linux下strncasecmp的实现

看来strcpy的实现确实是太low了，腾讯的笔试字符串知识点考察就变了。

 

Linux下不区分大小写的字符串比较函数原型：

int strncasecmp(const char *s1, const char *s2, register size_t n);

 

返回值：

如果相等返回0

如果s1>s2，返回正整数

如果s1

 

static const unsigned char charmap[] = 

{

'\000', '\001', '\002', '\003', '\004', '\005', '\006', '\007',

'\010', '\011', '\012', '\013', '\014', '\015', '\016', '\017',

'\020', '\021', '\022', '\023', '\024', '\025', '\026', '\027',

'\030', '\031', '\032', '\033', '\034', '\035', '\036', '\037',

'\040', '\041', '\042', '\043', '\044', '\045', '\046', '\047',

'\050', '\051', '\052', '\053', '\054', '\055', '\056', '\057',

'\060', '\061', '\062', '\063', '\064', '\065', '\066', '\067',

'\070', '\071', '\072', '\073', '\074', '\075', '\076', '\077',

'\100', '\141', '\142', '\143', '\144', '\145', '\146', '\147',

'\150', '\151', '\152', '\153', '\154', '\155', '\156', '\157',

'\160', '\161', '\162', '\163', '\164', '\165', '\166', '\167',

'\170', '\171', '\172', '\133', '\134', '\135', '\136', '\137',

'\140', '\141', '\142', '\143', '\144', '\145', '\146', '\147',

'\150', '\151', '\152', '\153', '\154', '\155', '\156', '\157',

'\160', '\161', '\162', '\163', '\164', '\165', '\166', '\167',

'\170', '\171', '\172', '\173', '\174', '\175', '\176', '\177',

'\200', '\201', '\202', '\203', '\204', '\205', '\206', '\207',

'\210', '\211', '\212', '\213', '\214', '\215', '\216', '\217',

'\220', '\221', '\222', '\223', '\224', '\225', '\226', '\227',

'\230', '\231', '\232', '\233', '\234', '\235', '\236', '\237',

'\240', '\241', '\242', '\243', '\244', '\245', '\246', '\247',

'\250', '\251', '\252', '\253', '\254', '\255', '\256', '\257',

'\260', '\261', '\262', '\263', '\264', '\265', '\266', '\267',

'\270', '\271', '\272', '\273', '\274', '\275', '\276', '\277',

'\300', '\341', '\342', '\343', '\344', '\345', '\346', '\347',

'\350', '\351', '\352', '\353', '\354', '\355', '\356', '\357',

'\360', '\361', '\362', '\363', '\364', '\365', '\366', '\367',

'\370', '\371', '\372', '\333', '\334', '\335', '\336', '\337',

'\340', '\341', '\342', '\343', '\344', '\345', '\346', '\347',

'\350', '\351', '\352', '\353', '\354', '\355', '\356', '\357',

'\360', '\361', '\362', '\363', '\364', '\365', '\366', '\367',

'\370', '\371', '\372', '\373', '\374', '\375', '\376', '\377',

};

int strncasecmp(const char *s1, const char *s2, register size_t n)

{

register unsigned char u1, u2;

for (; n != 0; --n) 

{

u1 = (unsigned char) *s1++;

u2 = (unsigned char) *s2++;

if (charmap[u1] != charmap[u2]) 

{

return charmap[u1] - charmap[u2];

}

if (u1 == '\0') 

{

return 0;

}

}

return 0;

}

 

这里的charmap数组下标是对应了ASCII的十进制值，数组值则对应了ASCII的八进制值，同时只要注意到从\141开始会有重复，没错，就是A-Z去重复a-z的八进制值。这样就可以忽略大小写进行比较了。这个代码的实现是取自linux的strncasecmp.c文件。

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