fallocate(2)



FALLOCATE(2)               Linux Programmer's Manual              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, off_t len);

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  subsec-
       tions 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 op-
       timally 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 off-
       set+len is greater than the file size.  Preallocating zeroed blocks be-
       yond 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 guarantee 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.

       Because allocation is done in block size chunks, fallocate() may  allo-
       cate a larger range of disk space than was specified.

   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 filesystem  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 following filesystems:

       *  XFS (since Linux 2.6.38)

       *  ext4 (since Linux 3.0)

       *  Btrfs (since Linux 3.7)

       *  tmpfs(5) (since Linux 3.5)

   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  completion 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 filesystem 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 ftruncate(2) to truncate a
       file.

       No other flags may be  specified  in  mode  in  conjunction  with  FAL-
       LOC_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 specified range, blocks are preallocated for
       the  regions that span the holes in the file.  After a successful call,
       subsequent reads from this range will return zeros.

       Zeroing is done within the  filesystem  preferably  by  converting  the
       range  into  unwritten extents.  This approach means that the specified
       range will not be physically zeroed out on the device (except for  par-
       tial blocks at the either end of the range), and I/O is (otherwise) re-
       quired 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 following 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 up-
       ward (i.e., to a higher file offset) by len bytes.   Inserting  a  hole
       inside a file increases the file size by len bytes.

       This  mode has the same limitations as FALLOC_FL_COLLAPSE_RANGE regard-
       ing 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), ftrun-
       cate(2) should be used.

       No other flags may be  specified  in  mode  in  conjunction  with  FAL-
       LOC_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 er-
       rno 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 specified by off-
              set 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.

       EINVAL mode  contains  one of FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_IN-
              SERT_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 FAL-
              LOC_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_IN-
              SERT_RANGE,  but  the  file referred to by fd is currently being
              executed.

VERSIONS
       fallocate() is available on Linux since kernel 2.6.23.  Support is pro-
       vided  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 4.16 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/.

Linux                             2018-04-30                      FALLOCATE(2)

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