Question

is there difference between with or without * for function pointer in C?

my function pointer declaration like this

typedef void (*DListVisitNode) (Node*, void*);
void DListTraverse( NodeList* , DListVisitNode , void*);

i have code like these

void print_index( Node* node, void* ctx)
{
    printf("index:%d\n", node->index);
}

void* print_content( Node* node, void* ctx)
{
    printf("content:%s\n", node->content);
}
void DListTraverse(NodeList* nodelist, DListVisitNode visit_func, void* ctx)
{
    Node* cur_node = nodelist->headnode;
    while( cur_node != NULL)
    {
        visit_func( cur_node, ctx );
        cur_node = cur_node->nextnode;
    }
}

DListTraverse( nodelist, print_content, NULL );
DListTraverse( nodelist, print_index, NULL );

both of DListTraverse works, but the one with * throws warning like this

warning: passing argument 2 of ‘DListTraverse’ from incompatible pointer type

i would simply delete the * afterward, but what’s the difference between them?

Answer 1

print_content is defined as returning a void* i.e. a generic raw pointer.

print_index is defined as returning void i.e. without any results.

These are different signatures. Only print_index matches DListVisitNode.

My coding style is to define signature thru typedef like

  typedef void signature_t (int);

Notice that no pointer is involved above. This names the signature of functions with one int argument and no results.

then, when needing pointers to such functions of above signature, use signature_t*

What is true is that the name of a function is like the name of an array; the language implicitly convert these to pointers. So DListTraverse(nodelist, print_content, NULL) is understood like DListTraverse(nodelist, &print_content, NULL)

You should enable all warnings on your compiler; with gcc that means giving -Wall -Wextra as program arguments to the compiler.