blocks|key|1766144|text|看看strings.h中的strcasecmp()。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1766145|entityMap|0|LINK|mutability|MUTABLE|url|https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_72/rtref/strcasecmp.htm^0|2|9|D|C|D|C|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@$9|S|A|T|1|U]]|E|$]]|$1|F|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

Take a look at <a href="https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_72/rtref/strcasecmp.htm" rel="noreferrer"><code>strcasecmp()</code></a> in <code>strings.h</code>.

blocks|key|3876779|text|我会使用stricmp()。它比较两个字符串而不考虑大小写。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3876780|请注意，在某些情况下，将字符串转换为小写可能会更快。|3876781|entityMap^0|4|9|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|N|8|@]|D|@]|E|$]]]|I|$]]

I would use <code>stricmp()</code>. It compares two strings without regard to case.

Note that, in some cases, converting the string to lower case can be faster.

blocks|key|3876801|text|在C标准中没有这样做的函数。符合POSIX的Unix系统要求在头strings.h中有strcasecmp；微软系统有stricmp。为了便于移植，可以编写自己的代码：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3876802|int+strcicmp(char+const+*a,+char+const+*b)
{
++++for+(;;+a%2B%2B,+b%2B%2B)+{
++++++++int+d+=+tolower((unsigned+char)*a)+-+tolower((unsigned+char)*b);
++++++++if+(d+!=+0+%7C%7C+!*a)
++++++++++++return+d;
++++}
}|code-block|syntax|javascript|3876803|但请注意，这些解决方案都不能处理UTF-8字符串，只能处理ASCII字符串。|3876804|entityMap|0|LINK|mutability|MUTABLE|url|http://pubs.opengroup.org/onlinepubs/9699919799/functions/strcasecmp.html^0|W|9|17|A|1N|7|17|A|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|F|3|G|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|15|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

There is no function that does this in the C standard. Unix systems that comply with POSIX are required to have <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strcasecmp.html" rel="noreferrer"><code>strcasecmp</code></a> in the header <code>strings.h</code>; Microsoft systems have <code>stricmp</code>. To be on the portable side, write your own:

<pre><code>int strcicmp(char const *a, char const *b)
{
 for (;; a++, b++) {
 int d = tolower((unsigned char)*a) - tolower((unsigned char)*b);
 if (d != 0 || !*a)
 return d;
 }
}
</code></pre>

But note that none of these solutions will work with UTF-8 strings, only ASCII ones.

blocks|key|5293891|text|如果库中没有，你可以从here得到一个想法，如何实现一个有效的想法|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|5293892|它使用一个表来存储全部256个字符。|5293893|该表中除字母外的所有字符都使用其ascii代码。|5293894|。用于大写字母代码的|unordered-list-item|5293895|-该表列出了小写符号的代码。|5293896|5293897|然后，我们只需要遍历一个字符串并比较给定字符的表格单元格：|5293898|const+char+*cm+=+charmap,
++++++++*us1+=+(const+char+*)s1,
++++++++*us2+=+(const+char+*)s2;
while+(cm[*us1]+==+cm[*us2%2B%2B])
++++if+(*us1%2B%2B+==+'\0')
++++++++return+(0);
return+(cm[*us1]+-+cm[*--us2]);|code-block|syntax|javascript|5293899|entityMap|0|LINK|mutability|MUTABLE|url|http://www.opensource.apple.com/source/Libc/Libc-167/string.subproj/strcasecmp.c^0|B|4|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@$A|13|B|14|1|15]]|C|$]]|$1|D|3|E|5|6|7|16|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|17|8|@]|9|@]|C|$]]|$1|H|3|I|5|J|7|18|8|@]|9|@]|C|$]]|$1|K|3|L|5|J|7|19|8|@]|9|@]|C|$]]|$1|M|3|-4|5|6|7|1A|8|@]|9|@]|C|$]]|$1|N|3|O|5|6|7|1B|8|@]|9|@]|C|$]]|$1|P|3|Q|5|R|7|1C|8|@]|9|@]|C|$S|T]]|$1|U|3|-4|5|6|7|1D|8|@]|9|@]|C|$]]]|V|$W|$5|X|Y|Z|C|$10|11]]]]

You can get an idea, how to implement an efficient one, if you don't have any in the library, from <a href="http://www.opensource.apple.com/source/Libc/Libc-167/string.subproj/strcasecmp.c" rel="nofollow">here</a>

It use a table for all 256 chars. 

