C (void *) used as polymorphic function pointer

Question

I'm trying to create a system call handler, and I'm not sure how to store it.

I'm using the following typedef to store a (void *) pointer, which should receive the address of the function and an integer arg_no representing the number of arguments. Then, I create an array of this type.

typedef struct
{
  void *foo;
  int arg_no;
}td_sys_call_handler;

td_sys_call_handler ish[SYSCALL_HANDLER_NUM];

I'm trying to initialize the array in the following manner.

  ish[0].foo  = void     (*halt) (void);                  ish[0].arg_no  = 0;
  ish[1].foo  = void     (*exit) (int status) NO_RETURN;  ish[1].arg_no  = 1;
  ish[2].foo  = pid_t    (*exec) (const char *file);      ish[2].arg_no  = 1;
  ish[3].foo  = int      (*wait) (pid_t);                 ish[3].arg_no  = 1;
  ish[4].foo  = bool     (*create) (const char *file, unsigned initial_size);
                                                          ish[4].arg_no  = 2;
  ish[5].foo  = bool     (*remove) (const char *file);    ish[5].arg_no  = 1;
  ish[6].foo  = int      (*open) (const char *file);      ish[6].arg_no  = 1;
  ish[7].foo  = int      (*filesize) (int fd);            ish[7].arg_no  = 1;
  ish[8].foo  = int      (*read) (int fd, void *buffer, unsigned length);
                                                          ish[8].arg_no  = 3;
  ish[9].foo  = int      (*write) (int fd, const void *buffer, unsigned length);
                                                          ish[9].arg_no  = 3;
  ish[10].foo = void     (*seek) (int fd, unsigned position);
                                                          ish[10].arg_no = 2;
  ish[11].foo = unsigned (*tell) (int fd);                ish[11].arg_no = 1;

But all the assignments from the function pointer to the void pointer produce the following error:

../../userprog/syscall.c: In function ‘syscall_init’:
../../userprog/syscall.c:76:17: error: expected expression before ‘void’
../../userprog/syscall.c:77:17: error: expected expression before ‘void’
../../userprog/syscall.c:78:17: error: expected expression before ‘pid_t’
../../userprog/syscall.c:79:17: error: expected expression before ‘int’
../../userprog/syscall.c:80:17: error: expected expression before ‘_Bool’
../../userprog/syscall.c:82:17: error: expected expression before ‘_Bool’
../../userprog/syscall.c:83:17: error: expected expression before ‘int’
../../userprog/syscall.c:84:17: error: expected expression before ‘int’
../../userprog/syscall.c:85:17: error: expected expression before ‘int’
../../userprog/syscall.c:87:17: error: expected expression before ‘int’
../../userprog/syscall.c:89:17: error: expected expression before ‘void’
../../userprog/syscall.c:91:17: error: expected expression before ‘unsigned’

I was under the impression that void* is the only instance of polymorphism in the language and that it can point to anything. However, it appears that I'm wrong.

So which is the type of the pointer which can store the address of any function type?

Also, can you give me a good reference about C polymorphism? I've looked in many books but as far as I've seen the polymorphism chapter is very thin.

Thank you.

Answer 1

Yes, you are wrong.

void * pointers can point at any kind of data, but in C code (functions) are not data.

It's not valid to even cast between void * and function pointers: even though on most contemporary computers it will work as expected, the language does not guarantee that.

I don't understand from your code how you intended the "overloading" to be used in practice, how do you expect to call through the foo pointer? Just having the expected number of arguments is not enough, arguments have types and thus are handled differently in the function call.

Answer 2

The notation you need casts the system call function pointer to void *:

ish[0].foo  = (void *)halt;

The C standard does not guarantee that pointers to functions will fit into pointers to data such as void *; fortunately for you, POSIX steps in and does guarantee that pointers to functions are the same size as pointers to data.

Answer 3

Your syntax is wrong. You should declare your function pointer first. Then you can use the address of the function pointer to assign to the pointer.

void (*halt) (void) = halt_sys_call_function;
ish[0].foo  = &halt; ish[0].arg_no  = 0;

C doesn't support traditional inheritance relationships in a direct way, but it does guarantee that the address of a structure is also the address of the first member of the structure. This can be used to emulate polymorphism in C. I described a similar approach in an answer I wrote about dynamic dispatch in C.

Answer 4

Consider a struct formatted to hold each function specifically:

typedef struct 
{

  void     (*halt) (void);                  
  void     (*exit) (int status);  
  pid_t    (*exec) (const char *file);      
  int      (*wait) (pid_t);                 
  bool     (*create) (const char *file, unsigned initial_size);
  bool     (*remove) (const char *file);    
  int      (*open) (const char *file);      
  int      (*filesize) (int fd);            
  int      (*read) (int fd, void *buffer, unsigned length);
  int      (*write) (int fd, const void *buffer, unsigned length);  
  void     (*seek) (int fd, unsigned position);   
  unsigned (*tell) (int fd);                

} myFuncs;

OR

This is messy and VERY unmaintable, but if you did cast each pointer to a void*, using void *addressOfWait = (void*)&wait;, then you could re-cast to the correct function pointer type before calling:

int (*waitFunctionPointer)(pid_t) = addressOfWait;

Then you could call that pointer:

waitFunctionPointer((pid_t) 1111); //wait for process with pid of 1111

Answer 5

I'll ask for @problemPotato's forgiveness for filching his structure definition:

typedef struct 
{
   void     (*halt) (void);                  
   void     (*exit) (int status);  
   pid_t    (*exec) (const char *file);      
   int      (*wait) (pid_t);                 
   bool     (*create) (const char *file, unsigned initial_size);
   bool     (*remove) (const char *file);    
   int      (*open) (const char *file);      
   int      (*filesize) (int fd);            
   int      (*read) (int fd, void *buffer, unsigned length);
   int      (*write) (int fd, const void *buffer, unsigned length);  
   void     (*seek) (int fd, unsigned position);   
   unsigned (*tell) (int fd);                
} fs_ops;

