I have made my own malloc implementation using this resource as a guide https://danluu.com/malloc-tutorial/ I was hoping to receive some feed back on how I can improve upon it, and If I did any major mistakes. I know my implementation has a lot of fragmentation and I pretty much no idea on how to fix it.


  1. Feed back on my _malloc and _free implementation.
  2. Does anyone have any good debugger that can be used to debugg x86_64 nasm code
bits 64

%define NULL 0
%define SYSCALL_BRK 12

struc block_meta
   .next resq 1               ;pointer to the next block of "block_mata" struct
   .size resq 1               ;how many bytes can this block hold
   .free resb 1               ;is this block free (0 == no its not free) (1 == yes its is free)
META_SIZE equ 17              ;the size of block_meta in bytes

section .data
   global_base dq NULL        ;pointer to the first "block_meta" struct
   current_sbrk dq 0

section .text
global _start
global _malloc

    push 400
    call _malloc                       ;allocationg 100 dwords aka 400 bytes(array of 100 dwords). rax contains pointer
    mov r15, rax                       ;saving pointer of array

    ;test program where we loop through the array and store 0 - 99 in each pos
    xor ebx, ebx
       mov [r15 + rbx * 4], ebx
       inc ebx
       cmp ebx, 100                    ;when ebx reaches 100 we have reached the end of the array
       jl ._L1

    xor ebx, ebx
    ;print out the array
       mov eax, [r15 + rbx * 4]
       push rax
       call _printInt
       add rsp, 8
       call _endl
       inc ebx
       cmp ebx, 100
       jl ._L2

    push r15
    call _free

    add rsp, 16                         ;clear the stack
    mov rax, 60                         ;SYSCALL_EXIT
    mov rdi, 0

;(first)last argument pused onto the stack must be the amount of bytes
;if successfull then rax will contain pointer to the memory
   push rbp
   mov rbp, rsp

   ;actual code
   cmp qword[rbp + 16], 0        ;compare with first argument
   jle ._mallocEpilog            ;if zero of negetive exit

   cmp qword[global_base], NULL  ;if the global_base pointer is "NULL" aka 0 allocate space
   jz  ._setGlobal_Base

   ;if global_base is not "NULL"
   push qword[rbp + 16]          ;how many bytes big does the block need to be
   push qword[global_base]       ;pointer to "meta_data" struct
   call ._findFreeBlock
   test rax, rax                 ;if zero no block was found. need to call ._requestSpace if zero
   jz ._needMoreSpace

   ;found free block
   mov rdx, rax                  ;save the pointer to memory block
   add rdx, block_meta.free      ;set the block to be not free
   mov byte[rdx], 0
   jmp ._mallocExit

   call ._requestSpace            ;we did not find a big enoug block. so make sapce
   jmp ._mallocExit

   ._setGlobal_Base:               ;will be used first time malloc is called
   push qword[rbp + 16]            ;how many bytes does the user want to reserve
   push NULL                       ;the global_base pointer has not been set
   call ._requestSpace
   mov [global_base], rax          ;save the pointer

   add rsp, 16                     ;clean the stack
   add rax, META_SIZE              ;add offset because of the "meta_block" struct

   pop rbp

;(fist)last agument on the stack must be pointer to the last "block_meta" struct
;second argument must be the size in bytes that need to be allocated
;returns pointer to memory block in rax
   push rbp
   mov rbp, rsp

   mov rdi, [rbp + 24]        ;how many bytes for the user
   add rdi, META_SIZE         ;extra bytes for meta data
   push rdi
   call ._sbrk                ;rax will contain the pointer
   add rsp, 8                 ;clear stack

   mov r8,  block_meta.next   ;putting the offsets in the register for later use
   mov r9,  block_meta.size
   mov r10, block_meta.free

   mov qword[rax + r8], NULL  ;just setting it to NULL to get rid of garbage data for the next

   cmp qword[rbp + 16], NULL  ;the last "block_meta" pointer is NULL then jmp
   jz ._fillMetaData

   mov rcx, [rbp + 16]        ;the current last "block_meta" struct in the list
   mov qword[rcx + r8], rax   ;mov pointer of allocated memory into next pointer of struct

