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1) I am currently trying to understand the following code, but I can't understand what void(*func)(void) means, I can understand that I am trying to save a the address of function named "function" from list0513, at void pointer func, but what does the casting (void) just before the equal sign mean?

// list0513.c
#include <dlfcn.h>
int main(void)
 
 void *handle = dlopen("./list0513.so", RTLD_LAZY);
 void (*func)(void) = dlsym(handle, "function");
 (*func)();
 dlclose (handle);
 return 0;
 

According to the book, the function called "function" is called from the following script

// list0513dl.c
#include <stdio.h>
void function(void)

 printf("Hello Worldn");

2) but how do I make a list0513.so file? the only files I've made are .c files...
Thanks for reading this.

The declaration reads as follows:

 func — func
 *func — is a pointer to
 (*func)( ) — a function taking
 (*func)(void) — no parameters
void (*func)(void) — returning void

The func pointer is then initialized with the result of the dlsym call, which returns the address of the function ”function” in the library list0513.so.

General declaration rules for pointer types:

T *p; // p is a pointer to T
T *p[N]; // p is an array of pointer to T
T (*p)[N]; // p is a pointer to an array of T
T *f(); // f is a function returning a pointer to T
T (*f)(); // f is a pointer to a function returning T

In both declarations and expressions, the postfix [] subscript and () function call operators have higher precedence than unary *, so *f() is parsed as *(f()) (function returning pointer). To declare a pointer to an array or function, the * has to be explicitly grouped with the array or function declarator.

Declarations can get pretty complex - you can have an array of pointers to functions:

T (*a[N])(); // a is an array of pointers to functions returning T

or functions returning pointers to arrays:

T (*f())[N]; // f is a function returning a pointer to an array

or even pointers to arrays of pointers to functions returning pointers to arrays:

T (*(*(*a)[N])())[M];

You probably won’t see anything that hairy in the wild, though (unless you run across some old code of mine).

It is omitted a declare of function type. The full or expended version should like this:

// list0513.c
#include <dlfcn.h>
int main(void)
 
 void *handle = dlopen("./list0513.so", RTLD_LAZY);
 typedef void(*FUNC)();
 FUNC func = dlsym(handle, "function");
 func(); // call function
 dlclose (handle);
 return 0;