#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>

#define HAVE_POSIX_FALLOCATE 1

int
os_resize_anonymous_file(int fd, off_t size)
{
#ifdef HAVE_POSIX_FALLOCATE
	/* 
	 * Filesystems that do support fallocate will return EINVAL or
	 * EOPNOTSUPP. In this case we need to fall back to ftruncate
	 */
	errno = posix_fallocate(fd, 0, size);
	if (errno == 0)
		return 0;
	else if (errno != EINVAL && errno != EOPNOTSUPP) {
		warn("posix_fallocate failed");
		return -1;
	}
#endif
	if (ftruncate(fd, size) < 0) {
		warn("ftruncate failed");
		return -1;
	}

	return 0;
}

int
main(void)
{
	int fd;
	off_t size = 12345;

	fd = shm_open(SHM_ANON, O_CREAT | O_RDWR | O_CLOEXEC, 0600); // shm_open is always CLOEXEC
	if (fd < 0)
		err(1, "shm_open(SHM_ANON) failed"); 

	if (os_resize_anonymous_file(fd, size) < 0) {
		close(fd);
		return -1;
	}

	return 0;
}