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Linux 2.6 - man page for fallocate (linux section 2)

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

       fallocate - manipulate file space

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

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

       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 spec-
       ified 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  opti-
       mizing 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  deallo-
       cates 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.

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

       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().

	      The  filesystem  containing the file referred to by fd does not support this opera-
	      tion; 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

       ESPIPE fd refers to a pipe or FIFO.

       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.

       fallocate() is Linux-specific.

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

       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

Linux					    2013-11-08				     FALLOCATE(2)

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