Example code for explaining how malloc and free work in C language.

vanilla_hater.c

#include <sys/mman.h>

void* c_malloc(size_t size) {
    void* ptr;
    
    asm volatile (
        "mov $9, %%rax\n"
        "mov $0, %%rdi\n"
        "mov %1, %%rsi\n"
        "mov $3, %%rdx\n"
        "mov $34, %%r10\n"
        "mov $-1, %%r8\n"
        "mov $0, %%r9\n"
        "syscall\n"
        "mov %%rax, %0\n"
        : "=r"(ptr)
        : "r"(size)
        : "rax", "rdi", "rsi", "rdx", "r10", "r8", "r9"
    );

    return ptr;
}

void c_free(void* ptr, size_t size) {
    asm volatile (
        "mov $11, %%rax\n"
        "mov %0, %%rdi\n"
        "mov %1, %%rsi\n"
        "syscall\n"
        :
        : "r"(ptr), "r"(size)
        : "rax", "rdi", "rsi"
    );
}

void print_point(void* ptr) {
    unsigned long long addr = (unsigned long long)ptr;
    
    char output[16];
    int i = 0;
    
    for (int j = 0; j < 16; j++) {
        unsigned long long nibble = (addr >> (60 - 4 * j)) & 0xF;
        output[i++] = nibble < 10 ? '0' + nibble : 'a' - 10 + nibble;
    }
    
    asm volatile (
        "mov $1, %%rax\n"
        "mov $1, %%rdi\n"
        "movq %0, %%rsi\n"
        "mov $16, %%rdx\n"
        "syscall\n"
        :
        : "r"(output)
        : "rax", "rdi", "rsi", "rdx"
    );
}


void print_value(int* ptr) {
    int value = *ptr;

    char output[16];
    int i = 0;
    
    if (value < 0) {
        output[i++] = '-';
        value = -value;
    }

    do {
        output[i++] = '0' + (value % 10);
        value /= 10;
    } while (value != 0 && i < 16);
    
    int j = 0;
    if (output[0] == '-') j = 1;

    while (j < i / 2) {
        char temp = output[j];
        output[j] = output[i - j - 1];
        output[i - j - 1] = temp;
        j++;
    }

    asm volatile (
        "mov $1, %%rax\n"
        "mov $1, %%rdi\n"
        "movq %0, %%rsi\n"
        "mov $16, %%rdx\n"
        "syscall\n"
        :
        : "r"(output)
        : "rax", "rdi", "rsi", "rdx"
    );
}

void print_newline() {
    char newline = '\n';

    asm volatile (
        "mov $1, %%rax\n"
        "mov $1, %%rdi\n"
        "movq %0, %%rsi\n"
        "mov $1, %%rdx\n"
        "syscall\n"
        :
        : "r"(&newline)
        : "rax", "rdi", "rsi", "rdx"
    );
}


int main() {
    // int* ptr = malloc(sizeof(int));
    size_t size = sizeof(int);
    int* ptr = c_malloc(size);

    //if (ptr == NULL) 
    if (ptr == MAP_FAILED) 
        return -1;

    *ptr = 33;
    
    print_value( ptr );
    print_newline();
    print_point( ptr );
    
    //free(ptr)
    c_free(ptr, size);

    //ptr null
    asm volatile (
        "movq $0, %0\n"
        : "=r"(ptr)
        :
        : "memory"
    );

    return 0;
}