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sync_file_range() - Unix, Linux System Call
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NAME
sync_file_range - sync a file segment with disk
SYNOPSIS
#define _GNU_SOURCE
#include <fcntl.h>
void sync_file_range(int fd, off64_t offset, off64_t nbytes,
unsigned int flags);
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DESCRIPTION
sync_file_range() permits fine control when synchronising the open file referred to by the
file descriptor
fd with disk.
offset is the starting byte of the file range to be synchronised.
nbytes specifies the length of the range to be synchronised, in bytes; if
nbytes is zero, then all bytes from
offset through to the end of file are synchronised.
Synchronisation 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:
Tag | Description |
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.
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SYNC_FILE_RANGE_WRITE | |
Initiate write-out of all dirty pages in the specified
range which are not presently submitted write-out.
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SYNC_FILE_RANGE_WAIT_AFTER | |
Wait upon write-out of all pages in the range
after performing any write.
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Specifying
flags as 0 is permitted, as a no-op.
NOTES
None of these operations write out the files metadata.
Therefore, unless the application is strictly performing overwrites of
already-instantiated disk blocks,
there are no guarantees that the data will be available after a crash.
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:
Tag | Description |
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.
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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.
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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.
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SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE | SYNC_FILE_RANGE_WAIT_AFTER | |
This is a traditional
fdatasync(2)
operation.
It 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.
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ERRORS
Tag | Description |
EBADF |
fd is not a valid file descriptor.
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EIO |
I/O error.
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EINVAL |
flags specifies an invalid bit; or
offset or
nbytes is invalid.
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ENOMEM |
Out of memory.
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ENOSPC |
Out of disk space.
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ESPIPE |
fd refers to something other than a regular file, a block device,
a directory, or a symbolic link.
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CONFORMING TO
This system call is Linux specific, and should be avoided
in portable programs.
SEE ALSO
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