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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.
932
The sys_brk() system call is provided by the kernel, to allocate memory without the need of moving it later. This call allocates memory right behind the application image in the memory. This system function allows you to set the highest available address in the data section.
Dynamic memory is allocated by either the malloc() or calloc() functions. These functions return pointers to the allocated memory. Once you have a block of memory of a certain initial size, you can change its size with the realloc() function. Dynamic memory is released with the free() function.
To allocate memory dynamically, library functions are malloc() , calloc() , realloc() and free() are used. These functions are defined in the <stdlib. h> header file.
Linux-based operating systems use a virtual memory system. Any address referenced by a user-space application must be translated into a physical address. This is achieved through a combination of page tables and address translation hardware in the underlying computer system.
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 If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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