Segmentation fault in malloc replication

I am trying to implement an example from the book The C programming language. The example replicates a simplified version of malloc. My implementation produces a Segmentation fault. The functions alloc and afree are basically copied from the book. I am trying to use those functions in main. As far as I understood, the *(pointer+i) expression gives me the value stored in the address next to the pointer as long as both addresses are in the same array. It should be implicitly satisfied but obviously, it is not enough.

How can I actually use the functions alloc and afree to create dynamic array?

#include <stdio.h>

int *alloc(int);
void afree(int *);

/* RETURNS SEGMENTATION FAULT, WHY? */
int main()

int *dyna_arr;

dyna_arr = alloc(50);

/* fill the array with integers */
for (int i = 0; i < 50; i++)

*(dyna_arr+i) = i;


for (int i=49; i>=0;i--)

printf(" %d", *(dyna_arr+i));


afree(dyna_arr);


#define ALLOCSIZE 10000

static int allocbuf[ALLOCSIZE];
static int *allocp =allocbuf; /* next free position in buffer */

/* alloc: return pointer to n ints */
int *alloc(int n)

if (allocbuf + ALLOCSIZE - allocp >= n) /* is there enough space in the buffer? */

allocp += n;
return allocp - n;

else /* not enough space */

return 0;



/* afree: free the storage pointed to by p */
/* Only possible in LIFO fashion */
void afree(int *p)

if (p >= allocbuf && p < allocbuf + ALLOCSIZE)

allocp = p;




2 Answers
2

The problem is

if (allocbuf + ALLOCSIZE - allocp <= n) /* is there enough space in the buffer? */


your comparison is wrong.
It should be available memory >= n but your checking if available memory <= n and returning 0;

available memory >= n

if available memory <= n

0

Below modification should work

if (((allocbuf + ALLOCSIZE) - allocp) >= n) /* is there enough space in the buffer? */


Your issue is in alloc(50); wich gives you 50 bytes of memory, not space for 50 int. You need to use alloc(50*sizeof(int));.

alloc(50);

int

alloc(50*sizeof(int));

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.