can you please explain in details this line of code inside struct:
There is a pointer to function but why would you reference it to struct?
void (*function)(struct Structure *);
what does this mean
(struct Structure *)?
(struct Structure *)
It means that the function have a struct Structure * argument. Actually it will make more sense with (struct Structure *variable of struct).
In this way, you can use a pointer to point a struct and should put the address of the struct variable which can be used in the function.
#include <stdio.h>
typedef struct circle{
int rad;
int area;
} Circle;
void ShowCircleInfo(Circle *info)
{
printf("rad value: %d\n", info->rad);
printf("area value: %d", info->area);
}
int main(void)
{
Circle circle_one;
circle_one.rad = 2;
circle_one.area = 3;
ShowCircleInfo(&circle_one);
return 0;
}
void (*function)(struct Structure *); declares function to be a pointer to a function that has a parameter of type struct Structure * and does not return a value.
For example
#include <stdio.h>
struct Structure {
int a;
void (*function)(struct Structure *);
};
void foo(struct Structure *a) {
if (a->function == NULL) a->function = foo;
a->a++;
printf("%d\n", a->a);
}
int main(void) {
struct Structure a = {42, foo};
struct Structure b = {0}; // don't call b.function just yet!!
a.function(&b); // foo(&b)
b.function(&a); // foo(&a)
}
See code running at https://ideone.com/7E74gb
In C, function pointer declarations have almost the same structure as function headers.
Only the function name will change to have some parantheses and a "*" in it, and the arguments won't have names, because only their types are important when using pointers (we don't access the values of the arguments, so we don't need their names).
They basically look like this:
<return_value> (*<function_name>)(<argument_list>)
So, for example, the function pointer for the function
void swap(int* a, int* b);
would be
void (*swap_ptr)(int*, int*);
Notice that the name of the pointer is in the place of the name of the function, and looks a bit odd compared to normal pointer declarations.
An excellent reading on this topic (you can skip the C++ stuff): https://www.cprogramming.com/tutorial/function-pointers.html
Related
I declared a struct like this one :
typedef struct s_data {
char buff[2048];
int len;
void *func[10];
struct data *next;
} t_data;
In my code, when passing a *data, I assigned some functions (just giving one so it is more understandable)
void init_data(t_data *data)
{
data->len = 0;
data->func[0] = &myfirstfunctions;
//doing the same for 9 others
}
My first function would be something taking as argument *data, and an int.
Then, I try to use this function in another function, doing
data->func[0](data, var);
I tried this and a couple of other syntaxes involving trying to adress (*func[0]) but none of them work. I kind of understood from other much more complex questions over there that I shouldn't store my function like this, or should cast it in another typedef, but I did not really understand everything as I am kind of new in programming.
void* can only be used reliably as a generic object pointer ("pointer to variables"). Not as a generic function pointer.
You can however convert between different function pointer types safely, as long as you only call the actual function with the correct type. So it is possible to do just use any function pointer type as the generic one, like this:
void (*func[10])(void);
...
func[0] = ((void)(*)(void))&myfirstfunction;
...
((whatever)func[0]) (arguments); // call the function
As you might note, the function pointer syntax in C is horrible. So I'd recommend using typedefs:
typedef void genfunc_t (void);
typedef int somefunc_t (whatever*); // assuming this is the type of myfirstfunction
Then the code turns far easier to read and write:
genfunc_t* func [10];
...
func[0] = (genfunc_t*)&myfirstfunction;
...
((somefunc_t*)func[0]) (arguments);
If all of your functions will have the same signature, you can do this like:
#include <stdio.h>
typedef void (*func)(void *, int);
struct s_data {
char buff[2048];
int len;
func f[10];
struct s_data *next;
};
static void
my_first_function(void *d, int x)
{
(void)d;
printf("%d\n", x + 2);
}
static void
init_data(struct s_data *data)
{
data->len = 1;
data->f[0] = my_first_function;
}
int
main(void)
{
struct s_data d;
init_data(&d);
d.f[0](NULL, 5);
return 0;
}
If your functions have different signatures, you will want to either use a union, or perhaps you will need several different members of the struct to store the function pointers.
