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)):

           _POSIX_C_SOURCE >= 200112L

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

       If the size of the file is less than offset+len, then the file  is  in-
       creased 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.

       EINTR  A signal was caught during execution.

       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  at-

       |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 an-
         other  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 fails 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  it-
       self  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)

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GNU                               2017-09-15                POSIX_FALLOCATE(3)
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