https://old.reddit.com/r/C_Programming/comments/18r7vwy/_/kf5fi0s/

arena.c

// Gradual-commit arena demonstration
// This is free and unencumbered software released into the public domain.
#include <stddef.h>
#include <string.h>

static void *os_reserve(ptrdiff_t);
static _Bool os_commit(void *, ptrdiff_t);

#define ARENA_PAGESIZE ((ptrdiff_t)1<<26)

typedef struct {
    char *begin;
    char *commit;
    char *end;
} arena;

static arena newarena(ptrdiff_t cap)
{
    arena a = {0};
    cap += -cap & (ARENA_PAGESIZE - 1);
    a.begin = a.commit = a.end = os_reserve(cap);
    if (a.begin) {
        a.end += cap;
    }
    return a;
}

static void *alloc(arena *a, ptrdiff_t size, ptrdiff_t align, ptrdiff_t count)
{
    ptrdiff_t padding   = -(size_t)a->begin & (align - 1);
    ptrdiff_t committed = a->commit - a->begin;
    if (count > (committed-padding)/size) {
        ptrdiff_t reserved = a->end - a->begin;
        if (count > (reserved-padding)/size) {
            return 0;
        }

        ptrdiff_t needed = size*count + padding - committed;
        needed += -needed & (ARENA_PAGESIZE - 1);
        if (!os_commit(a->commit, needed)) {
            return 0;
        }
        a->commit += needed;
    }

    void *ptr = a->begin + padding;
    a->begin += padding + size*count;
    return memset(ptr, 0, size*count);
}


// Test

static ptrdiff_t test(arena *a, unsigned long long rng)
{
    ptrdiff_t total = 0;
    void *save = a->begin;
    for (;;) {
        rng = rng*0x3243f6a8885a308d + 1;
        ptrdiff_t size  = 1 +  ((rng>>50) & ((1<<10) - 1));
        ptrdiff_t count = 1 +  ((rng>>40) & ((1<<10) - 1));
        ptrdiff_t align = 1 << ((rng>>30) & ((1<< 2) - 1));
        if (!alloc(a, size, align, count)) {
            break;
        }
        total += size * count;
    }
    a->begin = save;
    return total;  // dprintf LINE,"%f\n",total/1024./1024.
}

static void run(arena a)
{
    for (int i = 0; i < 8; i++) {
        test(&a, i+1);
    }
}


#ifdef _WIN32
// $ cc -g3 -nostartfiles -o arena.exe arena.c
// $ cl /Z7 arena.c /link /subsystem:console kernel32.lib libvcruntime.lib
#define W32(r) __declspec(dllimport) r __stdcall
W32(void)   ExitProcess(int);
W32(void *) VirtualAlloc(void *, ptrdiff_t, int, int);

#define MEM_COMMIT     0x1000
#define MEM_RESERVE    0x2000
#define PAGE_NOACCESS  0x0001
#define PAGE_READWRITE 0x0004

static void *os_reserve(ptrdiff_t cap)
{
    return VirtualAlloc(0, cap, MEM_RESERVE, PAGE_NOACCESS);
}

static _Bool os_commit(void *ptr, ptrdiff_t len)
{
    return VirtualAlloc(ptr, len, MEM_COMMIT, PAGE_READWRITE);
}

void mainCRTStartup(void)
{
    arena a = newarena((ptrdiff_t)1 << 33);
    run(a);
    ExitProcess(0);
}


#else  // POSIX
// $ cc -g3 -o arena arena.c
#include <sys/mman.h>

static void *os_reserve(ptrdiff_t cap)
{
    void *r = mmap(0, cap, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
    return r==MAP_FAILED ? 0 : r;
}

static _Bool os_commit(void *ptr, ptrdiff_t len)
{
    return !mprotect(ptr, len, PROT_READ|PROT_WRITE);
}

int main(void)
{
    arena a = newarena((ptrdiff_t)1 << 33);
    run(a);
}
#endif