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 2017-09-15 "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 .PP
  40 .BI "int ffs(int " i );
  41 .PP
  42 .B #include <string.h>
  43 .PP
  44 .BI "int ffsl(long int " i );
  45 .PP
  46 .BI "int ffsll(long long int " i );
  47 .fi
  48 .PP
  49 .in -4n
  50 Feature Test Macro Requirements for glibc (see
  51 .BR feature_test_macros (7)):
  52 .in
  53 .PP
  54 .PD 0
  55 .ad l
  56 .BR ffs ():
  57 .RS 4
  58 .TP 4
  59 Since glibc 2.12:
  60     _XOPEN_SOURCE >= 700
  61     || ! (_POSIX_C_SOURCE\ >=\ 200809L)
  62     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
  63     || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
  64 .TP
  65 Before glibc 2.12:
  66 none
  67 .RE
  68 .PP
  69 .BR ffsl (),
  70 .BR ffsll ():
  71 .RS 4
  72 .PD 0
  73 .TP 4
  74 Since glibc 2.27:
  75 .\" glibc commit 68fe16dd327c895c08b9ee443b234c49c13b36e9
  76     _DEFAULT_SOURCE
  77 .TP
  78 Before glibc 2.27:
  79     _GNU_SOURCE
  80 .PD
  81 .RE
  82 .ad
  83 .PD
  84 .SH DESCRIPTION
  85 The
  86 .BR ffs ()
  87 function returns the position of the first
  88 (least significant) bit set in the word \fIi\fP.
  89 The least significant bit is position 1 and the
  90 most significant position is, for example, 32 or 64.
  91 The functions
  92 .BR ffsll ()
  93 and
  94 .BR ffsl ()
  95 do the same but take
  96 arguments of possibly different size.
  97 .SH RETURN VALUE
  98 These functions return the position of the first bit set,
  99 or 0 if no bits are set in
 100 .IR i .
 101 .SH ATTRIBUTES
 102 For an explanation of the terms used in this section, see
 103 .BR attributes (7).
 104 .TS
 105 allbox;
 106 lbw22 lb lb
 107 l l l.
 108 Interface       Attribute       Value
 109 T{
 110 .BR ffs (),
 111 .BR ffsl (),
 112 .BR ffsll ()
 113 T}      Thread safety   MT-Safe
 114 .TE
 115 .SH CONFORMING TO
 116 .BR ffs ():
 117 POSIX.1-2001, POSIX.1-2008, 4.3BSD.
 118 .PP
 119 The
 120 .BR ffsl ()
 121 and
 122 .BR ffsll ()
 123 functions are glibc extensions.
 124 .SH NOTES
 125 BSD systems have a prototype in
 126 .IR <string.h> .
 127 .SH SEE ALSO
 128 .BR memchr (3)