linux/Documentation/vm/hugepage-mmap.c

/*
 * hugepage-mmap:
 *
 * Example of using huge page memory in a user application using the mmap
 * system call.  Before running this application, make sure that the
 * administrator has mounted the hugetlbfs filesystem (on some directory
 * like /mnt) using the command mount -t hugetlbfs nodev /mnt. In this
 * example, the app is requesting memory of size 256MB that is backed by
 * huge pages.
 *
 * For the ia64 architecture, the Linux kernel reserves Region number 4 for
 * huge pages.  That means that if one requires a fixed address, a huge page
 * aligned address starting with 0x800000... will be required.  If a fixed
 * address is not required, the kernel will select an address in the proper
 * range.
 * Other architectures, such as ppc64, i386 or x86_64 are not so constrained.
 */

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

#define FILE_NAME "/mnt/hugepagefile"
#define LENGTH (256UL*1024*1024)
#define PROTECTION (PROT_READ | PROT_WRITE)

/* Only ia64 requires this */
#ifdef __ia64__
#define ADDR (void *)(0x8000000000000000UL)
#define FLAGS (MAP_SHARED | MAP_FIXED)
#else
#define ADDR (void *)(0x0UL)
#define FLAGS (MAP_SHARED)
#endif

static void check_bytes(char *addr)
{
	printf("First hex is %x\n", *((unsigned int *)addr));
}

static void write_bytes(char *addr)
{
	unsigned long i;

	for (i = 0; i < LENGTH; i++)
		*(addr + i) = (char)i;
}

static void read_bytes(char *addr)
{
	unsigned long i;

	check_bytes(addr);
	for (i = 0; i < LENGTH; i++)
		if (*(addr + i) != (char)i) {
			printf("Mismatch at %lu\n", i);
			break;
		}
}

int main(void)
{
	void *addr;
	int fd;

	fd = open(FILE_NAME, O_CREAT | O_RDWR, 0755);
	if (fd < 0) {
		perror("Open failed");
		exit(1);
	}

	addr = mmap(ADDR, LENGTH, PROTECTION, FLAGS, fd, 0);
	if (addr == MAP_FAILED) {
		perror("mmap");
		unlink(FILE_NAME);
		exit(1);
	}

	printf("Returned address is %p\n", addr);
	check_bytes(addr);
	write_bytes(addr);
	read_bytes(addr);

	munmap(addr, LENGTH);
	close(fd);
	unlink(FILE_NAME);

	return 0;
}