show difference between mmap() and malloc()

results.txt

$ gcc -DUSE_MMAP=0 tmp/rss.c && ./a.out
rss 360448
==============
rss 360448
rss 1429504
rss 552960
==============
rss 552960
rss 1359872
rss 1359872
==============
rss 1359872
rss 1359872
rss 1359872
==============
rss 1359872
rss 1359872
rss 1359872
==============
rss 1359872
rss 1359872
rss 1359872

$ gcc -DUSE_MMAP=1 tmp/rss.c && ./a.out
rss 360448
==============
rss 360448
rss 1429504
rss 536576
==============
rss 536576
rss 1343488
rss 536576
==============
rss 536576
rss 1343488
rss 536576
==============
rss 536576
rss 1343488
rss 536576
==============
rss 536576
rss 1343488
rss 536576

rss.c

/*
 * Copyright (c) 2012, Ben Noordhuis <info@bnoordhuis.nl>
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#ifndef __linux__
#error "Linux only."
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>

static unsigned long rss(void)
{
  int fd = open("/proc/self/stat", O_RDONLY);
  if (fd == -1) return 0;
  char buf[1024];
  ssize_t n = read(fd, buf, sizeof(buf) - 1);
  if (n == -1) return close(fd), 0;
  buf[n] = '\0';
  const char *s = buf;
  int i;
  for (i = 0; i < 23; i++) { // XXX doesn't handle spaces in process name
    while (*s != ' ' && *s != '\0') s++;
    if (*s == ' ') s++;
  }
  unsigned long rv = 0;
  if (sscanf(s, "%lu", &rv) != 1) rv = 0;
  close(fd);
  return rv * sysconf(_SC_PAGESIZE);
}

int main(void)
{
  printf("rss %lu\n", rss());
  int i;
  for (i = 0; i < 5; i++) {
    size_t size = 1024*1024;
#if USE_MMAP
    int prot = PROT_READ|PROT_WRITE;
    int flags = MAP_PRIVATE|MAP_ANONYMOUS;
    void *addr = mmap(NULL, size, prot, flags, -1, 0);
    if (addr == (void *) MAP_FAILED) perror("mmap"), exit(1);
#else
    void *addr = malloc(size);
    if (addr == NULL) perror("malloc"), exit(1);
#endif
    printf("==============\n");
    printf("rss %lu\n", rss());
    memset(addr, 0, size);
    printf("rss %lu\n", rss());
#if USE_MMAP
    munmap(addr, size);
#else
    free(addr);
#endif
    printf("rss %lu\n", rss());
  }
  return 0;
}