The problem is that you haven't actually declared an array of function pointers. What you actually did is an array of pointers to void.
The syntax of declaring a pointer to function is as following:
function_return_type (*pointer_name)(arg1_type,arg2_type,...);
Then you can create an array of pointers to functions:
function_return_type (*arr_name[])(arg1_type, arg2_type,...)
Therefore, the declaration of your structure should look like this:
typedef void (*pointer_to_function)(void *, int);
struct s_data {
char buff[2048];
int len;
pointer_to_function array_of_pointeters[10];
struct s_data *next;
};
Good luck:)
I try to define function pointers as the members of the structure which can access and modify other parameters of the structure (like functions of a class in OOP).
For this, I have a structure with two function pointers:
typedef struct{
int a;
int b;
int (*init)(); // line 4
int (*multiply)(); // line 5
}STR_X2;
where the function pointers are defined as follows:
void init(STR_X2* self , int _a , int _b){
self->a = _a;
self->b = _b;
printf("Init a:%d, b:%d \n",self->a,self->b);
}
int multiply(STR_X2* self){
printf("Multiply a:%d, b:%d, res:%d\n",self->a,self->b,self->a*self->b);
return self->a*self->b;
}
and then I use the structure in main() function as follows:
int main(void) {
STR_X2* val2;
val2->init = init;
val2->multiply = multiply;
val2->init(val2,7,5);
printf("result:%d\n",val2->multiply(val2));
return EXIT_SUCCESS;
}
both function pointers have one argoment of type STR_X2. But logically I cannot define this argument because the structure STR_X2is not defined at line 4 and 5. My question: Is this way of defining function pointer (without argument) safe?Is there any better alternative to access the structure member in such function pointers?
Basically what you want here is a forward declaration of a structure type. You can achieve this by using structure tags. For example:
typedef struct STR_X2_S STR_X2;
struct STR_X2_S {
int a;
int b;
int (*init)(STR_X2 *self, int _a, int _b);
int (*multiply)(STR_X2 *self);
};
I am having a problem with declaring a struct with an array of function pointers (vTable) in C because if I declare the function pointer first and a parameter needs to be a self-referencial "this" pointer to itself, the struct hasn't yet been declared. If I declare the function pointer AFTER the struct, then the function type hasn't been declared so the compiler complains when I set up the struct:
#include <stdio.h>
#include <stdlib.h>
typedef int (*math_operation) (struct _MyClass *this,int a, int b);
typedef struct _MyClass{
int number;
char name[50];
math_operation *vTable[50];
} MyClass;
int main(void)
{
MyClass *test;
return(EXIT_SUCCESS);
}
What is the proper way to create an array of function pointer which have a "this" pointer to the parent struct?
You just need a forward declaration of the struct in the global namespace:
struct MyClass_;
typedef int math_operation(struct MyClass_ *this, int a, int b);
typedef struct MyClass_{
int number;
char name[50];
math_operation *vTable[50];
} MyClass;
Things to note:
I fixed the tag identifier, so it won't tread on the C standard.
I changed the function pointer typedef into a function type typedef. You already defined vTable as an array of pointers to math_operation. One pointer declarator was superfluous. This also has the nice utility of allowing you to declare functions by their intended purpose, and have the compiler type check it:
math_operation add;
// .. Later
int add(struct MyClass_ *this, int a, int b) {
return a + b;
}
Can we have functions in structures in C language?
Could someone please give an example of how to implement it and explain?
No, structures contain data only. However, you can define a pointer to a function inside of a struct as below:
struct myStruct {
int x;
void (*anotherFunction)(struct foo *);
}
The answer is no, but there is away to get the same effect.
Functions can only be found at the outermost level of a C program. This improves run-time speed by reducing the housekeeping associated with function calls.
