The following is an example code snippet for setting the fixed extent size of the MY_PREFERRED_EXTSIZE attribute on a new file, MY_FILE, assuming MY_PREFFERED_EXTSIZE is multiple of the file system block size:

#include <sys/fs/vx_ioctl.h>
struct vx_ext myext;
fd = open(MY_FILE, O_CREAT, 0644);
myext.ext_size = MY_PREFERRED_EXTSIZE;
myext.reserve = 0;
myext.flags = 0;
error = ioctl(fd, VX_SETEXT, &myext);

The following is an example code snippet for preallocating MY_FILESIZE_IN_BYTES bytes of space on the new file, MY_FILE, assuming that the target file system block size is THIS_FS_BLOCKSIZE:

#include <sys/fs/vx_ioctl.h>
struct vx_ext myext;
fd = open(MY_FILE, O_CREAT, 0644);
myext.ext_size =0;
myext.reserve = (MY_FILESIZE_IN_BYTES + THIS_FS_BLOCKSIZE) 
/THIS_FS_BLOCKSIZE;
myext.flags = VX_CHGSIZE;
error = ioctl(fd, VX_SETEXT, &myext);