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FDISCARD(2)                   System Calls Manual                  FDISCARD(2)

     posix_fallocate, fdiscard - allocate or discard backing store for files

     Standard C Library (libc, -lc)

     #include <fcntl.h>

     posix_fallocate(int fd, off_t pos, off_t length);

     #include <unistd.h>

     fdiscard(int fd, off_t pos, off_t length);

     The posix_fallocate() call allocates backing store for the file
     referenced by fd in the region starting at pos bytes from the start of
     the file and continuing for length bytes more.  If the region extends
     past the current end of file, the file size is increased to cover the

     The fdiscard() call discards backing store for the file referenced by fd
     in the region starting at pos bytes from the start of the file and
     continuing for length bytes more.  The file size is not affected.

     Both calls operate on the basis of file system blocks, so
     posix_fallocate() may allocate more physical space than requested and
     fdiscard() may discard less physical space than requested.

     When posix_fallocate() is applied to an unallocated region in a regular
     file (a "hole"), the hole is filled and the visible contents are
     unaffected; both holes and newly allocated regions read as all zeros.  If
     posix_fallocate() is applied to an already-allocated region in a regular
     file, it has no effect.

     When fdiscard() is applied to a regular file, a hole is created and any
     data in the affected region is thrown away.  Subsequent reads of the
     region return zeros.

     If fdiscard() is applied to a device, and the device supports an
     underlying discard operation, that operation is invoked.  For example,
     ATA flash devices and solid-state disks support an operation called TRIM
     that discards blocks at the device level.  The behavior of blocks
     discarded at this level is implementation-defined; as devices vary,
     specific behavior should not be relied upon.  Subsequent reads of the
     same block may return zeros; such reads may also, however, continue to
     return the previously written data, or return other data, or return
     indeterminate garbage; or may switch between any of these behaviors at
     unpredictable points later on.

     For both calls, the file fd must be open for writing and may not be a
     directory or socket.

     Because there is no way for posix_fallocate() to report a partial
     failure, errors may require some or all of the work it has already done
     to be unwound, which may be expensive.  It is recommended to set the file
     length first with ftruncate(2) and only then allocate space within the
     file using posix_fallocate().

     Depending on the implementation, even a failing call to posix_fallocate()
     may allocate some space to the target file.  Such a call will not,
     however, change the file size.

     Furthermore, in some implementations, the space reservations created by
     posix_fallocate() may not be persistent after a crash or reboot if the
     space reserved has not yet been written to.

     If successful, the posix_fallocate() function will return zero.
     Otherwise an error number will be returned, without setting errno.

     If successful, the fdiscard() function will return zero.  Otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     [EBADF]            The file handle fd is invalid or not open for writing.

     [EDQUOT]           Allocating the requested blocks would exceed the
                        user's quota.

     [EINVAL]           The position and/or length values are negative.

     [EIO]              A hardware-level I/O error occurred.

     [EISDIR]           The selected file is a directory.

     [ENOSPC]           There was no space in the file system to complete the


     The posix_fallocate() and fdiscard() function calls appeared in
     NetBSD 7.0.  Similar functions appeared previously in Linux.  The
     posix_fallocate() function is expected to conform to IEEE Std 1003.1-2004

NetBSD 9.99                      June 30, 2016                     NetBSD 9.99