fallocate

       #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
       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 off-
       set+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.

       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 zeroes.

       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
       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() will fail 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 zeroes space in the byte range starting at offset and continu-
       ing 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 zeroes.

       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)
       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 following filesystems:

       *  XFS (since Linux 3.15)

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

       *  SMB3 (since Linux 3.17)

   Increasing file space
       Specifying  the FALLOC_FL_INSERT_RANGE flag (available since Linux 4.1)
       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
       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.

       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  FAL-
              LOC_FL_INSERT_RANGE and also other flags;  no  other  flags  are
              permitted      with     FALLOC_FL_COLLAPSE_RANGE     or     FAL-
              LOC_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().
       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     FAL-
              LOC_FL_INSERT_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 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-07-23                      FALLOCATE(2)
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