mirrored ring buffer allocator

gistfile1.m

#include <mach/mach.h>
#include <mach/vm_map.h>

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


void *allocate_pair(size_t howmuch)
{
#define CHECK_ERR do { \
    if(err != KERN_SUCCESS) { \
        fprintf(stderr, "%d: got error %d\n", __LINE__, err); \
        exit(1); \
    } \
} while(0)
    
    void *mem = NULL;
    
    int success = 0;
    while(!success)
    {
        kern_return_t err = vm_allocate(mach_task_self(), (vm_address_t *)&mem, howmuch * 2, VM_FLAGS_ANYWHERE);
        CHECK_ERR;
        
        void *target = mem + howmuch;
        err = vm_deallocate(mach_task_self(), (vm_address_t)target, howmuch);
        CHECK_ERR;
        
        vm_prot_t cur_protection, max_protection;
        err = vm_remap(mach_task_self(),
                       (vm_address_t *)&target,
                       howmuch,
                       0, // mask
                       0, // anywhere
                       mach_task_self(),
                       (vm_address_t)mem,
                       0, // copy
                       &cur_protection,
                       &max_protection,
                       VM_INHERIT_COPY);
        if(err == KERN_PROTECTION_FAILURE)
        {
            err = vm_deallocate(mach_task_self(), (vm_address_t)mem, howmuch);
            CHECK_ERR;
        }
        else
        {
            success = 1;
            CHECK_ERR;
        }
    }
    return mem;
}

int main(void)
{
    int len = 4096;
    char *mem = allocate_pair(len);
    printf("%d: equal=%s\n", __LINE__, memcmp(mem, mem + len, len) == 0 ? "YES" : "NO");
    
    int i;
    for(i = 0; i < len; i++)
        mem[i] = i;
    printf("%d: equal=%s\n", __LINE__, memcmp(mem, mem + len, len) == 0 ? "YES" : "NO");
    
    return 0;
}