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

NAME

     sync_file_range - sync a file segment with disk

SYNOPSIS

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

     int sync_file_range(int fd, off64_t offset, off64_t nbytes,
                         unsigned int flags);

DESCRIPTION

     sync_file_range() permits fine control when synchronizing the open file
     referred to by the file descriptor fd with disk.

     offset is the starting byte of  the  file  range  to  be  synchronized.
     nbytes  specifies the length of the range to be synchronized, in bytes;
     if nbytes is zero, then all bytes from offset through  to  the  end  of
     file  are synchronized.  Synchronization is in units of the system page
     size: offset is rounded down to a page boundary;  (offset+nbytes-1)  is
     rounded up to a page boundary.

     The flags bit-mask argument can include any of the following values:

     SYNC_FILE_RANGE_WAIT_BEFORE
            Wait  upon  write-out  of  all pages in the specified range that
            have already been submitted to the device driver  for  write-out
            before performing any write.

     SYNC_FILE_RANGE_WRITE
            Initiate  write-out  of  all  dirty pages in the specified range
            which are not presently submitted  write-out.   Note  that  even
            this  may  block if you attempt to write more than request queue
            size.

     SYNC_FILE_RANGE_WAIT_AFTER
            Wait upon write-out of all pages in the range  after  performing
            any write.

     Specifying flags as 0 is permitted, as a no-op.

 Warning
     This  system call is extremely dangerous and should not be used in por-
     table programs.  None of these operations writes out the  file's  meta-
     data.   Therefore,  unless the application is strictly performing over-
     writes of already-instantiated disk blocks,  there  are  no  guarantees
     that the data will be available after a crash.  There is no user inter-
     face to know if a write is purely an overwrite.  On  filesystems  using
     copy-on-write  semantics  (e.g.,  btrfs) an overwrite of existing allo-
     cated blocks is impossible.  When writing into preallocated space, many
     filesystems  also  require  calls  into the block allocator, which this
     system call does not sync out to disk.  This system call does not flush
     disk  write caches and thus does not provide any data integrity on sys-
     tems with volatile disk write caches.

 Some details
     SYNC_FILE_RANGE_WAIT_BEFORE and SYNC_FILE_RANGE_WAIT_AFTER will  detect
     any  I/O  errors  or  ENOSPC  conditions  and  will return these to the
     caller.

     Useful combinations of the flags bits are:

     SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE
            Ensures that all pages in the specified range which  were  dirty
            when  sync_file_range()  was  called are placed under write-out.
            This is a start-write-for-data-integrity operation.

     SYNC_FILE_RANGE_WRITE
            Start write-out of all dirty pages in the specified range  which
            are  not  presently  under  write-out.   This is an asynchronous
            flush-to-disk  operation.   This  is  not  suitable   for   data
            integrity operations.

     SYNC_FILE_RANGE_WAIT_BEFORE (or SYNC_FILE_RANGE_WAIT_AFTER)
            Wait  for  completion of write-out of all pages in the specified
            range.     This    can    be    used    after     an     earlier
            SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE operation to
            wait for completion of that operation, and obtain its result.

     SYNC_FILE_RANGE_WAIT_BEFORE       |       SYNC_FILE_RANGE_WRITE       |
     SYNC_FILE_RANGE_WAIT_AFTER
            This is a write-for-data-integrity operation  that  will  ensure
            that  all  pages  in  the  specified range which were dirty when
            sync_file_range() was called are committed to disk.

RETURN VALUE

     On success, sync_file_range() returns 0; on failure -1 is returned  and
     errno is set to indicate the error.

ERRORS

     EBADF  fd is not a valid file descriptor.

     EINVAL flags  specifies an invalid bit; or offset or nbytes is invalid.

     EIO    I/O error.

     ENOMEM Out of memory.

     ENOSPC Out of disk space.

     ESPIPE fd refers to something  other  than  a  regular  file,  a  block
            device, or a directory.

VERSIONS

     sync_file_range() appeared on Linux in kernel 2.6.17.

CONFORMING TO

     This  system  call is Linux-specific, and should be avoided in portable
     programs.

NOTES

 sync_file_range2()
     Some architectures (e.g., PowerPC, ARM) need  64-bit  arguments  to  be
     aligned  in  a  suitable pair of registers.  On such architectures, the
     call signature of sync_file_range() shown in the SYNOPSIS would force a
     register  to  be wasted as padding between the fd and offset arguments.
     (See syscall(2) for details.)  Therefore, these architectures define  a
     different system call that orders the arguments suitably:

         int    sync_file_range2(int    fd,    unsigned    int    flags,
         off64_t offset, off64_t nbytes);

     The behavior of this system call  is  otherwise  exactly  the  same  as
     sync_file_range().

     A  system  call with this signature first appeared on the ARM architec-
     ture in Linux 2.6.20, with  the  name  arm_sync_file_range().   It  was
     renamed  in  Linux 2.6.22, when the analogous system call was added for
     PowerPC.  On architectures  where  glibc  support  is  provided,  glibc
     transparently     wraps     sync_file_range2()     under    the    name
     sync_file_range().