libowfat/array/iarray_allocate.c

#include "likely.h"
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>
#include "iarray.h"
#ifdef __dietlibc__
#include <sys/atomic.h>
#else
#define __CAS(ptr,oldval,newval) __sync_val_compare_and_swap(ptr,oldval,newval)
#endif

static iarray_page* new_page(size_t pagesize) {
#ifdef __MINGW32__
  void* x=malloc(pagesize);
#else
  void* x=mmap(0,pagesize,PROT_READ|PROT_WRITE,MAP_ANONYMOUS|MAP_PRIVATE,-1,0);
#endif
  if (x==MAP_FAILED) return 0;
  return (iarray_page*)x;
}

void* iarray_allocate(iarray* ia,size_t pos) {
  size_t index,prevlen=ia->len;
  /* first, find the linked list of pages */
  iarray_page** p=&ia->pages[pos%(sizeof(ia->pages)/sizeof(ia->pages[0]))];
  /* this is here so we don't munmap and then re-mmap pages when a
   * certain path makes it necessary to mmap several pages into the
   * linked list and we are competing with another thread that does the
   * same thing */
  iarray_page* newpage=0;
  /* since we have a fan-out of 16, on page 0 the elements are 0, 16, 32, ...
   * so we divide pos by the fan-out here */
  size_t realpos=pos;
  pos /= sizeof(ia->pages)/sizeof(ia->pages[0]);
  /* now walk the linked list of pages until we reach the one we want */
  for (index=0; ; index+=ia->elemperpage) {
    if (!*p) {
      if (!newpage)
	if (!(newpage=new_page(ia->bytesperpage))) return 0;
      if (__CAS(p,0,newpage)==0)
	newpage=0;
    }
    if (index+ia->elemperpage>pos)
      break;
    p=&(*p)->next;
  }
  if (newpage)
#ifdef __MINGW32__
    free(newpage);
#else
    munmap(newpage,ia->bytesperpage);
#endif
  {
    size_t l;
    do {
      l=__CAS(&ia->len,prevlen,realpos);
    } while (l<realpos);
  }
  return &(*p)->data[(pos-index)*ia->elemsize];
}