salloc – Simple Memory Allocator using brk

salloc is a minimal implementation of malloc written in x86-64 assembly (GAS), using the Linux brk syscall to allocate dynamic memory in C programs. This project is intended for learning how heap memory works at a low level and understanding memory allocation without relying on the standard C library.

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Features

• Dynamic memory allocation using the program break.
• Returns a valid pointer to allocated memory.
• Minimal: does not support free or advanced block management.
• Implemented in x86-64 GAS assembly.
• Easy to integrate with C programs.

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How it Works

1. Reads the current program break with brk(NULL) → old_break.

2. Adds the requested size → new_break.

3. Calls brk(new_break):

   • If the kernel allows it, the break moves → memory allocated.
   • If it fails, returns NULL.

4. Returns old_break as a pointer to the newly allocated memory.

Note: Memory pages may not be physically mapped until accessed (lazy allocation).

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Function Prototype

void* salloc(size_t size);

• size – the number of bytes to allocate.
• Returns a pointer to the allocated memory or NULL on failure.

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Example Usage in C

#include <stdio.h>
#include <stdint.h>

void* salloc(size_t size); // assembly function declaration

int main()
{
    char* p = (char*)salloc(100);  // allocate 100 bytes
    if (!p)
    {
        printf("Allocation failed!\n");
        return 1;
    }

    for (int i = 0; i < 100; i++)
        p[i] = (char)(i % 256);

    for (int i = 0; i < 10; i++)
        printf("%d ", (unsigned char)p[i]);
    printf("\n");

    return 0;
}

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Compilation

1. Assemble the .s file:

gcc -c salloc.s -o salloc.o -no-pie

2. Compile the C file and link:

gcc -no-pie test.c salloc.o -o test

3. Run:

./test

-no-pie is required because we are working with direct linear addresses of the heap.

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Technical Notes

• Target architecture: x86-64 Linux (System V ABI)

• Arguments are passed in registers:

  • %rdi – first argument (size)
  • %rsi, %rdx – used internally

• Return value:

  • %rax – pointer to allocated memory or NULL if allocation fails

• Does not manage memory blocks; multiple calls simply move the program break forward.