GCC compilation error, braces around scalar initializer [-Werror]

Question

I'm new in C programming and I try to create a struct containing a char and a function pointer, and that is the problem.

I want to change that code:

switch (flag) {
    case 'd':
    case 'i':
    return (my_put_nbr(va_arg(ap, int)));
    case 'p':
    return (my_putstr("0x") + my_l_base_changer(va_arg(ap, char *),
    16));
    case 'u':
    return (my_put_unbr(va_arg(ap, unsigned int)));
    case 'b':
    return (my_u_base_changer(va_arg(ap, unsigned int), 2, 0));
    case 'o':
    return (my_u_base_changer(va_arg(ap, unsigned int), 8, 0));
    case 'x':
    return (my_u_base_changer(va_arg(ap, unsigned int), 16, 0));
    case 'X':
    return (my_u_base_changer(va_arg(ap, unsigned int), 16, 1));
    default:
    return (0);
}

in that kind of code:

typedef struct fptr
{
   char op;
   int (*ptr)(va_list);
} fptr;

fptr  fptrs[]=
{
   { 's', ft_print_nb_spec },
   { 'S', ft_print_nb_up },
   { 0, NULL }
};
  int i = 0;
  while (fptrs[i++].op != flag && fptrs[i].op != 0);
  if (fptrs[i].op != 0)
  {
     fptrs[i].ptr();
  }

And my struct contain a function pointer to functions who get differents prototypes

My struct is like that :

typedef struct action {
    char flag;
    void *function;
} action;

And my code who bug like that :

action registers[] = {
    { 'd', (int *)(*my_put_nbr)(int) },
    { 'i', (int)(*my_put_nbr)(va_arg(ap, int)) },
    { 'p', (int)(*my_print_ptr)(va_list ap) },
    { 'u', (int)(*my_put_unbr)(va_arg(ap, unsigned int)) },
    { 'b', (int)(*my_u_base_changer)(va_arg(ap, unsigned int), 2, 0) },
    { 'o', (int)(*my_u_base_changer)(va_arg(ap, unsigned int), 8, 0) },
    { 'x', (int)(*my_u_base_changer)(va_arg(ap, unsigned int), 8, 0) },
    { 'X', (int)(*my_u_base_changer)(va_arg(ap, unsigned int), 8, 1) },
    { 0, NULL }
};

And when I run gcc got that:

flag_nbr.c:13:28: error: expected expression before ‘int’
{ 'd', (int)(my_put_nbr)(int) },
                         ^~~
flag_nbr.c:13:28: error: missing initializer for field ‘function’ of ‘printf_action {aka struct <anonymous>}’ [-Werror=missing-field-initializers]
In file included from flag_nbr.c:8:0:
../include/my.h:21:8: note: ‘function’ declared here
void *function;
      ^~~~~~~~
flag_nbr.c:14:10: error: initialization makes pointer from integer without a cast [-Werror=int-conversion]
{ 'i', (int)(my_put_nbr)(va_arg(ap, int)) },
        ^

And I don't understand that error...

What you have as function pointers actually look more like function calls. When assigning a function pointer, you only specify the name of the function. Perhaps you should explain exactly what you're trying to accomplish by doing this. — dbush, Nov 17 '17 at 16:08
I want a list of structs that contains a flag and the pointer to the function related to the flag, so with that use I want to "emule" a switch case or a forest of if, with only ONE while and ONE if, like that: int i = -1; while (fptrs[++i].op != flag && fptrs[i].op != 0); if (fptrs[i].op != 0) { fptrs[i].ptr(); } — Pierre MARTY, Nov 17 '17 at 16:13
You can't call a function through a `void *`. You need to know exactly what the function signature is to call it, and you need to pass it parameters. You're better of using a `switch` statement to determine what to call and how. — dbush, Nov 17 '17 at 16:17
What you have currently is clear and concise. I would recommend leaving it as without trying to overcomplicate things with function pointers. — dbush, Nov 17 '17 at 16:37
By doing that I'd want to understand how the function pointers works ^^ And overall function pointers permites to get a more maintainable code — Pierre MARTY, Nov 17 '17 at 17:06

Sourav Ghosh · Accepted Answer · 2017-11-17T15:48:01.597

2

When in doubt, check the data types!!

You define the variable as an array of pointers to the action type.

What you pass in the initializers are the brace enclosed list which is certainly no match for a pointer to the action type.

In other words, you try to pass a brace enclosed list while a pointer was expected. Now, an initializer for a pointer is supposed to be a singular value (or scalar, as you may call it), so a brace enclosure is not MANDATORY.

While this is perfectly valid, most of the times, this may be caused by an issue where you write something you did not intend to write (example: your case!!) and thus, gcc produces a warning for this.

You may want to change your array type to hold structures of type action, not pointers to them.

edited Nov 17 '17 at 15:48

answered Nov 17 '17 at 15:42

Sourav Ghosh

133,132
16
183
261

Okay found it, but, What is a scalar-initializer in C ? – Pierre MARTY Nov 17 '17 at 15:49
I suspect that fixing that will raise many more issues with the way the function pointers (if they can be called that) are used. – dbush Nov 17 '17 at 15:50
@dbush well, there's a bigger issue at hand, How does OP intend to convert the FP's to their original types? – Sourav Ghosh Nov 17 '17 at 15:51
@SouravGhosh Exactly. Function pointers are not easy for newbies to understand. – dbush Nov 17 '17 at 15:55
I know but I know how to do a switch for this case but that took a lot of code line and function pointers are more useful – Pierre MARTY Nov 17 '17 at 16:00

Prawin · Answer 2 · 2017-11-17T15:53:26.953

0

You're using typedef to give a name to structure which already has the same name,check that. This may not be the main cause.

edited Nov 17 '17 at 15:53

answered Nov 17 '17 at 15:51

Prawin

59
5

and that is supposed to be an issue? – Sourav Ghosh Nov 17 '17 at 15:51
Nope. I too am new to programming – Prawin Nov 17 '17 at 15:53
Well, welcome. Also you may consider deleting the answer as it may not be particularly helpful,as you might have understood. – Sourav Ghosh Nov 17 '17 at 15:56

GCC compilation error, braces around scalar initializer [-Werror]

2 Answers2