FALLOCATE(2)              (2019-11-19)               FALLOCATE(2)

     NAME
          fallocate - manipulate file space

     SYNOPSIS
          #define _GNU_SOURCE             /* See feature_test_macros(7) */
          #include <fcntl.h>

          int fallocate(int fd, int mode, off_t offset

     DESCRIPTION
          This is a nonportable, Linux-specific system call.  For the
          portable, POSIX.1-specified method of ensuring that space is
          allocated for a file, see posix_fallocate(3).

          fallocate() allows the caller to directly manipulate the
          allocated disk space for the file referred to by fd for the
          byte range starting at offset and continuing for len bytes.

          The mode argument determines the operation to be performed
          on the given range.  Details of the supported operations are
          given in the subsections below.

        Allocating disk space
          The default operation (i.e., mode is zero) of fallocate()
          allocates the disk space within the range specified by
          offset and len. The file size (as reported by stat(2)) will
          be changed if offset+len is greater than the file size.  Any
          subregion within the range specified by offset and len that
          did not contain data before the call will be initialized to
          zero.  This default behavior closely resembles the behavior
          of the posix_fallocate(3) library function, and is intended
          as a method of optimally implementing that function.

          After a successful call, subsequent writes into the range
          specified by offset and len are guaranteed not to fail
          because of lack of disk space.

          If the FALLOC_FL_KEEP_SIZE flag is specified in mode, the
          behavior of the call is similar, but the file size will not
          be changed even if offset+len is greater than the file size.
          Preallocating zeroed blocks beyond the end of the file in
          this manner is useful for optimizing append workloads.

          If the FALLOC_FL_UNSHARE flag is specified in mode, shared
          file data extents will be made private to the file to guar-
          antee that a subsequent write will not fail due to lack of
          space.  Typically, this will be done by performing a copy-
          on-write operation on all shared data in the file.  This
          flag may not be supported by all filesystems.

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          Because allocation is done in block size chunks, fallocate()
          may allocate a larger range of disk space than was speci-
          fied.

        Deallocating file space
          Specifying the FALLOC_FL_PUNCH_HOLE flag (available since
          Linux 2.6.38) in mode deallocates space (i.e., creates a
          hole) in the byte range starting at offset and continuing
          for len bytes.  Within the specified range, partial filesys-
          tem blocks are zeroed, and whole filesystem blocks are
          removed from the file.  After a successful call, subsequent
          reads from this range will return zeros.

          The FALLOC_FL_PUNCH_HOLE flag must be ORed with
          FALLOC_FL_KEEP_SIZE in mode; in other words, even when
          punching off the end of the file, the file size (as reported
          by stat(2)) does not change.

          Not all filesystems support FALLOC_FL_PUNCH_HOLE; if a
          filesystem doesn't support the operation, an error is
          returned.  The operation is supported on at least the fol-
          lowing filesystems:

          *  XFS (since Linux 2.6.38)

          *  ext4 (since Linux 3.0)

          *  Btrfs (since Linux 3.7)

          *  tmpfs(5) (since Linux 3.5)

          *  gfs2(5) (since Linux 4.16)

        Collapsing file space
          Specifying the FALLOC_FL_COLLAPSE_RANGE flag (available
          since Linux 3.15) in mode removes a byte range from a file,
          without leaving a hole.  The byte range to be collapsed
          starts at offset and continues for len bytes.  At the com-
          pletion of the operation, the contents of the file starting
          at the location offset+len will be appended at the location
          offset, and the file will be len bytes smaller.

          A filesystem may place limitations on the granularity of the
          operation, in order to ensure efficient implementation.
          Typically, offset and len must be a multiple of the filesys-
          tem logical block size, which varies according to the
          filesystem type and configuration.  If a filesystem has such
          a requirement, fallocate() fails with the error EINVAL if
          this requirement is violated.

          If the region specified by offset plus len reaches or passes
          the end of file, an error is returned; instead, use

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          ftruncate(2) to truncate a file.

          No other flags may be specified in mode in conjunction with
          FALLOC_FL_COLLAPSE_RANGE.

          As at Linux 3.15, FALLOC_FL_COLLAPSE_RANGE is supported by
          ext4 (only for extent-based files) and XFS.

        Zeroing file space
          Specifying the FALLOC_FL_ZERO_RANGE flag (available since
          Linux 3.15) in mode zeros space in the byte range starting
          at offset and continuing for len bytes.  Within the speci-
          fied range, blocks are preallocated for the regions that
          span the holes in the file.  After a successful call, subse-
          quent reads from this range will return zeros.

          Zeroing is done within the filesystem preferably by convert-
          ing the range into unwritten extents.  This approach means
          that the specified range will not be physically zeroed out
          on the device (except for partial blocks at the either end
          of the range), and I/O is (otherwise) required only to
          update metadata.

