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       fallocate - manipulate file space


       #include <linux/falloc.h>

       long fallocate(int fd, int mode, loff_t offset, loff_t len);


       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.  Currently only one flag is supported for mode:

              This flag allocates and  initializes  to  zero  the  disk  space
              within   the  range  specified  by  offset  and  len.   After  a
              successful  call,  subsequent  writes  into   this   range   are
              guaranteed   not   to  fail  because  of  lack  of  disk  space.
              Preallocating zeroed blocks beyond the end of the file is useful
              for  optimizing append workloads.  Preallocating blocks does not
              change the file size (as reported by stat(2)) even if it is less
              than offset+len.

       If  FALLOC_FL_KEEP_SIZE  flag  is  not  specified  in mode, the default
       behavior is almost same as when  this  flag  is  specified.   The  only
       difference  is that on success, the file size will be changed if offset
       + len is greater than the file size.   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.

       Because allocation is  done  in  block  size  chunks,  fallocate()  may
       allocate a larger range than that which was specified.


       fallocate()  returns  zero  on  success, or an error number on failure.
       Note that errno is not set.


       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

       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.

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


       fallocate() is available on Linux since kernel 2.6.23.


       fallocate() is Linux-specific.


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


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