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FALLOCATE(2)               Linux Programmer's Manual              FALLOCATE(2)



NAME
       fallocate - manipulate file space

SYNOPSIS
       #define _GNU_SOURCE
       #include <fcntl.h>

       int fallocate(int fd, int mode, off_t offset, off_t len);

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

       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 and initializes
       to zero 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.   This  default  behavior  closely resembles the behavior of the posix_fallo-
       cate(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 man-
       ner is useful for optimizing append workloads.

       Because  allocation is done in block size chunks, fallocate() may allocate 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 suc-
       cessful 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  doesn't  support
       the operation, an error is returned.

RETURN VALUE
       fallocate() returns zero on success, and -1 on failure.

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

       EFBIG  offset+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.

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

       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 The  file system containing the file referred to by fd does not support this
              operation.

       EOPNOTSUPP
              The mode is not supported by the file system containing the file referred to
              by fd.

VERSIONS
       fallocate()  is  available  on  Linux  since kernel 2.6.23.  Support is provided by
       glibc since version 2.10.

CONFORMING TO
       fallocate() is Linux-specific.

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-03-13                      FALLOCATE(2)

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