map.c

#include <sys/mman.h>
#include <sys/fcntl.h>
#include <unistd.h>
#include <stddef.h>
#include <sys/stat.h>

#define round_up(x, y) (((x) + (y) - 1) & ~((y) - 1))

static int pagesize;

static void __attribute__((constructor)) init_ps(void)
{
	pagesize = sysconf(_SC_PAGESIZE);
}

void *mapfile(char *fn, size_t *size)
{
	int fd = open(fn, O_RDWR);
	if (fd < 0)
		return NULL;
	struct stat st;
	void *map = (void *)-1L;
	if (fstat(fd, &st) >= 0) {
		*size = st.st_size;
		map = mmap(NULL, round_up(st.st_size, pagesize),
			   PROT_READ|PROT_WRITE,
			   MAP_PRIVATE, fd, 0);
	}
	close(fd);
	return map != (void *)-1L ? map : NULL;
}

void unmapfile(void *map, size_t size)
{
	munmap(map, round_up(size, pagesize));
}