c function pointers explanation

Question

I have found some code that I need to use for my application but there are two lines in it I can't figure out what exactly do they do and how... Please, either explain them to me or direct me to a link so I can read more about it.

Dict* dcreate(hash_size size, hash_size (*hashfunc) (const char *));

Here I guess it is passing a function as a parameter with it's parameter in the following bracket!?

hash_size i = dict->hashfunc(key) % dict->size;

and here, my guess is as good as my dog's!

The hashfunc:

static hash_size def_hashfunc(const char* key){ hash_size s = 0; while(*key){ s += (unsigned char) *key++; } return s; }

Thanks.

What they said. Also, you have "hash" in your tags, but it doesn't matter what the functions do; this mechanism is the same for non-hash-related functions! — Mr Lister
– Mr Lister, Commented Dec 26, 2011 at 9:15

Snowball · Accepted Answer · 2011-12-26 09:11:23Z

For the first line, your guess is correct. That is the header for a function that accepts two arguments, one of which is of the hash_size type, and another which is a pointer to a function whose argument is a const char* and returns a hash_size.

In the second line, dict appears to be a pointer to a struct, so dict->hashfunc(key) calls the function hashfunc, a pointer to which is stored in the dict struct. The last part (... % dict->size) is just the modulo operation.

Alok Save · Accepted Answer · 2011-12-26 09:06:43Z

hash_size (*hashfunc) (const char *)

Is a Function Pointer.

hashfunc is a pointer to a function which receives a const char * as an argument and returns a type hash_size.

MByD · Accepted Answer · 2011-12-26 09:07:46Z

It only passes a function pointer as a parameter. The following is the definition of the function type that should be passed:

hash_size (*hashfunc) (const char *)

E.g. a function that receives a const char * and returns hash_size.

pinxue · Accepted Answer · 2011-12-26 09:12:01Z

It is a function pointer

hash_size (*hashfunc) (const char *)

So that dcreate() will allocate an Dict and fill its field hashfunc like:

Dict * dict = (Dict *) malloc(sizeof(Dict)); dict->hashfunc = hashfunc;

Then you may call dict->hasfunc(const char *), it will return hash_size.

hash_size i = dict->hashfunc(key) % dict->size;

It is indeed:

hash_size i = ( dict->hashfunc(key) ) % ( dict->size ) ;

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