NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | SEE ALSO | COLOPHON

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

NAME         top

       fallocate - manipulate file space

SYNOPSIS         top

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

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

DESCRIPTION         top

       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.

       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 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
       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 (since Linux 3.5)

RETURN VALUE         top

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

ERRORS         top

       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
              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)).  Or: mode specifies FALLOC_FL_PUNCH_HOLE and the
              file referred to by fd is marked append-only (see chattr(1)).

       ESPIPE fd refers to a pipe or FIFO.

VERSIONS         top

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

CONFORMING TO         top

       fallocate() is Linux-specific.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 3.64 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                            2014-01-24                     FALLOCATE(2)