initialization of function pointer

**tilman** · 04-28-2013

Hello

I would like to initialize an arry containing function pointers with adresses of functions that have a variable number of arguments.

Below the code that works in principle. I would however get rid of the warning message during compilation pointing to the initialzation of the funtion pointers in the array. How do I need to cast the pointers to the functions ?

Code:

gcc func_ptr_init.c
func_ptr_init.c:28: warning: initialization from incompatible pointer type
func_ptr_init.c:32: warning: initialization from incompatible pointer type

Code:

#include<stdio.h>

unsigned char func1_ptr(unsigned int* if_desc,  int* result_code)
{
        *if_desc = 1;
        *result_code = 1;
        return(0);
}

unsigned char func2_ptr(unsigned int* if_desc,  int* result_code)
{
        *if_desc = 2;
        *result_code = 2;
        return(0);
}
struct _server_command_table_t
{
        char*           command_string;
        unsigned char   (*callback_func)(unsigned int* if_desc, ...);
};


struct _server_command_table_t table[] =
{
        {
                "String1",
                &func1_ptr
        },
        {
                "String2",
                &func2_ptr
        }
};

int main(int argc,char **argv)
{
        int interface;
        int result;

        printf("%s\n",table[0].command_string);
        printf("%d\n",table[0].callback_func(&interface,&result));
        printf("interface=%d, result=%d\n",interface,result);

        return(0);
};

Thanks
Tilman

**ZuK** · 04-28-2013

use a typedef

Code:

#include<stdio.h>
 
unsigned char func1_ptr(unsigned int* if_desc,  int* result_code)
{
        *if_desc = 1;
        *result_code = 1;
        return(0);
}
 
unsigned char func2_ptr(unsigned int* if_desc,  int* result_code)
{
        *if_desc = 2;
        *result_code = 2;
        return(0);
}

typedef unsigned char   (*callback_func)(unsigned int* if_desc, ...);

struct _server_command_table_t
{
        char*           command_string;
        callback_func   callback;
};
 

 
struct _server_command_table_t table[] =
{
        {
                "String1",
                (callback_func)&func1_ptr
        },
        {
                "String2",
                (callback_func)&func2_ptr
        }
};
 
int main(int argc,char **argv)
{
        int interface;
        int result;
 
        printf("%s\n",table[1].command_string);
        printf("%d\n",table[1].callback(&interface,&result,23));
        printf("interface=%d, result=%d\n",interface,result);
 
        return(0);
};

Kurt

**laserlight** · 04-28-2013

I think the appropriate solution is to change the signatures of func1_ptr and func2_ptr so that they are functions that accept a variable number of arguments, i.e., change their type to match the function pointer member of struct _server_command_table_t.

Casting would work if you are certain that the functions are compatible with functions that accept a variable number of arguments, but frankly, I'm not sure if they are.

**grumpy** · 04-28-2013

Originally Posted by laserlight

Casting would work if you are certain that the functions are compatible with functions that accept a variable number of arguments, but frankly, I'm not sure if they are.

In older versions of C, predating C89, there was such compatibility. However, that was no longer guaranteed as of the first C standard, because of tightening up of various rules related to type safety (and also explicitly allowing compiler implementers more flexibility in how arguments are passed to functions).

Among other things, that is why modern compilers refuse to allow implicit conversion between pointers to functions if there is any difference in signature, as in the example here.

Using an explicit type conversion between different pointers to function (aka typecast) will bludgeon the compiler into submission but the end result - when the function is eventually called - will be undefined behaviour.

**tilman** · 04-28-2013

Hello

thanks for you suggestions.
typedef works, of course, and is probably the most readeable solution. I will use it in my program.
I still wonder however, how I would cast it ? I am just getting syntax errors..

Thanks
Tilman

**laserlight** · 04-28-2013

Originally Posted by tilman

typedef works, of course, and is probably the most readeable solution. I will use it in my program.
I still wonder however, how I would cast it ? I am just getting syntax errors..

grumpy noted in post #3 that the functions that you have in mind are not compatible with functions that accept a variable number of arguments. As such, my concern in post #2 applies. Therefore, you should not use it in your program, i.e., ignore ZuK's suggestion.

Rather, do as I suggested: change the signatures of func1_ptr and func2_ptr so that they are functions that accept a variable number of arguments. If necessary, write wrapper functions that themselves take a variable number of arguments but which call the functions that you have in mind.

**grumpy** · 04-28-2013

Originally Posted by tilman

I still wonder however, how I would cast it ?

I have to say, you are brave.

Explicit type conversions (aka typecasts) should be a last resort to problem solving. In contrast, you are reaching for typecasts automatically to solve little concerns like syntax errors. In C programming circles, doing that is considered to be either very brave or very foolish. I have given you the benefit of the doubt in describing you as brave.

If you have to use typecasts to stop a compiler complaining about syntax errors, that is a positive sign your design and code is broken. Even if you successfully build your program after introducing typecasts, odds are it won't work.

Given that your func1_ptr and func2_ptr have identical signatures, it would be appropriate that your callback function have the same signature. That way, you can solve your problem without using typecasts at all.

**Salem** · 04-28-2013

The unadorned pointer cast looks like this - it's not something you want to be copy/pasting a lot of times.
(unsigned char(*)(unsigned int* if_desc, ...))&func2_ptr

> printf("%d\n",table[1].callback(&interface,&result,23));
Take heed of the other advice, and consider a different approach.
No function posted deals with the 3rd parameter passed here.

**Shurik** · 04-28-2013

@tilman

Why you didn't write

Code:

struct _server_command_table_t
{
        char*           command_string;
        unsigned char   (*callback_func)(unsigned int* if_desc, int* result_code);
};

?

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