Thread: Malloc help

Hi, I have an assignment to make a memory allocator. I used my C book as a template of how to make it and I am running into 2 problems. One being whenever I try to debug it with gdb it instantly quits with this message: Program exited with code 0304.
When I try to implement my malloc function in another program I made earlier I get this thing:
*** glibc detected *** ./a.out: free(): invalid pointer: 0x095812d9 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6[0x972ff1]
/lib/tls/i686/cmov/libc.so.6[0x9746f2]
/lib/tls/i686/cmov/libc.so.6(cfree+0x6d)[0x9777cd]
./a.out[0x80487c8]
./a.out[0x804862a]
./a.out[0x8048830]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6)[0x91eb56]
./a.out[0x8048581]
======= Memory map: ========
003d8000-003d9000 r-xp 00000000 00:00 0 [vdso]
00655000-00670000 r-xp 00000000 07:00 556017 /lib/ld-2.10.1.so
00670000-00671000 r--p 0001a000 07:00 556017 /lib/ld-2.10.1.so
00671000-00672000 rw-p 0001b000 07:00 556017 /lib/ld-2.10.1.so
00908000-00a46000 r-xp 00000000 07:00 580508 /lib/tls/i686/cmov/libc-2.10.1.so
00a46000-00a47000 ---p 0013e000 07:00 580508 /lib/tls/i686/cmov/libc-2.10.1.so
00a47000-00a49000 r--p 0013e000 07:00 580508 /lib/tls/i686/cmov/libc-2.10.1.so
00a49000-00a4a000 rw-p 00140000 07:00 580508 /lib/tls/i686/cmov/libc-2.10.1.so
00a4a000-00a4d000 rw-p 00000000 00:00 0
00bd7000-00bf3000 r-xp 00000000 07:00 555973 /lib/libgcc_s.so.1
00bf3000-00bf4000 r--p 0001b000 07:00 555973 /lib/libgcc_s.so.1
00bf4000-00bf5000 rw-p 0001c000 07:00 555973 /lib/libgcc_s.so.1
08048000-08049000 r-xp 00000000 07:00 294736 /home/gene/NetBeansProjects/Malloc/a.out
08049000-0804a000 r--p 00000000 07:00 294736 /home/gene/NetBeansProjects/Malloc/a.out
0804a000-0804b000 rw-p 00001000 07:00 294736 /home/gene/NetBeansProjects/Malloc/a.out
09581000-095a2000 rw-p 00000000 00:00 0 [heap]
b7600000-b7621000 rw-p 00000000 00:00 0
b7621000-b7700000 ---p 00000000 00:00 0
b773b000-b773c000 rw-p 00000000 00:00 0
b774d000-b774f000 rw-p 00000000 00:00 0
bfc5b000-bfc70000 rw-p 00000000 00:00 0 [stack]
Aborted

This is my header file

Code:

#ifndef _FUNCTIONS_H
#define	_FUNCTIONS_H

struct hdr {
    unsigned int len :29;
    unsigned int excess :2;
    unsigned int allocated :1;
    struct hdr *next;
};

typedef struct hdr header;

void *myMalloc(size_t size);
void myFree(void *p);
static header *moreMem(unsigned int);

#endif

My functions:

Code:

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

static int msize = 1024;
static header head;
static header *fblock = NULL;

void *myMalloc(size_t size) {
    header *prev, *p;
    moreMem(msize*sizeof(size_t));

    if ((prev = fblock) == NULL) {
        head.next = fblock = prev = &head;
        head.len = 0;
    }
        
    for (p = prev->next; ; prev = p, p = p->next) {
        if (size != 0) {
            if (p->len >= size) {
                if (p->len == size) {
                    prev->next = p->next;
                }

                else {
                    p->len -= size;
                    p += p->len;
                    p->len = size;
                }
                
                fblock = prev;
                p->allocated = 1;
                return (void*) (p+1);
            }
        
            if (p == fblock) {
                if (p == NULL) {
                    return NULL;
                }
            }
        }
        
        else if (size <= 0) {
            return NULL;
        }
    }
}

static header *moreMem(unsigned int size) {
    char *ptr;
    header *mem;
    if (size < msize) {
        size = msize;
    }

    ptr = malloc(size * sizeof(header));

    if (ptr == (char*)-1) {
        return NULL;
    }

    mem = (header*)ptr;
    mem->len = size;
    free ((void*)mem+1);
    return fblock;
}

void myFree(void *p) {

}

Any feedback is appreciated.

Why are you casting <free ((void*)mem+1);> to void* here? As far as I know you can use free ony any type of previously malloc-ed pointer.

Pretty sure that casts have a much higher precedence than addition so you are trying to do arithmetic on a void pointer. (I amnot sure if this is what claudiu was getting at or not)

Originally Posted by DeadPlanet

Pretty sure that casts have a much higher precedence than addition so you are trying to do arithmetic on a void pointer. (I amnot sure if this is what claudiu was getting at or not)

Yep. I was just surprised he said that the program works fine on another machine.

If I take off the +1 it gets rid of the error, but then it freezes on some function, and I have no way of finding out where it's messed up because gdb wont work for some reason.