man3/flockfile.3

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  25 .TH FLOCKFILE 3  2017-07-13 "" "Linux Programmer's Manual"
  26 .SH NAME
  27 flockfile, ftrylockfile, funlockfile \- lock FILE for stdio
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <stdio.h>
  31 .PP
  32 .BI "void flockfile(FILE *" filehandle );
  33 .BI "int ftrylockfile(FILE *" filehandle );
  34 .BI "void funlockfile(FILE *" filehandle );
  35 .fi
  36 .PP
  37 .in -4n
  38 Feature Test Macro Requirements for glibc (see
  39 .BR feature_test_macros (7)):
  40 .in
  41 .ad l
  42 .PP
  43 All functions shown above:
  44 .RS 4
  45 /* Since glibc 2.24: */ _POSIX_C_SOURCE\ >=\ 199309L
  46     || /* Glibc versions <= 2.23: */ _POSIX_C_SOURCE
  47     || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
  48 .RE
  49 .ad b
  50 .SH DESCRIPTION
  51 The stdio functions are thread-safe.
  52 This is achieved by assigning
  53 to each
  54 .I FILE
  55 object a lockcount and (if the lockcount is nonzero)
  56 an owning thread.
  57 For each library call, these functions wait until the
  58 .I FILE
  59 object
  60 is no longer locked by a different thread, then lock it, do the
  61 requested I/O, and unlock the object again.
  62 .PP
  63 (Note: this locking has nothing to do with the file locking done
  64 by functions like
  65 .BR flock (2)
  66 and
  67 .BR lockf (3).)
  68 .PP
  69 All this is invisible to the C-programmer, but there may be two
  70 reasons to wish for more detailed control.
  71 On the one hand, maybe
  72 a series of I/O actions by one thread belongs together, and should
  73 not be interrupted by the I/O of some other thread.
  74 On the other hand, maybe the locking overhead should be avoided
  75 for greater efficiency.
  76 .PP
  77 To this end, a thread can explicitly lock the
  78 .I FILE
  79 object,
  80 then do its series of I/O actions, then unlock.
  81 This prevents
  82 other threads from coming in between.
  83 If the reason for doing
  84 this was to achieve greater efficiency, one does the I/O with
  85 the nonlocking versions of the stdio functions: with
  86 .BR getc_unlocked (3)
  87 and
  88 .BR putc_unlocked (3)
  89 instead of
  90 .BR getc (3)
  91 and
  92 .BR putc (3).
  93 .PP
  94 The
  95 .BR flockfile ()
  96 function waits for
  97 .I *filehandle
  98 to be
  99 no longer locked by a different thread, then makes the
 100 current thread owner of
 101 .IR *filehandle ,
 102 and increments
 103 the lockcount.
 104 .PP
 105 The
 106 .BR funlockfile ()
 107 function decrements the lock count.
 108 .PP
 109 The
 110 .BR ftrylockfile ()
 111 function is a nonblocking version
 112 of
 113 .BR flockfile ().
 114 It does nothing in case some other thread
 115 owns
 116 .IR *filehandle ,
 117 and it obtains ownership and increments
 118 the lockcount otherwise.
 119 .SH RETURN VALUE
 120 The
 121 .BR ftrylockfile ()
 122 function returns zero for success
 123 (the lock was obtained), and nonzero for failure.
 124 .SH ERRORS
 125 None.
 126 .SH ATTRIBUTES
 127 For an explanation of the terms used in this section, see
 128 .BR attributes (7).
 129 .TS
 130 allbox;
 131 lbw29 lb lb
 132 l l l.
 133 Interface       Attribute       Value
 134 T{
 135 .BR flockfile (),
 136 .BR ftrylockfile (),
 137 .BR funlockfile ()
 138 T}      Thread safety   MT-Safe
 139 .TE
 140 .SH CONFORMING TO
 141 POSIX.1-2001, POSIX.1-2008.
 142 .SH AVAILABILITY
 143 These functions are available when
 144 .B _POSIX_THREAD_SAFE_FUNCTIONS
 145 is defined.
 146 .SH SEE ALSO
 147 .BR unlocked_stdio (3)