mmap.c

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

#define CHECK(thing) if (!(thing)) { perror(#thing); exit(1); }
#define MAX_PAGE_IN 104857600
#define MIN(a,b) ((a)<(b)?(a):(b))

static void get_rss(const char *tag) {
	char buf[128];
	snprintf(buf, sizeof(buf), "ps hu %d | cut -c-80", getpid());
	printf("%-25s ==> ", tag); fflush(stdout);
	system(buf);
}

int main(int argc, const char **argv) {
	if (argc != 2) {
		fprintf(stderr, "Usage:\n  %s filename\n", argv[0]);
		return 1;
	}

	// open it
	int fd = open(argv[1], O_RDONLY);
	CHECK(fd != -1);
	struct stat st;
	CHECK(fstat(fd, &st) != -1);

	// map it
	get_rss("before mmap");
	void *map = mmap(NULL, st.st_size, PROT_READ, MAP_SHARED, fd, 0);
	get_rss("after mmap");
	CHECK(map != (void*)-1);

	// does WILLNEED do anything?  seems like no.
	unsigned int sum = 0;
	madvise(map, st.st_size, MADV_WILLNEED);
	sum += ((unsigned char*)map)[0];
	sleep(1);
	get_rss("after WILLNEED");

	// walk it.  add it to sum and print at the end so the optimizer doesn't get clever.
	int i;
	int limit = MIN(st.st_size, MAX_PAGE_IN);
	madvise(map, st.st_size, MADV_SEQUENTIAL);
	for (i=0; i<limit; i++) {
		sum += ((unsigned char*)map)[i];
	}
	get_rss("after touching");

	// give it back!
	// here are other flags you can pass to madvise:
	// MADV_NORMAL, MADV_RANDOM, MADV_SEQUENTIAL, MADV_WILLNEED, MADV_DONTNEED
	madvise(map, st.st_size, MADV_DONTNEED);
	get_rss("after DONTNEED");

	printf("%d\n", sum);
	return 0;
}