Void pointer to a struct

Question

I am in initial phase of learning C language. I have one doubt regarding a statement including void pointer.

void (*myvar)(const struct foo *);

Can anyone please help me about the above line. What is the exact meaning of above line and how do use this pointer in the code.

Answer 1

In your question,

void (*myvar)(const struct charpp *); ^{Follow the NOTE}

is a function pointer, not a simple pointer -to -struct . You can find more information regarding this here .

Also, do check this answer for a clear idea of the usage.

Note: char is a reserved keyword [datatype] in c. It cannot be used as variable name, as you've used in your code. Change so something else.

Answer 2

That should be a variable declaration of a pointer to a function taking a const char * as argument and not returning any value. I say should because you have an error in the declaration of the argument type (there's no such thing as const struct char * ).

If you fix the error, and assuming you mean const char * , then you can use it like the following code:

#include <stdio.h>

void my_print_line(const char *str)
{
    printf("%s\n", str);
}

int main(void)
{
    void (*myvar)(const char *);

    myvar = &my_print_line;

    myvar("Hello world!");
}

Answer 3

It is a pointer to a function. You can break and understand the rule easily if you follow the Spiral Rule . Take a look at Example2. Now you can easily, put your declaration instead of the example and break it part by part to get better understanding.

             +--------------------+
             | +---+              |
             | |+-+|              |
             | |^ ||              |
        char *(*fp)( int, float *);
         ^   ^ ^  ||              |
         |   | +--+|              |
         |   +-----+              |
         +------------------------+

Also, struct char will generate an error.