man2/fdatasync.2

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   3 .\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
   4 .\" Copyright (C) 1996 Markus Kuhn.
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   6 .\" [This version merged from two independently written pages - aeb]
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  28 .\" 1996-04-12  Andries Brouwer <aeb@cwi.nl>
  29 .\" 1996-04-13  Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
  30 .\"
  31 .TH FDATASYNC 2 1996-04-13 "Linux" "Linux Programmer's Manual"
  32 .SH NAME
  33 fdatasync \- synchronize a file's in-core data with that on disk
  34 .SH SYNOPSIS
  35 .B #include <unistd.h>
  36 .sp
  37 .BI "int fdatasync(int " fd );
  38 .SH DESCRIPTION
  39 .BR fdatasync ()
  40 flushes all data buffers of a file to disk (before the system
  41 call returns).
  42 It resembles
  43 .BR fsync (2)
  44 but is not required to update metadata such as the file last access time.
  45
  46 Applications that access databases or log files often write a tiny
  47 data fragment (e.g., one line in a log file) and then call
  48 .BR fsync (2)
  49 immediately in order to ensure that the written data is physically
  50 stored on the harddisk.
  51 Unfortunately,
  52 .BR fsync (2)
  53 will always initiate two write operations: one for the newly written
  54 data and another one in order to update the modification time stored
  55 in the inode.
  56 If the modification time is not a part of the transaction
  57 concept
  58 .BR fdatasync ()
  59 can be used to avoid unnecessary inode disk write operations.
  60 .SH "RETURN VALUE"
  61 On success, zero is returned.
  62 On error, \-1 is returned, and
  63 .I errno
  64 is set appropriately.
  65 .SH ERRORS
  66 .TP
  67 .B EBADF
  68 .I fd
  69 is not a valid file descriptor open for writing.
  70 .TP
  71 .B EIO
  72 An error occurred during synchronization.
  73 .TP
  74 .BR EROFS ", " EINVAL
  75 .I fd
  76 is bound to a special file which does not support synchronization.
  77 .SH "CONFORMING TO"
  78 POSIX.1-2001.
  79 .SH AVAILABILITY
  80 On POSIX systems on which
  81 .BR fdatasync ()
  82 is available,
  83 .B _POSIX_SYNCHRONIZED_IO
  84 is defined in
  85 .I <unistd.h>
  86 to a value greater than 0.
  87 (See also
  88 .BR sysconf (3).)
  89 .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
  90 .\" -1: unavailable, 0: ask using sysconf().
  91 .\" glibc defines them to 1.
  92 .SH NOTES
  93 In Linux 2.2 and earlier,
  94 .BR fdatasync ()
  95 is equivalent to
  96 .BR fsync (2),
  97 and so has no performance advantage.
  98 .SH "SEE ALSO"
  99 .BR fsync (2),
 100 .BR sync_file_range (2)
 101 .br
 102 B.O. Gallmeister, POSIX.4, O'Reilly, pp. 220-223 and 343.