Say you have matching functions, declared like:

int      ext5_open(const char * file);
unsigned ext5_tell (int fd);

then you can define and initialize a variable like (the bare name of the function is a pointer to it):

fs_ops ext5_ops = {
   .open = ext5_open,
   .tell = ext5_tell,
};

Fields that aren't initialized get NULL (i.e., pointer to no function). You can change the value of a field, ask if it is set (if(ext5_ops.seek == NULL) ...), and call the function:

retval = ext5_ops.(*ext5_open)("/tmp/junk");

(the parenteses around (*ext5_open) are because * (pointer indirection) binds less strongly than function call).

Answer 6

A function pointer can be converted into a void *, but it's a little trickier to convert it back to correct function-pointer type in order to call it. It should be possible by using a union. You'll need a separate union-member of the correct type for type of function that you want to store. And, as user4815162342 notes in a comment, you'll need to manage all the various combinations, probably with an enum.

typedef struct
{
  union {
    void *vp;
    void (*v__v)(void);
    void (*v__i)(int);
    pid_t (*pid__ccp)(const char *);
    int (*i__pid)(pid_t);
    bool (*b__ccp_u)(const char *, unsigned);
    bool (*b__ccp)(const char *);
    int (*i__ccp)(const char *);
    int (*i__i)(int);
    int (*i__i_vp_u)(int, void *, unsigned);
    int (*i__i_cvp_u)(int, const void *, unsigned);
    void (*v__i_u)(int, unsigned);
    unsigned (*u__i)(int);
  } fp;
  int arg_no;
}td_sys_call_handler;

The idea here is to try to encode the types into the identifiers, as a kind of "apps-Hungarian". This way, the meaning of any of these identifiers is directly visible.

It may be easier to generate these pointers and the associated enum at the same time. I think the easiest way to manage this part is with my favorite trick, X-Macros. Warning: it just gets more and more weird.

#define function_types(_) \
    _(v__v, void, void) \
    _(v__i, void, int) \
    _(pid_ccp, pid_t, const char *) \
    _(i__pid, int, pid_t) \
    _(b__ccp_u, const char *, unsigned) \
    _(b__ccp, const char *) \
    _(i__ccp, const char *) \
    _(i__i, int) \
    _(i__i_vp_u, int, void *, unsigned) \
    _(i__i_cvp_u, int, const void *, unsigned) \
    _(v__i_u, int, unsigned) \
    _(u__i, unsigned, int) \
    /* end function_types */

This "master"-macro is a comma separated table of tokens which is passed, row by row, to the _ underscore macro, which is passed-in.

Now the struct type can be constructed by writing additional macros to use the rows, these are passed-in as _ to the table macro to instantiate the template:

#define create_function_pointer(id, ret, ...) \
    ret (*id)(__VA_ARGS__);

#define create_function_type_id(id, ret, ...) \
    f__ ## id

typedef struct {
    union {
        void *vp;
        function_types(create_function_pointer)
    } fp;
    int arg_no;
    enum {
        function_types(create_function_type_id)
    } type;
} td_sys_call_handler;

Now an array of these structs can be populated:

td_sys_call_handler ish[SYSCALL_HANDLER_NUM];
int i=0;

ish[i++]  = (td_sys_call_handler){ halt,     0, f__v__v };
ish[i++]  = (td_sys_call_handler){ exit,     1, f__v__i };
ish[i++]  = (td_sys_call_handler){ exec,     1, f__pid__ccp };
ish[i++]  = (td_sys_call_handler){ wait,     1, f__i__pid };
ish[i++]  = (td_sys_call_handler){ create,   2, f__b__ccp_u };
ish[i++]  = (td_sys_call_handler){ remove,   1, f__b__ccp };
ish[i++]  = (td_sys_call_handler){ open,     1, f__i__ccp };
ish[i++]  = (td_sys_call_handler){ filesize, 1, f__i__i };
ish[i++]  = (td_sys_call_handler){ read,     3, f__i__i_vp_u };
ish[i++]  = (td_sys_call_handler){ write,    3, f__i__i_cvp_u };
ish[i++]  = (td_sys_call_handler){ seek,     2, f__v__i_u };
ish[i++]  = (td_sys_call_handler){ tell,     1, f__u__i };

Now, calling a function given one of these structs will require (as you surmised) a switch, with a separate case for each signature. It needs to crack the arguments using stdarg and the call with the appropriate union member function pointer.

void make_sys_call(td_sys_call_handler ish, ...){
    va_list ap;
    int i;
    const char *ccp;
    pid_t pid;
    bool b;
    void *vp;
    unsigned u;
    const void *cvp;
    va_start(ap, ish);
    switch(ish.type) {
    case f__v__f: ish.fp.v__v();
                  break;
    case f__v__i: i = va_arg(int);
                  ish.fp.v__i(i);
                  break;
    case f__pid__ccp: ccp = va_arg(const char *);
                      ish.fp.pid__ccp(ccp);
                      break;
    // etc.
    }
    va_end(ap);
}

It will not be possible to return different types directly. You will either need to allocate a union type variable to hold the return value and return that, or something even more insane. An external stack data type could hold unions of the various return types. Depending on profiling results, it may be appropriate to consider this instead of returning the unions.

HTH.