VOP_ADVISE(9)          FreeBSD Kernel Developer's Manual         VOP_ADVISE(9)

NAME
     VOP_ADVISE - apply advice about use of file data

SYNOPSIS
     #include <sys/param.h>
     #include <sys/vnode.h>

     int
     VOP_ADVISE(struct vnode *vp, off_t start, off_t end, int advice);

DESCRIPTION
     This call applies advice for a range of a file's data.  It is used to
     implement the posix_fadvise system call.

     Its arguments are:

     vp      The vnode of the file.

     start   The start of the range of file data.

     end     The end of the range of file data.  A value of OFF_MAX indicates
             that the advice is to be applied up to the end of the file.

     advice  The type of operation to apply to the file data.  Possible values
             are:

             POSIX_FADV_WILLNEED  Initiate an asynchronous read of the file
                                  data if it is not already resident.

             POSIX_FADV_DONTNEED  Decrease the in-memory priority of clean
                                  file data or discard clean file data.

     If the start and end offsets are both zero, then the operation should be
     applied to the entire file.  Note that this call is advisory only and may
     perform the requested operation on a subset of the requested range
     (including not performing it at all) and still return success.

LOCKS
     The file should be unlocked on entry.

RETURN VALUES
     Zero is returned if the call is successful, otherwise an appropriate
     error code is returned.

ERRORS
     [EINVAL]           An invalid value was given for advice.

SEE ALSO
     vnode(9)

FreeBSD 13.1-RELEASE-p6       September 26, 2015       FreeBSD 13.1-RELEASE-p6