Question

I have a variable k of type const char *, and a function in glib with the prototype

void g_hash_table_replace(GHashTable *hash_table,
                          gpointer key,
                          gpointer value);

gpointer is defined simply as

typedef void* gpointer;

I know that in this case it is, in fact, okay to pass in k as the key in g_hash_table_replace, however gcc gives me the error

service.c:49:3: warning: passing argument 2 of ‘g_hash_table_replace’ discards ‘const’ qualifier from pointer target type [enabled by default]
/usr/include/glib-2.0/glib/ghash.h:70:13: note: expected ‘gpointer’ but argument is of type ‘const char *’

this is with gcc 4.6.0. With 4.5.0 and earlier, a simple cast to (char *) sufficed to supress this warning, but gcc seems to have gotten ‘smarter’. I’ve tried (char *)(void *)k, but it still knows that the variable was originally const. What is the best way to silence this warning without calling strdup(3) on k?

Answer 1

I just tried this with gcc 4.6.1.

#include <glib/ghash.h>
#include <stdio.h>
#include <unistd.h>

const char *k="Testing";

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

    int val = 1024;

    GHashTable *hash_table=NULL;
    g_hash_table_replace(hash_table,(gpointer) (intptr_t)k, &val);

    return 0;
}

Without casts, the error is as you describe above. But if I cast the const char* to intptr_t first as shown above, the warning is suppressed. Can you confirm that you still experience the error with my code sample?