1FFS(3) Linux Programmer's Manual FFS(3)
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6 ffs, ffsl, ffsll - find first bit set in a word
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9 #include <strings.h>
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11 int ffs(int i);
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13 #include <string.h>
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15 int ffsl(long i);
16 int ffsll(long long i);
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18 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20 ffs():
21 Since glibc 2.12:
22 _XOPEN_SOURCE >= 700
23 || ! (_POSIX_C_SOURCE >= 200809L)
24 || /* Glibc since 2.19: */ _DEFAULT_SOURCE
25 || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
26 Before glibc 2.12:
27 none
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29 ffsl(), ffsll():
30 Since glibc 2.27:
31 _DEFAULT_SOURCE
32 Before glibc 2.27:
33 _GNU_SOURCE
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36 The ffs() function returns the position of the first (least signifi‐
37 cant) bit set in the word i. The least significant bit is position 1
38 and the most significant position is, for example, 32 or 64. The func‐
39 tions ffsll() and ffsl() do the same but take arguments of possibly
40 different size.
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43 These functions return the position of the first bit set, or 0 if no
44 bits are set in i.
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47 For an explanation of the terms used in this section, see at‐
48 tributes(7).
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50 ┌────────────────────────────────────────────┬───────────────┬─────────┐
51 │Interface │ Attribute │ Value │
52 ├────────────────────────────────────────────┼───────────────┼─────────┤
53 │ffs(), ffsl(), ffsll() │ Thread safety │ MT-Safe │
54 └────────────────────────────────────────────┴───────────────┴─────────┘
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57 ffs(): POSIX.1-2001, POSIX.1-2008, 4.3BSD.
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59 The ffsl() and ffsll() functions are glibc extensions.
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62 BSD systems have a prototype in <string.h>.
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65 memchr(3)
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68 This page is part of release 5.13 of the Linux man-pages project. A
69 description of the project, information about reporting bugs, and the
70 latest version of this page, can be found at
71 https://www.kernel.org/doc/man-pages/.
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75GNU 2021-03-22 FFS(3)