   ._fillMetaData:            ;setting all the other fields in the struct
   mov rdi, [rbp + 24]        ;how many bytes for the user
   mov qword[rax + r9], rdi   ;setting the size field of the struct
   mov byte[rax + r10], 0     ;setting the free field to be 0 of struct

   pop rbp

;(fist)last argument on the stack must be pointer to "block_meta" struct
;second argument is how big the block needs to be
;if successfull then rax will contain pointer to the block
;if failure the rax will contain pointer to the last block of "block_meta" struct
   push rbp
   mov rbp, rsp

   mov rax, [rbp + 16]         ;pointer to the "block_meta" struct
   mov rdx, [rbp + 24]         ;how big do you need the block to be
   mov r8,  block_meta.next    ;offset
   mov r9,  block_meta.size
   mov r10, block_meta.free
   jmp ._findFreeBlockLoopCond

      mov [rbp + 16], rax      ;save current pointer in argument 1
      mov rax, [rax + r8]      ;go to the next "block_meta" struct
      test rax, rax            ;if rax is zero we have reached the end of the linked list. exit
      jz ._findFreeBlockExit
      cmp byte[rax + r10], 0   ;if zero then block is not empty. loop again
      jz ._findFreeBlockLoop
      cmp [rax + r9], rdx      ;if the current block has does not have enough space loop again.
      jl ._findFreeBlockLoop

   pop rbp

;(fist)last argument must be how much space do you want to reserve
;return pointer in rax
   push rbp
   mov rbp, rsp

   ;actual code
   mov rax, SYSCALL_BRK       ;using brk to get initilial address
   mov rdi, [current_sbrk]    ;starts at 0. gets updated later
   mov r8,  rax               ;save for later

   mov rax, SYSCALL_BRK
   mov rdi, [rbp + 16]        ;first argument (how many bytes)
   add rdi, r8                ;needs to start at teh address we saved

   mov [current_sbrk], rax    ;next time will start at this address

   mov rax, r8                ;restore pointer to the memory

   pop rbp

;(first)last arguemnt on the stack must be the pointer you want to deallocate memory for
   push rbp
   mov rbp, rsp

   ;I will be calling the pointer in rax to be the "original block"
   mov rax, [rbp + 16]         ;pointer to memory that needs to be deallocated
   sub rax, META_SIZE          ;offset to find the "block_meta" struct

   mov rcx, rax
   add rcx, block_meta.free    ;offset to set free to be 1
   mov byte[rcx], 1

   pop rbp
;print methods for testing!
%define STDIN  0
%define STDOUT 1
%define STDERR 2

%define SYSCALL_READ     0
%define SYSCALL_WRITE    1
%define SYSCALL_EXIT     60

section .data
   endl db 10
   endlLength equ $ - endl

;no input needed
;just an end line "method"
   mov rax, SYSCALL_WRITE
   mov rdi, STDOUT
   mov rsi, endl
   mov edx, endlLength
 ;last value pushed to stack will be printed
   push rbp
   mov rbp, rsp
   ;save registers
   push rbx

   ;actual code
   mov rsi, rsp
   mov rax, [rbp + 16]         ;get the value that user wants to print
   mov rbx, 10                 ;will be used to divide by 10 later
   xor rcx, rcx

   cmp rdx, -1                 ;check to see if negetive
   jne _divisionLoop           ;if not negetive jump

   ;print negetive sign
   dec rsi
   mov [rsi], byte '-'
   mov rax, SYSCALL_WRITE
   mov rdi, STDOUT
   mov rdx, 1
   inc rsi

   ;convert to positive number
   mov rax, [rbp + 16]         ;get the value that needs to be printed
   neg rax                     ;make it a positive
   xor rcx, rcx

      xor rdx, rdx
      div rbx                    ;divides number by 10 to move over last digit into rdx reg
      add dl, '0'                ;add the '0' to ascii to convert into ascii val
      dec rsi
      mov [rsi], dl
      inc rcx                    ;count for how many digits added to stack
   test rax, rax
   jnz _divisionLoop             ;jump if the division did not result in a zero

   ;print all the values
   mov rax, SYSCALL_WRITE
   mov rdi, STDOUT
   mov rdx, rcx

   ;restore register
   pop rbx
   pop rbp

If I did any major mistakes.

Depends on whether you want to follow the Linux 64-bit calling conventions or not. Currently you are using the stack for parameter passing between your functions, where the normal calling convention places the first 6 parameters in the registers RDI, RSI, RDX, RCX, R8, and R9. I believe that your code would benefit from following the conventions.

Where you must follow the conventions is in your sbrk function that does 2 Linux syscalls. In between calls you've preserved a value in the r8 register but you are forgetting that the Linux 64-bit calling conventions say that this is a scratch register that the SYSCALL_BRK is free to clobber! Just save the value on the stack.

Erratum: Apparently SYSCALL is not a "call" in the traditional sense and only ever clobbers RAX, RCX, and R11. That means that your choice of R8 is fine. See Peter Cordes' answer on the matter.

