g_hash_table_insert() and local string array

patiobarbecue · **Joined:** Sun Dec 16, 2012 9:09 pm **Posts:** 12

Dear there,

According to the manual:

Quote:

Note that neither keys nor values are copied when inserted into the GHashTable, so they must exist for the lifetime of the GHashTable. This means that the use of static strings is OK, but temporary strings (i.e. those created in buffers and those returned by GTK+ widgets) should be copied with g_strdup() before being inserted.

I am confused why the following code works? After atable_new() exist, shouldn't keys[] and values[] on stack also gone?

Code:

 
//tmp1.h
#ifndef _TMP1_
#define _TMP1_
#include <glib.h>
#include <gtk/gtk.h>
typedef void atable;
atable *atable_new();
void atable_free(atable *table);
gchar *atable_lookup(atable* table, gpointer key);
#endif

Code:

 
//tmp1.c
#include "tmp1.h"
atable *atable_new(){
  gchar *keys[]={"sam", "jack"};
  gchar *values[]={"happy", "sad"};
  GHashTable *table;
  table = g_hash_table_new(g_str_hash, g_str_equal);
  int i;
  for (i=0;i<2;i++)
    g_hash_table_insert(table, (gpointer)keys[i], (gpointer)values[i]);
  return (atable *)table;
}
void atable_free(atable *table){
 g_hash_table_destroy((GHashTable *)table);
}
gchar *atable_lookup(atable* table, gpointer key){
 return (gchar *) g_hash_table_lookup((GHashTable *)table, (gpointer)key);
}

Code:

 
//tmpmain.c
#include <stdio.h>
#include "tmp1.h"
int main(){
  atable *table;
  table = atable_new();
  printf("%s\n", atable_lookup(table, "sam"));
  printf("%s\n", atable_lookup(table, "jack"));
  atable_free(table);
}

I compiled tmpmain and the output is:
happy
sad

I thought this code should not work!

Paul Childs · **Posted:** Fri Dec 21, 2012 7:29 pm

I don't think the data values are actually on the stack.
Your c compiler will generally perform a number of optimisations. When it sees that the data in the atable_new function is not going to change and is identical for every atable_new call, it will leave it in static memory as that is faster and more memory efficient.

patiobarbecue · **Joined:** Sun Dec 16, 2012 9:09 pm **Posts:** 12

Yes, you are right! I knew the code were problemtic; just confused why it could possibly work. I actually used flag -O0. It seems gcc still did something behind the scene.

A slightly modified code did not work as expected:

./tmpmain
Segmentation fault (core dumped)

Code:

#include "tmp1.h"
atable *atable_new(char *key1, char *value1, char* key2, char *value2){

  GHashTable *table;
  table = g_hash_table_new(g_str_hash, g_str_equal);
  int i;
  g_hash_table_insert(table, (gpointer)key1, (gpointer)value1);
  g_hash_table_insert(table, (gpointer)key2, (gpointer)value2);
  return (atable *)table;
}
void atable_free(atable *table){
  g_hash_table_destroy((GHashTable *)table);
}
gchar *atable_lookup(atable* table, gpointer key){
  return (gchar *) g_hash_table_lookup((GHashTable *)table, (gpointer)key);
}

Code:

//tmp1.h
#ifndef _TMP1_
#define _TMP1_
#include <glib.h>
#include <gtk/gtk.h>
typedef void atable;
atable *atable_new();
void atable_free(atable *table);
gchar *atable_lookup(atable* table, gpointer key);
#endif

Code:

#include <stdio.h>
#include "tmp1.h"
int main(int argc, char **argv){
  atable *table;
  table = atable_new(argv[1], argv[2], argv[3], argv[4]);
  printf("%s\n", atable_lookup(table, "sam"));
  printf("%s\n", atable_lookup(table, "jack"));
  atable_free(table);
}

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g_hash_table_insert() and local string array

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