man3/ffs.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  25 .\" References consulted:
  26 .\"     Linux libc source code
  27 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  28 .\"     386BSD man pages
  29 .\" Modified Sat Jul 24 19:39:35 1993 by Rik Faith (faith@cs.unc.edu)
  30 .\"
  31 .\" Modified 2003 Walter Harms (walter.harms@informatik.uni-oldenburg.de)
  32 .\"
  33 .TH FFS 3 2015-08-08 "GNU" "Linux Programmer's Manual"
  34 .SH NAME
  35 ffs, ffsl, ffsll \- find first bit set in a word
  36 .SH SYNOPSIS
  37 .nf
  38 .B #include <strings.h>
  39 .sp
  40 .BI "int ffs(int " i );
  41 .sp
  42 .B #include <string.h>
  43 .sp
  44 .BI "int ffsl(long int " i );
  45 .sp
  46 .BI "int ffsll(long long int " i );
  47 .fi
  48 .sp
  49 .in -4n
  50 Feature Test Macro Requirements for glibc (see
  51 .BR feature_test_macros (7)):
  52 .in
  53 .PD 0
  54 .ad l
  55 .sp
  56 .BR ffs ():
  57 .RS 4
  58 .TP 4
  59 Since glibc 2.12:
  60 _POSIX_C_SOURCE\ >=\ 200809L
  61     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
  62     || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
  63 .TP
  64 Before glibc 2.12:
  65 none
  66 .RE
  67 .PP
  68 .BR ffsl (),
  69 .BR ffsll ():
  70 .RS 4
  71 _GNU_SOURCE
  72 .RE
  73 .ad
  74 .PD
  75 .SH DESCRIPTION
  76 The
  77 .BR ffs ()
  78 function returns the position of the first
  79 (least significant) bit set in the word \fIi\fP.
  80 The least significant bit is position 1 and the
  81 most significant position is, for example, 32 or 64.
  82 The functions
  83 .BR ffsll ()
  84 and
  85 .BR ffsl ()
  86 do the same but take
  87 arguments of possibly different size.
  88 .SH RETURN VALUE
  89 These functions return the position of the first bit set,
  90 or 0 if no bits are set in
  91 .IR i .
  92 .SH ATTRIBUTES
  93 For an explanation of the terms used in this section, see
  94 .BR attributes (7).
  95 .TS
  96 allbox;
  97 lbw22 lb lb
  98 l l l.
  99 Interface       Attribute       Value
 100 T{
 101 .BR ffs (),
 102 .BR ffsl (),
 103 .BR ffsll ()
 104 T}      Thread safety   MT-Safe
 105 .TE
 106 .SH CONFORMING TO
 107 .BR ffs ():
 108 POSIX.1-2001, POSIX.1-2008, 4.3BSD.
 109
 110 The
 111 .BR ffsl ()
 112 and
 113 .BR ffsll ()
 114 functions are glibc extensions.
 115 .SH NOTES
 116 BSD systems have a prototype in
 117 .IR <string.h> .
 118 .SH SEE ALSO
 119 .BR memchr (3)