to sign up. In strongly-typed languages like Java and Cvoid or Void as a return type for a method seem to mean:.
This method doesn't return anything. No return. You will not receive anything from this method. What's really strange is that in C, void as a return type or even as a method parameter type means:. It could really be anything. You'd have to read the source code to find out.
Good luck. If it's a pointer, you should really know what you're doing. Since C is older than Java and Cwhy did these languages adopt void as meaning "nothing" while C used it as "nothing or anything when a pointer "? The keyword void not a pointer means "nothing" in those languages. This is consistent. This is an unfortunate decision because as you mentioned, it does make void mean two different things.
I have not been able to find the historical reason behind reusing void to mean "nothing" and "anything" in different contexts, however C does this in several other places. For example, static has different purposes in different contexts. There is obviously precedent in the C language for reusing keywords this way, regardless of what one may think of the practice. Java and C are different enough to make a clean break to correct some of these issues.
Java and "safe" C also do not allow raw pointers and do not need easy C compatibility Unsafe C does allow pointers but the vast majority of C code does not fall into this category.
This allows them to change things up a bit without worrying about backwards compatibility. All pointers in C need to be able to be dereferenced. Remember C pointers don't carry any runtime type information, so the type must be known at compile time. Perhaps it would be more useful to think of void as the return type. Then your second method would read "a method that returns an untyped pointer.
From the online C standard :. A void expression has no value although it may have side effects. If I have a function defined to return voidlike so:. Yes, using the same keyword for two different concepts no value vs. In Java and Cvoid as a return type for a method seem to mean: this method doesn't return anything.
That statement is incorrect. They are completely different. A pointer is a value which may be dereferenced. Dereferencing a valid pointer gives a storage location of the pointed-to type.
Return pointer from functions in C
A void pointer is a pointer which has no particular pointed-to type; it must be converted to a more specific pointer type before the value is dereferenced to produce a storage location. Java does not have void pointers; C does. Why did Java and C adopt the convention that void is a valid return type, instead of, for instance, using the Visual Basic convention that functions are never void returning and subroutines are always void returning? To be familiar to programmers coming to Java or C from languages where void is a return type.
The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Also, is there any way of generalizing a function which can receive a pointer and store it in a void pointer and by using that void pointer, can we make a generalized function?
Also if there are good internet articles which explain the concept of a void pointer, then it would be beneficial if you could provide the URLs. Is it possible to dereference the void pointer without type-casting in C programming language No, void indicates the absence of type, it is not something you can dereference or assign to. You cannot just dereference it in a portable way, as it may not be properly aligned. It may be an issue on some architectures like ARM, where pointer to a data type must be aligned at boundary of the size of data type e.
Pointer arithmetic is not possible on pointers of void due to lack of concrete value underneath the pointer and hence the size. Pointer arithmetic is about changing pointer values by multiples of the sizeof the pointed-to objects. Something similar to getClass simply doesn't exist, since this information is nowhere to be found. For that reason, the kind of "generic" you are looking for always comes with explicit metainformation, like the int b in your example or the format string in the printf family of functions.
When a pointer variable is declared using keyword void — it becomes a general purpose pointer variable. Address of any variable of any data type char, int, float etc.
To make a general function. But I was getting error, so I came to know my understanding with void pointer is not correct :. So now I will move towards to collect points why is that so. We need to typecast the void pointer variable to dereference it.
This is because a void pointer has no data type associated with it. There is no way the compiler can know or guess?
So to take the data pointed to by a void pointer we typecast it with the correct type of the data holded inside the void pointers location. A void pointer can be really useful if the programmer is not sure about the data type of data inputted by the end user. In such a case the programmer can use a void pointer to point to the location of the unknown data type. The program can be set in such a way to ask the user to inform the type of data and type casting can be performed according to the information inputted by the user.
A code snippet is given below.