vmtest.c

#define _GNU_SOURCE
#include <assert.h>
#include <stdio.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
int main(void) {
  size_t page_size = sysconf(_SC_PAGESIZE);
  void* p = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
                           MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
  assert(p != MAP_FAILED);
  struct iovec in_iov[2];
  struct iovec out_iov[2];

  out_iov[0].iov_base = in_iov[0].iov_base = in_iov[1].iov_base = p;
  out_iov[0].iov_len = in_iov[0].iov_len = 1;

  for (int i = 1; i < 20; ++i) {
    for (int j = 1; j < 8; ++j) {
      int len = i+j;
      in_iov[1].iov_len = len;
      out_iov[1].iov_len = len;
      out_iov[1].iov_base = p + page_size - i;
      errno = 0;
      int nbytes = process_vm_readv(getpid(), out_iov, 2, in_iov, 2, 0);
      printf("Size %d copy at page_size - %d: %d\n", len, i, nbytes - 1);
    }
  }

  return 0;
}