Check if a pointer to function is initialized

Question

How can I check if a pointer to function was initialized? I can check for NULL, but if not null could be garbage, right?

I have the following:

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

typedef struct client_struct
{
    char *name;
    char *email;
    void (*print)(struct client_struct *c);
} client;

void print_client(client *c)
{

    if (c->print != NULL)
        c->print(c);
}

int main()
{
    client *c = (client *)malloc(sizeof(client));
    c->email = (char *)malloc(50 * sizeof(char));
    sprintf(c->email, "email@server.com");
    c->name = (char *)malloc(50 * sizeof(char));
    sprintf(c->name, "some name");

    //Uncommenting line below work as expected, otherwise segmentation fault
    //c->print = NULL;

    print_client(c);

    printf("\nEOF\n");
    int xXx = getchar();

    return 0;
}

How can I check if this pointer really points to function "void (*f)(client *)"? Comparing size doesn't work because could garbage in same size, correct? I would like a way to accomplish that preferably according to C standard.

Answer 1

As described in the comments, it is impossible to determine with 100% certainty whether a pointer is garbage.

To avoid such situation, you can provide a "constructor" function, like this:

struct client_struct* client_allocate()
{
    struct client_struct* object = malloc(sizeof *object);
    if (object)
    {
        object->name = NULL;
        object->email = NULL;
        object->print = NULL;
    }
    return object;
}

Then write in your documentation that the only valid way to create "clients" is by using your function. If you do this, you should also provide a destroy function, where you call free.

Suppose you add a new pointer to your struct one day. Then you update your client_allocate function, where you set this pointer to NULL, and the new pointer will always be properly initialized. There is no need to update all places in code where your struct is allocated, because now there is only one such place.

Answer 2

In c function pointers are no different than regular pointers and by standard they have one value that says the value should not be used and this is NULL.

The way you should work with pointers is to set them only to valid value or NULL. There is no other way you can be sure there is a OK value. And by definition every value that is not NULL should be considered valid.

Answer 3

Like pointed to in other comments and answers, there is not way to check a variable is initialized. That's why initializing vars to NULL and then checking is considered good practice.

If you really want to validate your function pointer is pointing to the correct place, you could export the function and load your pointer from the ELF symbols (see: http://gcc.gnu.org/wiki/Visibility)

Answer 4

Caveats

Checking if a pointer to a function is initialized with an valid function is not an easily solvable problem. Any solution, will not be portable across platforms, and is also dependent on the binary format (statically or dynamically linkable formats) that you end up with. There are ways to do this, with varying success, on different binary formats, however I am not going to go over every permutation. Hopefully this will get you going down that rabbit hole :-) and you can figure out the particular solution that works for you in your circumstances.

In order for some of the solutions to work you have to ensure that the linked binaries have exported symbols (it's possible to do it without, but it's a lot harder and I don't have the time). So when you're linking your program ensure that you have dynamic symbols enabled.

Having said that, here's an approach you can use on systems using dlfcn functions. (See History below)

More Caveats

As @Deduplicator points out in his comment below, there may be situations where 0xdeadbeef may arbitrarily happen to point to a valid function, in which case you may end up with a situation where you end up calling the wrong valid function. There are ways to mitigate that situation at either compile-time or runtime but you'll have to build the solution by hand. For example, C++ does it by mangling in namespace into the symbols. You could require that to happen. (I'll think of an interesting way to do this and post it)

Linux / SysV variants (Mac OSX included)

Use dladdr (SysV) (GNU has a dladdr1 as well) to determine which function does the address you provide fall within:

Example:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <dlfcn.h>


int is_valid_function_ptr( void *func)  {   
      Dl_info info;  
      int rc;      
    
      rc = dladdr(func, &info);   
    
      if (!rc) {
          /* really should do more checking here */
          return 0;
      }  

      return 1; /* you can print out function names and stuff here */    
}

void print(const char *value) {
    fprintf(stdout, "%s", value);
}

void call_the_printer(void (*foo)(), const char *value)
{
    if(is_valid_function_ptr(foo)) {
        foo(value);
    }
    else {
        fprintf(stderr, "The Beef is Dead!\n");
    }
}

int main() 
{
    void (*funcptr)() = (void (*)()) 0xdeadbeef; /* some dead pointer */
    call_the_printer(funcptr, "Hello Dead Beef\n");
    funcptr = print; /* actually a function */
    call_the_printer(funcptr, "Hello Printer\n");
    return 0;
}

NOTE Enable dynamic symbols for this to work

GCC/LLVM etc.

use -rdynamic or -Wl,--export-dynamic during the link process, so compile with:

gcc -o ex1 -rdynamic ex1.c

Windows

Windows does its own thing (as always) and I haven't tested any of these, but the basic concept should work:

Use GetModuleHandle and EnumCurrentProcess together to get loaded symbol information and run through the pointers in a loop to see they match any of the address therein.

The other way would be to use VirtualQuery and then cast mbi.AllocationBase to (HMODULE) and see if you get the path of your own binary back.

Answer 5

Always check for null parameters first of all.

void print_client(client *c) 
{
    if ((c != NULL) && (c->print != NULL)) 
    {
        c->print(c);
    }   
}

As for your question, nullify your client struct after it's been malloc'd. This way you can ensure that an unassigned function pointer shall indeed ==NULL.

client* create_client(void)
{
    client *c = malloc(sizeof(client));
    if (c != NULL)
    {
        memset(c, 0, sizeof(c))
    }

    return c;
}