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1 | .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) |
2 | .\" | |
5fbde956 | 3 | .\" SPDX-License-Identifier: Linux-man-pages-copyleft |
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4 | .\" |
5 | .\" References consulted: | |
6 | .\" Linux libc source code | |
7 | .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) | |
8 | .\" 386BSD man pages | |
9 | .\" Modified Sat Jul 24 18:55:27 1993 by Rik Faith (faith@cs.unc.edu) | |
45186a5d | 10 | .TH MEMCMP 3 2021-03-22 "Linux man-pages (unreleased)" |
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11 | .SH NAME |
12 | memcmp \- compare memory areas | |
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13 | .SH LIBRARY |
14 | Standard C library | |
8fc3b2cf | 15 | .RI ( libc ", " \-lc ) |
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16 | .SH SYNOPSIS |
17 | .nf | |
18 | .B #include <string.h> | |
68e4db0a | 19 | .PP |
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20 | .BI "int memcmp(const void *" s1 ", const void *" s2 ", size_t " n ); |
21 | .fi | |
22 | .SH DESCRIPTION | |
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23 | The |
24 | .BR memcmp () | |
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25 | function compares the first \fIn\fP bytes (each interpreted as |
26 | .IR "unsigned char" ) | |
27 | of the memory areas \fIs1\fP and \fIs2\fP. | |
47297adb | 28 | .SH RETURN VALUE |
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29 | The |
30 | .BR memcmp () | |
31 | function returns an integer less than, equal to, or | |
c13182ef | 32 | greater than zero if the first \fIn\fP bytes of \fIs1\fP is found, |
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33 | respectively, to be less than, to match, or be greater than the first |
34 | \fIn\fP bytes of \fIs2\fP. | |
847e0d88 | 35 | .PP |
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36 | For a nonzero return value, the sign is determined by the sign of |
37 | the difference between the first pair of bytes (interpreted as | |
38 | .IR "unsigned char" ) | |
39 | that differ in | |
40 | .I s1 | |
41 | and | |
42 | .IR s2 . | |
847e0d88 | 43 | .PP |
7f139a45 MH |
44 | If |
45 | .I n | |
46 | is zero, the return value is zero. | |
c4313c2c | 47 | .SH ATTRIBUTES |
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48 | For an explanation of the terms used in this section, see |
49 | .BR attributes (7). | |
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50 | .ad l |
51 | .nh | |
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52 | .TS |
53 | allbox; | |
c466875e | 54 | lbx lb lb |
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55 | l l l. |
56 | Interface Attribute Value | |
57 | T{ | |
c4313c2c | 58 | .BR memcmp () |
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59 | T} Thread safety MT-Safe |
60 | .TE | |
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61 | .hy |
62 | .ad | |
63 | .sp 1 | |
3113c7f3 | 64 | .SH STANDARDS |
62accc7c | 65 | POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD. |
f70fe9d7 | 66 | .SH NOTES |
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67 | Do not use |
68 | .BR memcmp () | |
69 | to compare security critical data, such as cryptographic secrets, | |
03417139 | 70 | because the required CPU time depends on the number of equal bytes. |
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71 | Instead, a function that performs comparisons in constant time is required. |
72 | Some operating systems provide such a function (e.g., NetBSD's | |
8d476c39 | 73 | .BR consttime_memequal ()), |
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74 | but no such function is specified in POSIX. |
75 | On Linux, it may be necessary to implement such a function oneself. | |
47297adb | 76 | .SH SEE ALSO |
fea681da | 77 | .BR bcmp (3), |
879091c9 | 78 | .BR bstring (3), |
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79 | .BR strcasecmp (3), |
80 | .BR strcmp (3), | |
81 | .BR strcoll (3), | |
82 | .BR strncasecmp (3), | |
83 | .BR strncmp (3), | |
84 | .BR wmemcmp (3) |