bsd/man/man2/fsync.2

   1 .\"     $NetBSD: fsync.2,v 1.4 1995/02/27 12:32:38 cgd Exp $
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  34 .\"     @(#)fsync.2     8.1 (Berkeley) 6/4/93
  35 .\"
  36 .Dd June 4, 1993
  37 .Dt FSYNC 2
  38 .Os BSD 4.2
  39 .Sh NAME
  40 .Nm fsync
  41 .Nd "synchronize a file's in-core state with that on disk"
  42 .Sh SYNOPSIS
  43 .Fd #include <unistd.h>
  44 .Ft int
  45 .Fo fsync
  46 .Fa "int fildes"
  47 .Fc
  48 .Sh DESCRIPTION
  49 .Fn fsync
  50 causes all modified data and attributes of
  51 .Fa fildes
  52 to be moved to a permanent storage device.
  53 This normally results in all in-core modified copies
  54 of buffers for the associated file to be written to a disk.
  55 .Pp
  56 Note that while
  57 .Fn fsync
  58 will flush all data from the host to the drive
  59 (i.e. the "permanent storage device"),
  60 the drive itself may not physically write the data
  61 to the platters for quite some time
  62 and it may be written in an out-of-order sequence.
  63 .Pp
  64 Specifically, if the drive loses power
  65 or the OS crashes,
  66 the application may find that only some or none of their data was written.
  67 The disk drive may also re-order the data
  68 so that later writes may be present, while earlier writes are not.
  69 .Pp
  70 This is not a theoretical edge case.
  71 This scenario is easily reproduced with real world workloads
  72 and drive power failures.
  73 .Pp
  74 For applications that require tighter guarantees
  75 about the integrity of their data,
  76 Mac OS X provides the F_FULLFSYNC fcntl.
  77 The F_FULLFSYNC fcntl asks the drive to flush all buffered data
  78 to permanent storage.
  79 Applications, such as databases,
  80 that require a strict ordering of writes
  81 should use F_FULLFSYNC to ensure that their data
  82 is written in the order they expect.
  83 Please see
  84 .Xr fcntl 2
  85 for more detail.
  86 .Pp
  87 .Sh RETURN VALUES
  88 .Rv -std fsync
  89 .Sh ERRORS
  90 The
  91 .Fn fsync
  92 system call will fail if:
  93 .Bl -tag -width Er
  94 .\" ==========
  95 .It Bq Er EBADF
  96 .Fa fildes
  97 is not a valid descriptor.
  98 .\" ==========
  99 .It Bq Er EINTR
 100 Its execution is interrupted by a signal.
 101 .\" ==========
 102 .It Bq Er EINVAL
 103 .Fa fildes
 104 refers to a file type (e.g., a socket)
 105 that does not support this operation.
 106 .\" ==========
 107 .It Bq Er EIO
 108 An I/O error occurred while reading from or writing to the file system.
 109 .El
 110 .Pp
 111 If a queued I/O operation fails,
 112 .Fn fsync
 113 may fail with any of the errors defined for
 114 .Xr read 2
 115 or
 116 .Xr write 2 .
 117 .Sh SEE ALSO
 118 .Xr fcntl 2 ,
 119 .Xr read 2 ,
 120 .Xr sync 2 ,
 121 .Xr write 2 ,
 122 .Xr sync 8 ,
 123 .Xr update 8
 124 .Sh HISTORY
 125 The
 126 .Fn fsync
 127 function call appeared in
 128 .Bx 4.2 .