example of using custom mmap allocator in R

mmap_alloc.c

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>

#include <Rinternals.h>
#include <R_ext/Rallocators.h>

static void *mmap_alloc(R_allocator_t *allocator, size_t length) {
   int fd = open((char*) allocator->data, O_RDWR|O_CREAT, 0666);
   if (fd < 0) return 0;
   /* some OSes require the file to actually exist with the needed size - ensure that */
   lseek(fd, length, SEEK_SET);
   if (write(fd, "", 1)) {}
   void *mem = mmap(0, length, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
   close(fd);
   Rprintf("mmap allocated '%s'\n", (char*) allocator->data);
   allocator->data = mem ? (((char*)mem) + length) : 0; 
   return mem;
}

static void mmap_free(R_allocator_t *allocator, void *mem) {
   Rprintf("mmap freed %p\n", mem);
   munmap(mem, allocator->data ? (((char*)allocator->data) - ((char*) mem)) : 0);
}

SEXP mmap_vector(SEXP template, SEXP sFile, SEXP length) {
   R_allocator_t allocator = { mmap_alloc, mmap_free, 0, 0 };
   allocator.data = (void*) CHAR(STRING_ELT(sFile, 0));
   return Rf_allocVector3(TYPEOF(template), asInteger(length), &allocator);
}

### v = .Call(1L, "test.bin", 1e4); v[1:10]=1:10; rm(v); gc()