Simple malloc function in c

Question

#define SIZE 7000

static char buf[SIZE]; 
static char *bufptr = buf;

struct node 
{
    int reg_num;
    int val;
    char var_name[30];
    char var_str[100];
    struct node *memroy;
    struct node *next;
};

struct node* add(struct node *head, int i)
{
    struct node *temp;

    if (head == NULL)
    {
        temp = (struct node *)malloc(sizeof(struct node));
        temp->next = NULL;
        temp->reg_num = i;
        head = temp;
    }
    else
    {
        head->next = add(head->next, i);
    }
    return head;
} 


void* malloc(int n) 
{
    if (buf + SIZE - bufptr >= n) 
    { 
        bufptr += n;
        return bufptr - n;
    }
    else
    {
        return NULL;
    }
}

When I run my programm it crashes during the assignment temp->next = NULL.

I think the problem is in my malloc function. I tested it with malloc in libraries and it worked correctly, but I not allowed to use libraries and must write a new malloc function.

I don't want to use sbrk() and brk() I need just a simple malloc just for allocation not free and realloc and... — moon, Jun 09 '15 at 12:51
Welcome to Stack Overflow! Standard warning: [Do not cast the result of malloc](http://stackoverflow.com/q/605845/1151654) — Eregrith, Jun 09 '15 at 12:58
That's a risky and inefficient way to append a node to a list. — chqrlie, Jun 09 '15 at 13:05
To avoid confusion, always use a distinguishable name for user defined functions. Don't use the same name as the library function itself. — Sourav Ghosh, Jun 09 '15 at 13:05
compiling using gcc with '-Wall -Wextra -pedantic' exposes several problems with the code: 1) conflict with built-in function malloc() 2) 'NULL' not defined 3) control reached end of non-void function without a return statement. — user3629249, Jun 10 '15 at 05:28

score 7 · Answer 1 · answered Jun 09 '15 at 12:58

7

You never check the return of your malloc yet you know it can return NULL;.
Check if temp is NULL before doing temp->next = NULL;

answered Jun 09 '15 at 12:58

Eregrith

4,263
18
39

score -2 · Answer 2 · answered Jun 09 '15 at 13:19

-2

My problem don't has relation with kind of pointer and returned value from
malloc().I have problem with size of buf[] and by
increment of size my problem solved.Tnx from every one.

answered Jun 09 '15 at 13:19

moon

1
2

1

Yeah, but you have to check if `malloc` didn't return NULL, otherwise your program will crash, when it will return NULL. Moreover you shouldn't really name your function `malloc`, as `stdlib` is often used library and you will have symbol clash. – zoska Jun 09 '15 at 13:46
1

Please don't comment your question in an answer. – Jabberwocky Jun 09 '15 at 14:02

Simple malloc function in c

2 Answers2