          If the FALLOC_FL_KEEP_SIZE flag is additionally specified in
          mode, the behavior of the call is similar, but the file size
          will not be changed even if offset+len is greater than the
          file size.  This behavior is the same as when preallocating
          space with FALLOC_FL_KEEP_SIZE specified.

          Not all filesystems support FALLOC_FL_ZERO_RANGE; if a
          filesystem doesn't support the operation, an error is
          returned.  The operation is supported on at least the fol-
          lowing filesystems:

          *  XFS (since Linux 3.15)

          *  ext4, for extent-based files (since Linux 3.15)

          *  SMB3 (since Linux 3.17)

          *  Btrfs (since Linux 4.16)

        Increasing file space
          Specifying the FALLOC_FL_INSERT_RANGE flag (available since
          Linux 4.1) in mode increases the file space by inserting a
          hole within the file size without overwriting any existing
          data.  The hole will start at offset and continue for len
          bytes.  When inserting the hole inside file, the contents of
          the file starting at offset will be shifted upward (i.e., to
          a higher file offset) by len bytes.  Inserting a hole inside
          a file increases the file size by len bytes.

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          This mode has the same limitations as
          FALLOC_FL_COLLAPSE_RANGE regarding the granularity of the
          operation.  If the granularity requirements are not met,
          fallocate() fails with the error EINVAL.  If the offset is
          equal to or greater than the end of file, an error is
          returned.  For such operations (i.e., inserting a hole at
          the end of file), ftruncate(2) should be used.

          No other flags may be specified in mode in conjunction with
          FALLOC_FL_INSERT_RANGE.

          FALLOC_FL_INSERT_RANGE requires filesystem support.
          Filesystems that support this operation include XFS (since
          Linux 4.1) and ext4 (since Linux 4.2).

     RETURN VALUE
          On success, fallocate() returns zero.  On error, -1 is
          returned and errno is set to indicate the error.

     ERRORS
          EBADF
               fd is not a valid file descriptor, or is not opened for
               writing.

          EFBIG
               offset+len exceeds the maximum file size.

          EFBIG
               mode is FALLOC_FL_INSERT_RANGE, and the current file
               size+len exceeds the maximum file size.

          EINTR
               A signal was caught during execution; see signal(7).

          EINVAL
               offset was less than 0, or len was less than or equal
               to 0.

          EINVAL
               mode is FALLOC_FL_COLLAPSE_RANGE and the range speci-
               fied by offset plus len reaches or passes the end of
               the file.

          EINVAL
               mode is FALLOC_FL_INSERT_RANGE and the range specified
               by offset reaches or passes the end of the file.

          EINVAL
               mode is FALLOC_FL_COLLAPSE_RANGE or
               FALLOC_FL_INSERT_RANGE, but either offset or len is not
               a multiple of the filesystem block size.

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          EINVAL
               mode contains one of FALLOC_FL_COLLAPSE_RANGE or
               FALLOC_FL_INSERT_RANGE and also other flags; no other
               flags are permitted with FALLOC_FL_COLLAPSE_RANGE or
               FALLOC_FL_INSERT_RANGE.

          EINVAL
               mode is FALLOC_FL_COLLAPSE_RANGE or
               FALLOC_FL_ZERO_RANGE or FALLOC_FL_INSERT_RANGE, but the
               file referred to by fd is not a regular file.

          EIO  An I/O error occurred while reading from or writing to
               a filesystem.

          ENODEV
               fd does not refer to a regular file or a directory.
               (If fd is a pipe or FIFO, a different error results.)

          ENOSPC
               There is not enough space left on the device containing
               the file referred to by fd.

          ENOSYS
               This kernel does not implement fallocate().

          EOPNOTSUPP
               The filesystem containing the file referred to by fd
               does not support this operation; or the mode is not
               supported by the filesystem containing the file
               referred to by fd.

          EPERM
               The file referred to by fd is marked immutable (see
               chattr(1)).

          EPERM
               mode specifies FALLOC_FL_PUNCH_HOLE or
               FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE and
               the file referred to by fd is marked append-only (see
               chattr(1)).

          EPERM
               The operation was prevented by a file seal; see
               fcntl(2).

          ESPIPE
               fd refers to a pipe or FIFO.

          ETXTBSY
               mode specifies FALLOC_FL_COLLAPSE_RANGE or
               FALLOC_FL_INSERT_RANGE, but the file referred to by fd
               is currently being executed.

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     VERSIONS
          fallocate() is available on Linux since kernel 2.6.23.  Sup-
          port is provided by glibc since version 2.10.  The
          FALLOC_FL_* flags are defined in glibc headers only since
          version 2.18.

     CONFORMING TO
          fallocate() is Linux-specific.

     SEE ALSO
          fallocate(1), ftruncate(2), posix_fadvise(3),
          posix_fallocate(3)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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