Question

if ((pos < n) && (key == ptr->keys[pos].value))
  {
    struct return_values* function(&ptr->keys[pos]);
  }

while compilation i get the error

 error: syntax error before '&' token in this line 
 struct return_values* function(&ptr->keys[pos]);

i am passing the address of ptr->keys[pos] to the function

struct return_values* function(struct classifier fun_ptr)

where struct return_values is the return type what is the error here h

Answer 1

You need to assign the result of functon to a variable:

struct return_values* values = function(&ptr->keys[pos]);