POSIX_FALLOCATE(3)         Linux Programmer's Manual        POSIX_FALLOCATE(3)

       posix_fallocate - allocate file space

       #include <fcntl.h>

       int posix_fallocate(int fd, off_t offset, off_t len);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           _XOPEN_SOURCE >= 600 || _POSIX_C_SOURCE >= 200112L

       The function posix_fallocate() ensures that disk space is allocated for
       the file referred to by the descriptor fd for the bytes  in  the  range
       starting  at  offset  and continuing for len bytes.  After a successful
       call to posix_fallocate(), subsequent writes to bytes in the  specified
       range are guaranteed not to fail because of lack of disk space.

       If  the  size  of  the  file  is less than offset+len, then the file is
       increased to this size; otherwise the file size is left unchanged.

       posix_fallocate() returns zero on success, or an error number on  fail-
       ure.  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.

       EINVAL offset  was  less than 0, or len was less than or equal to 0, or
              the underlying filesystem does not support the operation.

       ENODEV fd does not refer to a regular file.

       ENOSPC There is not enough space left on the device containing the file
              referred to by fd.

       ESPIPE fd refers to a pipe.

       posix_fallocate() is available since glibc 2.1.94.

       For   an   explanation   of   the  terms  used  in  this  section,  see

       |Interface         | Attribute     | Value                   |
       |posix_fallocate() | Thread safety | MT-Safe (but see NOTES) |

       POSIX.1-2008 says that an implementation shall give the EINVAL error if
       len was 0, or offset was less than 0.  POSIX.1-2001 says that an imple-
       mentation shall give the EINVAL error if len is less than 0, or  offset
       was less than 0, and may give the error if len equals zero.

       In the glibc implementation, posix_fallocate() is implemented using the
       fallocate(2) system call, which is MT-safe.  If the underlying filesys-
       tem  does not support fallocate(2), then the operation is emulated with
       the following caveats:

       * The emulation is inefficient.

       * There is a race condition where concurrent writes from another thread
         or process could be overwritten with null bytes.

       * There  is  a  race  condition where concurrent file size increases by
         another thread or process could  result  in  a  file  whose  size  is
         smaller than expected.

       * If  fd has been opened with the O_APPEND or O_WRONLY flags, the func-
         tion will fail with the error EBADF.

       In general, the emulation is not MT-safe.  On Linux,  applications  may
       use  fallocate(2)  if  they  cannot tolerate the emulation caveats.  In
       general, this is only recommended if the application plans to terminate
       the  operation  if  EOPNOTSUPP  is  returned, otherwise the application
       itself will need to implement a fallback with all the same problems  as
       the emulation provided by glibc.

       fallocate(1), fallocate(2), lseek(2), posix_fadvise(2)

       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

GNU                               2015-12-05                POSIX_FALLOCATE(3)
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