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FDISCARD(2) NetBSD System Calls Manual FDISCARD(2)
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posix_fallocate, fdiscard -- allocate or discard backing store for files
Standard C Library (libc, -lc)
posix_fallocate(int fd, off_t pos, off_t length);
fdiscard(int fd, off_t pos, off_t length);
The posix_fallocate() call allocates backing store for the file refer-
enced 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 region.
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 con-
tinuing 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 unaf-
fected; 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 underly-
ing discard operation, that operation is invoked. For example, ATA flash
devices and solid-state disks support an operation called TRIM that dis-
cards 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
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 fail-
ure, 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, how-
ever, 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. Other-
wise 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
[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
Modified for NetBSD
by Kimmo Suominen