man3/strcmp.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\" and Copyright 2020 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" %%%LICENSE_START(VERBATIM)
   5 .\" Permission is granted to make and distribute verbatim copies of this
   6 .\" manual provided the copyright notice and this permission notice are
   7 .\" preserved on all copies.
   8 .\"
   9 .\" Permission is granted to copy and distribute modified versions of this
  10 .\" manual under the conditions for verbatim copying, provided that the
  11 .\" entire resulting derived work is distributed under the terms of a
  12 .\" permission notice identical to this one.
  13 .\"
  14 .\" Since the Linux kernel and libraries are constantly changing, this
  15 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  16 .\" responsibility for errors or omissions, or for damages resulting from
  17 .\" the use of the information contained herein.  The author(s) may not
  18 .\" have taken the same level of care in the production of this manual,
  19 .\" which is licensed free of charge, as they might when working
  20 .\" professionally.
  21 .\"
  22 .\" Formatted or processed versions of this manual, if unaccompanied by
  23 .\" the source, must acknowledge the copyright and authors of this work.
  24 .\" %%%LICENSE_END
  25 .\"
  26 .\" References consulted:
  27 .\"     Linux libc source code
  28 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  29 .\"     386BSD man pages
  30 .\" Modified Sat Jul 24 18:08:52 1993 by Rik Faith (faith@cs.unc.edu)
  31 .\" Modified 2001-08-31, aeb
  32 .\"
  33 .TH STRCMP 3  2020-04-11 "" "Linux Programmer's Manual"
  34 .SH NAME
  35 strcmp, strncmp \- compare two strings
  36 .SH SYNOPSIS
  37 .nf
  38 .B #include <string.h>
  39 .PP
  40 .BI "int strcmp(const char *" s1 ", const char *" s2 );
  41 .PP
  42 .BI "int strncmp(const char *" s1 ", const char *" s2 ", size_t " n );
  43 .fi
  44 .SH DESCRIPTION
  45 The
  46 .BR strcmp ()
  47 function compares the two strings
  48 .I s1
  49 and
  50 .IR s2 .
  51 The locale is not taken into account (for a locale-aware comparison, see
  52 .BR strcoll (3)).
  53 The comparison is done using unsigned characters.
  54 .PP
  55 .BR strcmp ()
  56 returns an integer indicating the result of the comparison, as follows:
  57 .IP \(bu 2
  58 0, if the
  59 .I s1
  60 and
  61 .I s2
  62 are equal;
  63 .IP \(bu
  64 a negative value if
  65 .I s1
  66 is less than
  67 .IR s2 ;
  68 .IP \(bu
  69 a positive value if
  70 .I s1
  71 is greater than
  72 .IR s2 ;
  73 .PP
  74 The
  75 .BR strncmp ()
  76 function is similar, except it compares
  77 only the first (at most)
  78 .IR n
  79 bytes of
  80 .I s1
  81 and
  82 .IR s2 .
  83 .SH RETURN VALUE
  84 The
  85 .BR strcmp ()
  86 and
  87 .BR strncmp ()
  88 functions return an integer
  89 less than, equal to, or greater than zero if
  90 .I s1
  91 (or the first
  92 .I n
  93 bytes thereof) is found, respectively, to be less than, to
  94 match, or be greater than
  95 .IR s2 .
  96 .SH ATTRIBUTES
  97 For an explanation of the terms used in this section, see
  98 .BR attributes (7).
  99 .TS
 100 allbox;
 101 lbw19 lb lb
 102 l l l.
 103 Interface       Attribute       Value
 104 T{
 105 .BR strcmp (),
 106 .BR strncmp ()
 107 T}      Thread safety   MT-Safe
 108 .TE
 109 .SH CONFORMING TO
 110 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
 111 .SH NOTES
 112 POSIX.1 specifies only that:
 113 .RS
 114 .PP
 115 The sign of a non-zero return value shall be determined by the sign
 116 of the difference between the values of the first pair of bytes
 117 (both interpreted as type
 118 .IR "unsigned char" )
 119 that differ in the strings being compared.