As such, you cannot have a function inside of a struct (or inside of another function) but it is very common to have function pointers inside structures. For example:
#include <stdio.h>
int get_int_global (void)
{
return 10;
}
double get_double_global (void)
{
return 3.14;
}
struct test {
int a;
double b;
};
struct test_func {
int (*get_int) (void);
double (*get_double)(void);
};
int main (void)
{
struct test_func t1 = {get_int_global, get_double_global};
struct test t2 = {10, 3.14};
printf("Using function pointers: %d, %f\n", t1.get_int(), t1.get_double());
printf("Using built-in types: %d, %f\n", t2.a, t2.b);
return 0;
}
A lot of people will also use a naming convention for function pointers inside structures and will typedef their function pointers. For example you could declare the structure containing pointers like this:
typedef int (*get_int_fptr) (void);
typedef double (*get_double_fptr)(void);
struct test_func {
get_int_fptr get_int;
get_double_fptr get_double;
};
Everything else in the code above will work as it is. Now, get_int_fptr is a special type for a function returning int and if you assume that *_fptr are all function pointers then you can find what the function signature is by simply looking at the typedef.
No, it has to be implemented like this :
typedef struct S_House {
char* name;
int opened;
} House;
void openHouse(House* theHouse);
void openHouse(House* theHouse) {
theHouse->opened = 1;
}
int main() {
House myHouse;
openHouse(&myHouse);
return 0;
}
I have two functions, each taking a pointer to a different type:
void processA(A *);
void processB(B *);
Is there a function pointer type that would be able to hold a pointer to either function without casting?
I tried to use
typedef void(*processor_t)(void*);
processor_t Ps[] = {processA, processB};
but it didn't work (compiler complains about incompatible pointer initialization).
Edit: Another part of code would iterate through the entries of Ps, without knowing the types. This code would be passing a char* as a parameter. Like this:
Ps[i](data_pointers[j]);
Edit: Thanks everyone. In the end, I will probably use something like this:
void processA(void*);
void processB(void*);
typedef void(*processor_t)(void*);
processor_t Ps[] = {processA, processB};
...
void processA(void *arg)
{
A *data = arg;
...
}
If you typedef void (*processor_t)(); then this will compile in C. This is because an empty argument list leaves the number and types of arguments to a function unspecified, so this typedef just defines a type which is "pointer to function returning void, taking an unspecified number of arguments of unspecified type."
Edit: Incidentally, you don't need the ampersands in front of the function names in the initializer list. In C, a function name in that context decays to a pointer to the function.
It works if you cast them
processor_t Ps[] = {(processor_t)processA, (processor_t)processB};
By the way, if your code is ridden with this type of things and switch's all over the place to figure out which function you need to call, you might want to take a look at object oriented programming. I personally don't like it much (especially C++...), but it does make a good job removing this kind of code with virtual inheritance.
This can be done without casts by using a union:
typedef struct A A;
typedef struct B B;
void processA(A *);
void processB(B *);
typedef union { void (*A)(A *); void (*B)(B *); } U;
U Ps[] = { {.A = processA}, {.B = processB} };
int main(void)
{
Ps[0].A(0); // 0 used for example; normally you would supply a pointer to an A.
Ps[1].B(0); // 0 used for example; normally you would supply a pointer to a B.
return 0;
}
You must call the function using the correct member name; this method only allows you to store one pointer or the other in each array element, not to perform weird function aliasing.
Another alternative is to use proxy functions that do have the type needed when calling with a parameter that is a pointer to char and that call the actual function with its proper type:
typedef struct A A;
typedef struct B B;
void processA(A *);
void processB(B *);
typedef void (*processor_t)();
void processAproxy(char *A) { processA(A); }
void processBproxy(char *B) { processB(B); }
processor_t Ps[] = { processAproxy, processBproxy };
int main(void)
{
char *a = (char *) address of some A object;
char *b = (char *) address of some B object;
Ps[0](a);
Ps[1](b);
return 0;
}
I used char * above since you stated you are using it, but I would generally prefer void *.