Brainfuck Interpreter

#define BUF_SIZE 30000
#define CODE_SIZE 4096

#include <stdio.h>
#include <stdlib.h>

struct linked_list
{
    int index;
    struct linked_list *next;
};

struct linked_list *stack_add(struct linked_list *list, int data)
{
    struct linked_list *ptr = malloc(sizeof(struct linked_list));
    ptr->next = list;
    ptr->index = data;
    return ptr;
}

struct linked_list *stack_remove(struct linked_list *list)
{
    struct linked_list *ptr = list->next;
    free(list);
    return ptr;
}

int interpret(char *code, unsigned char *buf, int index)
{
    struct linked_list *brackets = NULL;
    int c_index = 0;

    while(code[c_index])
    {
        //printf("c_index = %d\tcode = %c\tindex = %d\tbuf = %d\n",c_index,code[c_index],index,buf[index]);
        switch(code[c_index++])
        {
            case '>':
                index++;
                break;
            case '<':
                index--;
                break;
            case '+':
                buf[index]++;
                break;
            case '-':
                buf[index]--;
                break;
            case '.':
                putchar(buf[index]);
                break;
            case ',':
                buf[index] = getchar();
                break;
            case '[':
                if(!buf[index])
                {
                    while(code[c_index++]!=']');
                    if((brackets!=NULL) && (brackets->index == index))
                        brackets = stack_remove(brackets);
                }
                else
                    if((brackets==NULL) || (brackets->index != index))
                        brackets = stack_add(brackets, index);
                break;
            case ']':
                while(code[--c_index]!='[');
                break;
        }
    }
    return index;
}

void execute(char *code)
{
    unsigned char buf[BUF_SIZE] = {0};
    interpret(code, buf, 0);
}

int main(int argc, char **argv)
{
    if(argc>1)
    {
        int size;
        FILE *file = fopen(argv[1], "r");
        char *code;
        if(file == NULL)
        {
            printf("Error, could not open file (%s)\n", argv[1]);
            return 0;
        }
        fseek(file, 0, SEEK_END);
        size = ftell(file);
        rewind(file);
        code = malloc(size);
        fread(code, size, 1, file);
        printf("loaded program: (%d bytes)\n%s\nStarting...\n", size, code);
        execute(code);
        return 0;
    }
    else
    {
        unsigned char buffer[BUF_SIZE] = {0};
        char code[CODE_SIZE];
        int buf_index = 0;
        printf("Brainfuck Interpreter Shell\n");
        while(1)
        {
            char open_loop = 0;
            int index = 0;
            printf("\n>> ");
            do
            {
                fgets(&code[index], CODE_SIZE, stdin);
                for(;code[index]!=0;index++)
                    if(code[index]=='[')
                        open_loop++;
                    else
                        if(code[index]==']')
                            open_loop--;
                if(open_loop)
                    printf(">>\t");
            } while(open_loop | !index);
            buf_index = interpret(code, buffer, buf_index);
        }
    }
    return 0;
}