How to Dynamically Allocate Memory Using Assembly and System Calls Under Linux

Question

I'm looking for some good code examples of dynamic memory allocation using an assembly language under Linux and using system calls, not malloc and friends.

What are some of the simplest but effective ways to do this?

On Intel 386+ computers.

Answer 1

brk(2). And take a look at ELF.

Answer 2

On Linux mmap2 is a sensible system call to use for this at a low level. It takes 6 arguments, so in IA32 you can call it using:

    mov eax, 192    ; mmap2
    xor ebx, ebx    ; addr = NULL
    mov ecx, 4096   ; len = 4096
    mov edx, $7     ; prot = PROT_READ|PROT_WRITE|PROT_EXEC
    mov esi, $22    ; flags = MAP_PRIVATE|MAP_ANONYMOUS
    mov edi, -1     ; fd = -1
    xor ebp, ebp    ; offset = 0 (4096*0)
    int $80         ; make call

(See the relevant kernel source for details on the parameter passing)

I built this with NASM and verified it worked using strace, which produced:

mmap2(NULL, 4096, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xf77ae000

Answer 3

An alternative to brk() is to use the mmap() system call, with MAP_ANONYMOUS | MAP_PRIVATE.

Answer 4

Use the brk system call to change the end of your data segment.

Take a look here: http://www.linuxjournal.com/article/6390 to understand what you're doing.