Receiving a Segmentation Fault when using this memory allocator - c

Question

I am trying to implement a memory allocator which can be called by multiple processes at once, so I used shared memory to keep the information of the first and last memory block alocated, both are put in the first 32 bytes of the shared memory and after that I tried to just have a simple allocation from the getpagesize() offset in the shared memory to see if it works but I keep getting a Segmentation Fault when using it to allocate a simple integer variable and I cannot figure out why.
#include <unistd.h>
#include <string.h>
#include <pthread.h>
/* Only for the debug printf */
#include <stdio.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
typedef char ALIGN[16];
union header {
struct {
size_t size;
unsigned is_free;
union header *next;
} s;
/* force the header to be aligned to 16 bytes */
ALIGN stub;
};
typedef union header header_t;
header_t *get_free_block(size_t size)
{
char shm_name[] = "memalloc";
int shm_fd;
shm_fd = shm_open(shm_name, O_CREAT|O_RDWR, S_IRUSR|S_IWUSR);
if (shm_fd < 0)
{
perror(NULL);
return NULL;
}
size_t shm_size = 10 * getpagesize();
if (ftruncate(shm_fd, shm_size) == -1)
{
perror(NULL);
shm_unlink(shm_name);
return NULL;
}
char* shm_ptr = mmap(0, shm_size, PROT_READ|PROT_WRITE, MAP_SHARED, shm_fd, 0);
if (shm_ptr == MAP_FAILED)
{
perror(NULL);
shm_unlink(shm_name);
return NULL;
}
header_t *head;
if (shm_ptr)
head = (header_t*)shm_ptr;
else
head = NULL;
header_t *tail;
if (head)
tail = (header_t*)(shm_ptr + 16);
else
tail = NULL;
header_t *curr = head;
while(curr) {
/* see if there's a free block that can accomodate requested size */
if (curr->s.is_free && curr->s.size >= size)
return curr;
curr = curr->s.next;
}
return NULL;
}
void *malloc(size_t size)
{
char shm_name[] = "memalloc";
int shm_fd;
shm_fd = shm_open(shm_name, O_CREAT|O_RDWR, S_IRUSR|S_IWUSR);
if (shm_fd < 0)
{
perror(NULL);
return NULL;
}
size_t shm_size = 10 * getpagesize();
if (ftruncate(shm_fd, shm_size) == -1)
{
perror(NULL);
shm_unlink(shm_name);
return NULL;
}
char* shm_ptr = mmap(0, 32, PROT_READ|PROT_WRITE, MAP_SHARED, shm_fd, 0);
if (shm_ptr == MAP_FAILED)
{
perror(NULL);
shm_unlink(shm_name);
return NULL;
}
header_t *head;
if (shm_ptr)
head = (header_t*)shm_ptr;
else
head = NULL;
header_t *tail;
if (head)
tail = (header_t*)(shm_ptr + 16);
else
tail = NULL;
size_t total_size;
void *block;
header_t *header;
if (!size)
return NULL;
header = get_free_block(size);
if (header) {
/* Woah, found a free block to accomodate requested memory. */
header->s.is_free = 0;
return (void*)(header + 1);
}
/* We need to get memory to fit in the requested block and header from OS. */
total_size = sizeof(header_t) + size;
int nr_pagesize = 1;
block = mmap(0, total_size, PROT_READ|PROT_WRITE, MAP_SHARED, shm_fd, nr_pagesize * getpagesize());
if (block == MAP_FAILED)
{
perror(NULL);
shm_unlink(shm_name);
return NULL;
}
header = block;
header->s.size = size;
header->s.is_free = 0;
header->s.next = NULL;
if (!head)
head = header;
if (tail)
tail->s.next = header;
tail = header;
return (void*)(header + 1);
}
Here is the test program:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main()
{
int *x = (int*)malloc(sizeof(int));
return 0;
}
The commands I used on terminal are:
gcc -o memalloc.so -fPIC -shared memalloc.c
export LD_PRELOAD=$PWD/memalloc.so
gcc helloworld.c -o hello
and after compiling the test program I get the Segmentation Fault.