Can't initialize array with struct containing function pointer -error: initializer element is not constant

Question

I'm trying to initialize an array of structs. The struct contains a function pointer as one of its data members. But my compiler is giving me an error complaining that The initializer element is not constant. How can I initialize the array with my declared function pointer?

typedef void (*write_func_ptr_t)( void**, size_t*, char*, const size_t);
typedef bool (*read_func_ptr_t)( char*, const void*, const size_t);

write_func_ptr_t generate_basic_msg_ptr;
read_func_ptr_t handle_basic_msg_ptr;
write_func_ptr_t generate_reg_msg_ptr;
read_func_ptr_t handle_reg_msg_ptr;

struct supported_msg_info
{
    const char* const type;
    const write_func_ptr_t write_func;
    const read_func_ptr_t read_func;
};

static struct supported_msg_info SUPP_MESSAGES[] = {
    { "basic", generate_basic_msg_ptr, handle_basic_msg_ptr },
    { "registration", generate_reg_msg_ptr, handle_reg_msg_ptr }
};

Initializers in C can't contain mutable variables, including function pointers. Even if they were declared const, they still wouldn't work because C initializer rules are very strict. — Austin Mullins
– Austin Mullins, Commented Feb 20, 2015 at 19:31
What are you trying to do? where are the definitions of the functions? — Iharob Al Asimi
– Iharob Al Asimi, Commented Feb 20, 2015 at 19:31
The definitions of the functions are going to be pulled in at runtime from a shared library using dlsym — agranum
– agranum, Commented Feb 20, 2015 at 19:40

Iharob Al Asimi · Accepted Answer · 2015-02-20 19:45:45Z

2

You can do that, add the functions declarations and make them match the function pointer types like this

void generate_basic_msg_ptr(void **, size_t *, char *, const size_t);
bool handle_basic_msg_ptr(char *, const void *, const size_t);
void generate_reg_msg_ptr(void **, size_t *, char *, const size_t);
bool handle_reg_msg_ptr(char *, const void *, const size_t);

But according to your comment

The definitions of the functions are going to be pulled in at runtime from a shared library using dlsym – agranum 59 secs ago

What you really need is this

struct supported_msg_info
{
    const char *const type;
    /* remove the const qualifier */
    write_func_ptr_t write_func;
    read_func_ptr_t read_func;
};

static struct supported_msg_info SUPP_MESSAGES[] = {
    {"basic", NULL, NULL},
    {"registration", NULL, NULL}
};

and then when you call dlsym you can do this

SUPP_MESSAGES[0].write_func = dlsym(dlhandle, "symbol_name");

the function poitner SUPP_MESSAGES[0].write_func will then be called with the signature specified by your struct definition and function pointers typedef's and remember that void * is convertible to any type of pointer without casting, so this is all you need.

edited Feb 20, 2015 at 19:45

answered Feb 20, 2015 at 19:34

Iharob Al Asimi

53.1k6 gold badges66 silver badges100 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

agranum Over a year ago

Your initial solution actually got me thinking that it would good to have a default implementation of the functions in this c file as a fallback in case the implementation did not exist in the .so I would be linking to at runtime. That way I could spit out a message indicating that it wasn't there and not crash. Thanks.

Eugene Sh. · Accepted Answer · 2015-02-20 19:41:49Z

0

generate_basic_msg_ptr and friends are just variables that are not even initialized(well, strictly speaking they might have been initialized to zeros, but it is probably not your intention). So they have no compile-time defined values, and obviously cannot be used for array initialization. You might want to declare a functions like void func(<whatever>);, and use func in the initialization.

edited Feb 20, 2015 at 19:41

answered Feb 20, 2015 at 19:32

Eugene Sh.

18.7k8 gold badges45 silver badges69 bronze badges

Collectives™ on Stack Overflow

Can't initialize array with struct containing function pointer -error: initializer element is not constant

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related