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Reactos
1/***
2*mbsdec.c - Move MBCS string pointer backward one charcter.
3*
4* Copyright (c) Microsoft Corporation. All rights reserved.
5*
6*Purpose:
7* Move MBCS string pointer backward one character.
8*
9*******************************************************************************/
10#ifndef _MBCS
11 #error This file should only be compiled with _MBCS defined
12#endif
13
14#include <corecrt_internal.h>
15#include <corecrt_internal_mbstring.h>
16#include <locale.h>
17#include <stddef.h>
18
19/***
20*_mbsdec - Move MBCS string pointer backward one charcter.
21*
22*Purpose:
23* Move the supplied string pointer backwards by one
24* character. MBCS characters are handled correctly.
25*
26*Entry:
27* const unsigned char *string = pointer to beginning of string
28* const unsigned char *current = current char pointer (legal MBCS boundary)
29*
30*Exit:
31* Returns pointer after moving it.
32* Returns nullptr if string >= current.
33*
34*Exceptions:
35* Input parameters are validated. Refer to the validation section of the function.
36*
37*******************************************************************************/
38
39extern "C" unsigned char * __cdecl _mbsdec_l(
40 const unsigned char *string,
41 const unsigned char *current,
42 _locale_t plocinfo
43 )
44{
45 const unsigned char *temp;
46
47 /* validation section */
48 _VALIDATE_RETURN(string != nullptr, EINVAL, nullptr);
49 _VALIDATE_RETURN(current != nullptr, EINVAL, nullptr);
50
51 if (string >= current)
52 return(nullptr);
53
54 _LocaleUpdate _loc_update(plocinfo);
55
56 if (_loc_update.GetLocaleT()->mbcinfo->ismbcodepage == 0)
57 return (unsigned char *)--current;
58
59 temp = current - 1;
60
61/* There used to be an optimisation here:
62 *
63 * If (current-1) returns true from _ismbblead, it is a trail byte, because
64 * current is a known character start point, and so current-1 would have to be a
65 * legal single byte MBCS character, which a lead byte is not. Therefore, if so,
66 * return (current-2) because it must be the trailbyte's lead.
67 *
68 * if ( _ismbblead(*temp) )
69 * return (unsigned char *)(temp - 1);
70 *
71 * But this is not a valid optimisation if you want to cope correctly with an
72 * MBCS string which is terminated by a leadbyte and a 0 byte, when you are passed
73 * an initial position pointing to the \0 at the end of the string.
74 *
75 * This optimisation is also invalid if you are passed a pointer to half-way
76 * through an MBCS pair.
77 *
78 * Neither of these are truly valid input conditions, but to ensure predictably
79 * correct behaviour in the presence of these conditions, we have removed
80 * the optimisation.
81 */
82
83/*
84 * It is unknown whether (current - 1) is a single byte character or a
85 * trail. Now decrement temp until
86 * a) The beginning of the string is reached, or
87 * b) A non-lead byte (either single or trail) is found.
88 * The difference between (current-1) and temp is the number of non-single
89 * byte characters preceding (current-1). There are two cases for this:
90 * a) (current - temp) is odd, and
91 * b) (current - temp) is even.
92 * If odd, then there are an odd number of "lead bytes" preceding the
93 * single/trail byte (current - 1), indicating that it is a trail byte.
94 * If even, then there are an even number of "lead bytes" preceding the
95 * single/trail byte (current - 1), indicating a single byte character.
96 */
97 while ( (string <= --temp) && (_ismbblead_l(*temp, _loc_update.GetLocaleT())) )
98 ;
99
100 return (unsigned char *)(current - 1 - ((current - temp) & 0x01) );
101}
102
103extern "C" unsigned char * (__cdecl _mbsdec)(
104 const unsigned char *string,
105 const unsigned char *current
106 )
107{
108 return _mbsdec_l(string, current, nullptr);
109}