func pointers

/*

build shared library:

gcc -fPIC -c testas.c
ld -shared -soname libtestas.so.1 -o libtestas.so.1.0 -lc testas.o

testas.c:

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

void _init()
{
    printf("init\n");
}

void _fini()
{
    printf("fini\n");
}

void fncG()
{
   printf("test 2\n");
}

*/

//main.c:

//-std=c99 -Wall -Wextra -Wpedantic -Wshadow
//linking: -ldl -Wl,-rpath=.

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <dlfcn.h>

void fncA()
{
   printf("test\n");
}

int fncB(int x, int y)
{
    return x + y;
}

void *fncC(void *p)
{

   return p;
}

int fnc(int x)
{
   return x+1;
}

char fncc(char c)
{
   return c+1;
}

//typedef char (*PFC)(); warning: return from incompatible pointer type
typedef char (*PFC)(char);

char testF1(char a)
{
   return a+1;
}

char testF2(char a)
{
   return a+2;
}

PFC pfAddr(bool b)
{
   if (b) return &testF1;
   return &testF2;
}


int main (int argc, char **argv)
{

   int result;

   //notes:
   //the unary operator & is optional
   //to make code more portable, you should always use &
   //when assigning an address of a function to a function pointer

   void(*pf_fncA)()=&fncA; //=fncA;
   int(*pf_fncB)()=&fncB;  //=fncB;
   void *(*pf_fncC)();


   (*pf_fncA)();

   pf_fncA();


   result = (*pf_fncB)(1,2);
   printf("%d\n",result);


   pf_fncC = &fncC; //=fncC;

   printf("%p\n",(void*)&result);

   void *pResult = pf_fncC(&result);

   printf("%p\n",pResult);

   int (*fp)(int);

   fp = fnc;

   printf("%d\n",fp(1));
   printf("%d\n",(*fp)(1));

   char (*fpc)(char);

   //fpc = fncc;
   fpc = &fncc;

   printf("%c\n",fpc('a'));
   printf("%c\n",(*fpc)('b'));


   PFC pFTest;

   pFTest = pfAddr(true);

   printf("%c\n",pFTest('a'));

   pFTest = pfAddr(false);

   printf("%c\n",pFTest('a'));

   char *err;
   void *lib = dlopen("libtestas.so.1.0", RTLD_NOW);

   if (!lib)
   {
      printf("%s\n", dlerror());
      return 1;
   }

   dlerror();

   printf("INFO: load lib OK.\n");

   //warning: ISO C forbids initialization between function pointer and ‘void *’
   //void(*pfVoid)() = dlsym(lib, "fncG");

   void(*pfVoid)();

   *(void **)(&pfVoid) = dlsym(lib, "fncG");

   if ( (err = dlerror()) != NULL)
   {
      printf("%s\n", err);
      dlclose(lib);
      return 1;
   }

   //function must be called before dlclose(lib), or there will be a segfault
   pfVoid();

   dlclose(lib);

   printf("INFO: exit OK.\n");

   return 0;
}

/*
output:

test
test
3
0x7ffe780bbff4
0x7ffe780bbff4
2
2
b
c
b
c
init
INFO: load lib OK.
test 2
fini
INFO: exit OK.


*/