Memory Allocator implementation segfault - c

Question

I am currently developing a memory allocator, I have come with this code a the moment but this code below segfault on almost every commands when i do a LD_PRELOAD of my implementation.
#include <pthread.h>
#include "malloc.h"
t_mem_map g_map;
int g_empty_map = TRUE;
static t_ctrl *get_free_block(size_t size)
{
t_ctrl *tmp;
tmp = g_map.head;
while (tmp != NULL)
{
if (tmp->is_free == TRUE && tmp->size < size)
{
tmp->is_free = FALSE;
return (tmp);
}
tmp = tmp->next;
}
return (NULL);
}
static void *init_map(size_t size)
{
t_ctrl *tmp;
g_map.map_size = DEFAULT_MAP_SIZE;
while (g_map.map_size < size + sizeof(t_ctrl))
g_map.map_size += DEFAULT_MAP_SIZE;
if ((g_map.head = (t_ctrl *)sbrk(g_map.map_size)) == (void *)-1)
return (NULL);
tmp = g_map.head;
tmp->next = NULL;
tmp->prev = NULL;
tmp->size = size;
tmp->is_free = FALSE;
g_map.free_space = g_map.map_size - size - sizeof(t_ctrl);
g_empty_map = FALSE;
unlock_thread();
return ((void *)((char *)tmp + sizeof(t_ctrl)));
}
static void *add_block(size_t size)
{
t_ctrl *tmp;
t_ctrl *new;
tmp = get_free_block(size);
if (tmp != NULL)
return ((void *)((char *)tmp + sizeof(t_ctrl)));
tmp = g_map.head;
while (tmp->next != NULL)
tmp = tmp->next;
new = (t_ctrl *)((char *)tmp + sizeof(t_ctrl) + tmp->size);
new->prev = tmp;
new->next = NULL;
tmp->next = new;
new->size = size;
new->is_free = FALSE;
g_map.free_space -= (new->size + sizeof(t_ctrl));
unlock_thread();
return ((void *)((char *)new + sizeof(t_ctrl)));
}
static void *resize_map(size_t size)
{
size_t size_shift;
size_shift = 0;
while (g_map.free_space < size + sizeof(t_ctrl))
{
g_map.map_size += DEFAULT_MAP_SIZE;
g_map.free_space += DEFAULT_MAP_SIZE;
size_shift += DEFAULT_MAP_SIZE;
}
if (sbrk(size_shift) == (void *)-1)
return (NULL);
return (add_block(size));
}
void *malloc(size_t size)
{
size_t a_size;
lock_thread();
a_size = ALIGN(size);
if (g_empty_map == TRUE)
return (init_map(a_size));
else
{
if ((a_size + sizeof(t_ctrl)) <= g_map.free_space)
return (add_block(a_size));
else
return (resize_map(a_size));
}
return (NULL);
}
Here is the malloc.h :
# include <stdio.h>
# include <stddef.h>
# include <unistd.h>
# define TRUE 0
# define FALSE 1
# define SUCCESS 0
# define FAILURE 1
# ifndef __X86_64__
# define ALIGNMENT (16)
# else
# define ALIGNMENT (8)
# endif
# define ALIGN(size) (((size) + (ALIGNMENT - 1)) &~ (ALIGNMENT - 1))
# define DEFAULT_MAP_SIZE (ALIGN(sysconf(_SC_PAGESIZE)))
typedef struct s_ctrl
{
size_t is_free;
size_t size;
struct s_ctrl *next;
struct s_ctrl *prev;
} t_ctrl;
typedef struct s_mem_map
{
size_t map_size;
size_t free_space;
// int free_blocks;
//int nb_blocks;
t_ctrl *head;
} t_mem_map;
With some research I found out that the segfault is likely coming from the while loop in the get_free_block() function but i can't understand why.
static t_ctrl *get_free_block(size_t size)
{
t_ctrl *tmp;
tmp = g_map.head;
while (tmp != NULL)
{
if (tmp->is_free == TRUE && tmp->size < size)
{
tmp->is_free = FALSE;
return (tmp);
}
tmp = tmp->next;
}
return (NULL);
}

Answer 1

Can't verify that this is the only problem, but you are checking tmp->size < size, when you probably should check tmp->size >= size instead.