Feed back on my _free implementation.

This is a truly minimalistic version of it. You're putting to much trust in the user (you yourself). How can you be sure that the address that they provide will point to a valid allocation?
A save way to do this is to follow the chain of allocations and only when you encounter the submitted address do you free that allocation.

Feed back on my _malloc implementation.

This on the other hand is an overly complex code that depends on the ingenious use of modifying and/or recycling of the stacked input parameters.
It is easy to lose your way in this code.

If you would only initialize once the global_base and current_sbrk variables at program startup, it would already bring down complexity a lot. e.g. It would eliminate the successive syscalls in ._sbrk.

And why not use the _malloc stackframe pointer RBP for the local subroutines ._requestSpace, ._findFreeBlock, and ._sbrk ? Then you can do without all of those prologs and epilogs.

I was hoping to receive some feed back on how I can improve upon it

The biggest improvement that you can make is a structural one and it will require you to rewrite the lot but in the end it will be very rewarding...

Although the tutorial used a linked list, that's not necessarily the best way to manage your meta data. Having both a pointer to the next block and also a block size, is like storing the same information twice (and keeping it up to date too!).
If you only maintain a next field then you get the size via:

size = next - (current + META_SIZE)

If you only maintain a size field than you get the next via:

next = current + META_SIZE + size

Do yourself a favor and only store the block size. It's the simpler solution.

Next comes alignment. Your current implementation uses a META_SIZE of 17 bytes. This is a real disaster when it comes to program performance! Always have the memory that the caller requests aligned at an optimal value like say qword or dqword. I would choose the latter and use next struc:

struc block_meta
   .size resq 1 ; how many bytes can this block hold
   .free resb 1 ; (0 == it's not free) (1 == it's free)
   .pad  resb 7
META_SIZE equ 16

This is how you can make the requested block size a muliple of 16 in accordance with the chosen META_SIZE (which has to be a power of 2):

; RDI is the requested allocation size
add rdi, META_SIZE - 1
and rdi, -META_SIZE 

Some code improvements include

Code like:

mov rdx, rax
add rdx, block_meta.free
mov byte[rdx], 0
mov r10, block_meta.free
cmp byte[rax + r10], 0

can be written like:

mov byte[rax + block_meta.free], 0
cmp byte[rax + block_meta.free], 0

And in your _printInt code you can shave off quite a few instructions by checking for a negative number the way I showed you in a previous answer of mine. By picking up the result from the test a second time after the digit were collected on the stack and prepending the "-" character, you can output the lot in one SYSCALL_WRITE operation instead of two.

And of course drop the redundant size tags, put your defines on top so you can use them everywhere, don't forget global _free, clear a register using xor edi, edi, etc ...

As an example this is how I would program ._findFreeBlock. The code traverses the memory between global_base and current_sbrk checking for a free block that is large enough. If found then RAX holds the address of the meta data, and if not found then RAX is zero.

; IN (rdi) OUT (rax) MOD (rdx)
    mov  rax, [global_base]
    jmp  .Start
    mov  rdx, [rax + block_meta.size]
    cmp  byte [rax + block_meta.free], 1 ; (1 == it's free)
    jne  .Next                           ; Block is not free
    cmp  rdx, rdi
    jae  .Exit                           ; Free block is large enough
    lea  rax, [rax + META_SIZE + rdx]
    cmp  rax, [current_sbrk]
    jb   .Loop
    xor  eax, eax                        ; Not found
  • 1
    \$\begingroup\$ Thank You! I will try to clear all the bugs/calling convention as you have pointed out and post the new version within a few days! \$\endgroup\$
    – Dagar
    Oct 25 '20 at 16:36
  • \$\begingroup\$ How can you be sure that the address that they provide will point to a valid allocation? - Because passing anything else would be undefined behaviour. Doing extra checking should be optional or a debug-build feature, except for problems you can detect for (nearly) free. Having a debug version with extra checking is certainly useful during development, but presumably extreme efficiency is the goal of hand-written asm. \$\endgroup\$ Nov 19 '20 at 19:35
  • \$\begingroup\$ Alignment: x86-64 System V has alignof(max_align_t) == 16. malloc(n) must be usable to store any standard-alignment object that can fit in it, so allocations of 16 bytes or later must always be aligned by 16. Unless you're eschewing the standard ABI altogether, then sure 8-byte alignment could be viable. \$\endgroup\$ Nov 19 '20 at 19:40

When a block is free'd, check the blocks either side. If an adjacent block is not allocated then merge it with the free'd block. This should help with fragmentation.


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