Access violation of dynamic array for hashmap - c

Question

I am having an issue with the table of a little hashmap I am trying to implement.
map.h
typedef struct Map Map;
Map *map_create();
int map_set(Map *map, char *key, void *val);
map.c
const int MAP_INITIAL_SIZE = 100;
typedef struct MapPair MapPair;
struct MapPair
{
char *key;
void *val;
};
struct Map
{
MapPair **table;
int count;
int limit;
};
Map *map_create(void)
{
Map *map = (Map*)malloc(sizeof(Map));
if (!map) return NULL;
map->table = (MapPair**)malloc(MAP_INITIAL_SIZE * sizeof(MapPair));
if (!map->table)
{
free(map);
return NULL;
}
map->count = 0;
map->limit = MAP_INITIAL_SIZE;
return map;
}
void add(MapPair **context, int start, MapPair *pair, int limit)
{
int i = start;
while (context[i] != NULL && strcmp(context[i]->key, pair->key) != 0) // crashing here
{
i++;
if (i == limit) i = 0;
}
context[i] = pair;
}
int map_set(Map *map, char *key, void *val)
{
if (map->count >= map->limit / 2)
{
if (!expand(map)) return 0;
}
MapPair *pair = (MapPair*)malloc(sizeof(MapPair));
if (!pair) return 0;
pair->key = key;
pair->val = val;
add(map->table, hash(key, map->limit), pair, map->limit);
++map->count;
return 1;
}
I was originally developing in pelles c but moved to vs2013 for the debugger when I was experiencing problems. Then in vs2013 the program would crash at the add function but not in pelles c. I am assuming it has something to do with my dynamic array that I plan to be able to expand later.
Can anybody tell me why the program seems to crash when I try to access an index of the dynamic array?

Answer 1

In add function you are checking the table, until you reach the NULL pointer:
while (context[i] != N ...
But when you allocate this table you never set any of those pointers to NULL:
map->table = (MapPair**)malloc(MAP_INITIAL_SIZE * sizeof(MapPair));
You should set them to NULL:
for( size_t i = 0 ; i < MAP_INITIAL_SIZE ; i++ )
map->table[i] = NULL ;
Otherwise you will go out of bounds of that array.

Answer 2

I didn't know Visual could compile pure C projects ! Anyway, your crash is caused by a magic string : http://en.wikipedia.org/wiki/Magic_number_(programming)
* 0xABABABAB : Used by Microsoft's HeapAlloc() to mark "no man's land" guard bytes after allocated heap memory
* 0xABADCAFE : A startup to this value to initialize all free memory to catch errant pointers
* 0xBAADF00D : Used by Microsoft's LocalAlloc(LMEM_FIXED) to mark uninitialised allocated heap memory
* 0xBADCAB1E : Error Code returned to the Microsoft eVC debugger when connection is severed to the debugger
* 0xBEEFCACE : Used by Microsoft .NET as a magic number in resource files
* 0xCCCCCCCC : Used by Microsoft's C++ debugging runtime library to mark uninitialised stack memory
* 0xCDCDCDCD : Used by Microsoft's C++ debugging runtime library to mark uninitialised heap memory
* 0xDEADDEAD : A Microsoft Windows STOP Error code used when the user manually initiates the crash.
* 0xFDFDFDFD : Used by Microsoft's C++ debugging heap to mark "no man's land" guard bytes before and after allocated heap memory
* 0xFEEEFEEE : Used by Microsoft's HeapFree() to mark freed heap memory
(SO source : In Visual Studio C++, what are the memory allocation representations?)
Unlike GCC (or pelles I imagine), Visual Studio set uninitialized heap array pointers as 0xCDCDCDCD, not NULL. So your check of context[i] != NULL returns true even though context is not initialized.
... And that's why explicit is always better than implicit.