gistfile1.txt

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

struct shm_t {
	uint32_t	head;
	uint32_t	alloc;
	char		data[0];
};

#define SHM_HEADER_SIZE 8
#define SHM_BLOCK_SIZE 8
#define SHM_BLOCK_ALIGNMENT 8
#define SHM_SIZE (1<<20)

void *shm_alloc(struct shm_t *shm, uint32_t size)
{
	uint32_t alloc, new;

	size = ((size+SHM_BLOCK_ALIGNMENT-1)&~(SHM_BLOCK_ALIGNMENT-1)) + SHM_BLOCK_SIZE;
	alloc = __atomic_load_n(&shm->alloc, __ATOMIC_RELAXED);
	for (;;) {
		new = alloc + size;
		if (new > SHM_SIZE-SHM_HEADER_SIZE)
			return 0;
		if (__atomic_compare_exchange_n(&shm->alloc, &alloc, new, 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED))
			return &shm->data[alloc + SHM_BLOCK_SIZE];
	}
}

void shm_commit(struct shm_t *shm, void *block)
{
	uint32_t *hdr;

	hdr = (uint32_t*)((char*)block-SHM_BLOCK_SIZE);
	*hdr = __atomic_load_n(&shm->head, __ATOMIC_ACQUIRE);
	while (!__atomic_compare_exchange_n(&shm->head, hdr, (char*)block - &shm->data[0], 1, __ATOMIC_ACQ_REL, __ATOMIC_ACQUIRE));
}

void shm_foreach(struct shm_t *shm, void (*f)(void*, void*), void *ctx)
{
	uint32_t pos;

	pos = __atomic_load_n(&shm->head, __ATOMIC_ACQUIRE);
	while (pos) {
		f(ctx, &shm->data[pos]);
		pos = *(uint32_t*)&shm->data[pos-SHM_BLOCK_SIZE];
	}
}

void print(void *ctx, void *b);

int main()
{
	struct shm_t *shm;
	void *b0, *b1, *b2;

	shm = calloc(SHM_SIZE, 1);
	b0 = shm_alloc(shm, 8);
	b1 = shm_alloc(shm, 12);
	b2 = shm_alloc(shm, 12);
	((uint32_t*)b0)[0] = 1;
	((uint32_t*)b0)[1] = 11;
	((uint32_t*)b1)[0] = 2;
	((uint32_t*)b1)[1] = 21;
	((uint32_t*)b1)[2] = 22;
	((uint32_t*)b2)[0] = 2;
	((uint32_t*)b2)[1] = 31;
	((uint32_t*)b2)[2] = 32;
	shm_commit(shm, b1);
	shm_commit(shm, b0);
	shm_commit(shm, b2);
	shm_foreach(shm, print, 0);
	return 0;
}

void print(void *ctx, void *b)
{
	uint32_t i, n;

	n = *(uint32_t*)b;
	printf("size=%d:", n);
	for (i = 0; i < n; i++)
		printf(" %d", ((uint32_t*)b)[i+1]);
	printf("\n");
}