What is the meaning of “array type has incomplete element type”?

Question

I have the following simple lines of code:

#include <glib.h>
#include <stdio.h>

void my_func () {
    GHashTable htbls[3]; /* ASSUME LINE NUMBER IS N */
    /* Do something */
}

int main (int argc, char *argv[]) {
    my_func ();
    return 0;
}

But

$gcc `pkg-config --cflags --libs glib-2.0` ./main.c

gives the following error:

./main.c:N: error: array type has incomplete element type

I don't understand why the element type is incomplete. GHashTable is completely specified in glib.h.

Thanks in advance for your help.

Answer 1

It presumably means that GHashTable is not completely defined in the headers you include. That is, there is likely a line in glib.h or one of the files it includes that reads something like:

typedef struct GHashTable GHashTable;

The structure tag could be different without changing my argument. There must be something similar because otherwise you'd get a different message about GHashTable not being recognized as a type name:

error: unknown type name 'GHashTable'

There is no extra information provided about the structure in <glib.h>; you don't need to know it to use it. The API for the hash table functions probably deal with GHashTable * values only, so you don't need to know what's inside, any more than you need to know what's inside a FILE * to be able to use it in your code (though macroized functions such as getchar() might need to know about the internals of FILE *; maybe a better analogy is DIR *, but that's a POSIX interface and not necessarily as well known).

It means you will need to use:

 GHashTable *htbls[3];

You can have arrays of pointers to incomplete types without problem.

Answer 2

Are you sure about that? Copy/paste the specification from glib.h. (My glib.h does not contain a definition for GHashTable).

You need to use a pointer, not a value:

GHashTable* htbls[3];

Answer 3

Q: Isn't GHashTable is completely specified in glib.h?

A: No.

If you want a hashtable, you should use g_hash_table_new(), g_hash_table_add () and friends:

• http://developer.gnome.org/glib/stable/glib-Hash-Tables.html

In other words, "struct _GHashTable" is opaque to your application.

If you really want an array of GHashTables, then you'll need an array of "GHashTable *". And you'll need to explicitly initialize each element of your array with its own, new hash table.