arena_alloc.c

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/mman.h>
#include <unistd.h>
#include <string.h>
#ifndef MAP_ANONYMOUS
#  ifdef MAP_ANON
#    define MAP_ANONYMOUS MAP_ANON
#  else
#    define MAP_ANONYMOUS 0x20   /* fallback: Linux value */
#  endif
#endif

typedef struct {
    uint8_t *base;
    size_t size;
    size_t used;
    size_t guard_size;
} arena_t;

static int arena_init(arena_t *a, size_t bytes)
{
    size_t pagesize = (size_t)sysconf(_SC_PAGESIZE);
    size_t total = (bytes + pagesize - 1) / pagesize * pagesize + pagesize;
    void *mem = mmap(NULL, total, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (mem == MAP_FAILED) return -1;
    a->base = mem;
    a->size = total - pagesize;
    a->used = 0;
    a->guard_size = pagesize;
    uint8_t *guard = a->base + a->size;
    if (mprotect(guard, pagesize, PROT_NONE) < 0) {
        munmap(mem, total);
        return -1;
    }
    return 0;
}

static void arena_destroy(arena_t *a)
{
    if (!a->base) return;
    size_t total = a->size + a->guard_size;
    munmap(a->base, total);
    a->base = NULL;
    a->size = 0;
    a->used = 0;
}

static void *arena_alloc(arena_t *a, size_t n)
{
    size_t aligned = (n + sizeof(void *) - 1) & ~(sizeof(void *) - 1);
    if (a->used + aligned > a->size) return NULL;
    void *p = a->base + a->used;
    a->used += aligned;
    return p;
}

static void arena_reset(arena_t *a)
{
    a->used = 0;
}

int main(void)
{
    arena_t a;
    if (arena_init(&a, 1 << 20) < 0) return 1;
    char *s1 = arena_alloc(&a, 32);
    char *s2 = arena_alloc(&a, 32);
    strcpy(s1, "hello from arena");
    strcpy(s2, "another string");
    printf("%s | %s\n", s1, s2);
    arena_reset(&a);
    char *s3 = arena_alloc(&a, 64);
    strcpy(s3, "after reset");
    printf("%s\n", s3);
    arena_destroy(&a);
    return 0;
}