force_4k.c

/*
 * example calling mmap on a file starting at offset 4096.  the
 * size mapped is the size of the file minus the 4096 bytes that
 * were skipped at the beginning.
 */
#include <sys/mman.h>
#include <sys/stat.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
	int fd;
	struct stat stbuf;
	char *pmaddr;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s filename\n", argv[0]);
		exit(1);
	}

	if ((fd = open(argv[1], O_RDWR)) < 0)
		err(1, "open: %s", argv[1]);

	if (fstat(fd, &stbuf) < 0)
		err(1, "fstat: %s", argv[1]);

	/* start mapping at offset 4k to force 4k pages */
	if ((pmaddr = (char *)mmap(NULL, stbuf.st_size - 4096,
				PROT_READ|PROT_WRITE,
				MAP_SHARED, fd, 4096)) == MAP_FAILED)
		err(1, "mmap: %s", argv[1]);

	close(fd);

	/* now pmaddr points to the mapped area... */

	printf("done.\n");
	exit(0);
}

Makefile

#
# Makefile for force_4k example
#
PROGS = force_4k
CFLAGS = -g -Wall -Werror -std=gnu99

all: $(PROGS)

force_4k: force_4k.o
	$(CC) -o $@ $(CFLAGS) $^ $(LIBS)

clean:
	$(RM) *.o a.out core

clobber: clean
	$(RM) $(PROGS) force_4k

.PHONY: all clean clobber