 120 .RE
 121 .PP
 122 In glibc, as in most other implementations,
 123 the return value is the arithmetic result of subtracting
 124 the last compared byte in
 125 .I s2
 126 from the last compared byte in
 127 .IR s1 .
 128 (If the two characters are equal, this difference is 0.)
 129 .SH EXAMPLES
 130 The program below can be used to demonstrate the operation of
 131 .BR strcmp ()
 132 (when given two arguments) and
 133 .BR strncmp ()
 134 (when given three arguments).
 135 First, some examples using
 136 .BR strcmp ():
 137 .PP
 138 .in +4n
 139 .EX
 140 $ \fB./string_comp ABC ABC\fP
 141 <str1> and <str2> are equal
 142 $ \fB./string_comp ABC AB\fP      # \(aqC\(aq is ASCII 67; \(aqC\(aq \- \(aq\0\(aq = 67
 143 <str1> is greater than <str2> (67)
 144 $ \fB./string_comp ABA ABZ\fP     # \(aqA\(aq is ASCII 65; \(aqZ\(aq is ASCII 90
 145 <str1> is less than <str2> (\-25)
 146 $ \fB./string_comp ABJ ABC\fP
 147 <str1> is greater than <str2> (7)
 148 $ .\fB/string_comp $\(aq\e201\(aq A\fP   # 0201 \- 0101 = 0100 (or 64 decimal)
 149 <str1> is greater than <str2> (64)
 150 .EE
 151 .in
 152 .PP
 153 The last example uses
 154 .BR bash (1)-specific
 155 syntax to produce a string containing an 8-bit ASCII code;
 156 the result demonstrates that the string comparison uses unsigned
 157 characters.
 158 .PP
 159 And then some examples using
 160 .BR strncmp ():
 161 .PP
 162 .in +4n
 163 .EX
 164 $ \fB./string_comp ABC AB 3\fP
 165 <str1> is greater than <str2> (67)
 166 $ \fB./string_comp ABC AB 2\fP
 167 <str1> and <str2> are equal in the first 2 bytes
 168 .EE
 169 .in
 170 .SS Program source
 171 \&
 172 .EX
 173 /* string_comp.c
 174
 175    Licensed under GNU General Public License v2 or later.
 176 */
 177 #include <stdio.h>
 178 #include <stdlib.h>
 179 #include <string.h>
 180
 181 int
 182 main(int argc, char *argv[])
 183 {
 184     int res;
 185
 186     if (argc < 3) {
 187         fprintf(stderr, "Usage: %s <str1> <str2> [<len>]\en", argv[0]);
 188         exit(EXIT_FAILURE);
 189     }
 190
 191     if (argc == 3)
 192         res = strcmp(argv[1], argv[2]);
 193     else
 194         res = strncmp(argv[1], argv[2], atoi(argv[3]));
 195
 196     if (res == 0) {
 197         printf("<str1> and <str2> are equal");
 198         if (argc > 3)
 199             printf(" in the first %d characters\en", atoi(argv[3]));
 200         printf("\en");
 201     } else if (res < 0) {
 202         printf("<str1> is less than <str2> (%d)\en", res);
 203     } else {
 204         printf("<str1> is greater than <str2> (%d)\en", res);
 205     }
 206
 207     exit(EXIT_SUCCESS);
 208 }
 209 .EE
 210 .SH SEE ALSO
 211 .BR bcmp (3),
 212 .BR memcmp (3),
 213 .BR strcasecmp (3),
 214 .BR strcoll (3),
 215 .BR string (3),
 216 .BR strncasecmp (3),
 217 .BR strverscmp (3),
 218 .BR wcscmp (3),
 219 .BR wcsncmp (3),
 220 .BR ascii (7)