<ul>
<li>in that table for all chars, except letters - used its ascii codes. </li>
<li>for upper case letter codes - the table list codes of lower cased symbols. </li>
</ul>

then we just need to traverse a strings and compare our table cells for a given chars:

<pre><code>const char *cm = charmap,
 *us1 = (const char *)s1,
 *us2 = (const char *)s2;
while (cm[*us1] == cm[*us2++])
 if (*us1++ == '\0')
 return (0);
return (cm[*us1] - cm[*--us2]);
</code></pre>

blocks|key|3876868|text|我发现了名为from的内置方法，它在标准头文件中包含额外的字符串函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3876869|下面是相关的签名：|3876870|int++strcasecmp(const+char+*,+const+char+*);
int++strncasecmp(const+char+*,+const+char+*,+size_t);|code-block|syntax|javascript|3876871|我还在xnu内核中找到了它的同义词(osfmk/device/subrs.c)，并且在下面的代码中实现了它，因此与原始的strcmp函数相比，您不会期望在数量上有任何行为变化。|3876872|tolower(unsigned+char+ch)+{
++++if+(ch+>=+'A'+&&+ch+<=+'Z')
++++++++ch+=+'a'+%2B+(ch+-+'A');
++++return+ch;
+}

int+strcasecmp(const+char+*s1,+const+char+*s2)+{
++++const+unsigned+char+*us1+=+(const+u_char+*)s1,
++++++++++++++++++++++++*us2+=+(const+u_char+*)s2;

++++while+(tolower(*us1)+==+tolower(*us2%2B%2B))
++++++++if+(*us1%2B%2B+==+'\0')
++++++++++++return+(0);
++++return+(tolower(*us1)+-+tolower(*--us2));
}|3876873|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I've found built-in such method named from which contains additional string functions to the standard header . 

Here's the relevant signatures : 

<pre><code>int strcasecmp(const char *, const char *);
int strncasecmp(const char *, const char *, size_t);
</code></pre>

I also found it's synonym in xnu kernel (osfmk/device/subrs.c) and it's implemented in the following code, so you wouldn't expect to have any change of behavior in number compared to the original strcmp function. 

<pre><code>tolower(unsigned char ch) {
 if (ch &gt;= 'A' &amp;&amp; ch &lt;= 'Z')
 ch = 'a' + (ch - 'A');
 return ch;
 }

int strcasecmp(const char *s1, const char *s2) {
 const unsigned char *us1 = (const u_char *)s1,
 *us2 = (const u_char *)s2;

 while (tolower(*us1) == tolower(*us2++))
 if (*us1++ == '\0')
 return (0);
 return (tolower(*us1) - tolower(*--us2));
}
</code></pre>

blocks|key|1766288|text|int+strcmpInsensitive(char*+a,+char*+b)
{
++++return+strcmp(lowerCaseWord(a),+lowerCaseWord(b));
}

char*+lowerCaseWord(char*+a)
{
++++char+*b=new+char[strlen(a)];
++++for+(int+i+=+0;+i+<+strlen(a);+i%2B%2B)
++++{
++++++++b[i]+=+tolower(a[i]);+++
++++}
++++return+b;
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1766289|祝好运|unstyled|1766290|Edit-lowerCaseWord函数使用获取一个char*变量，并返回该char*的小写数值。例如，"abcde“表示字符*的值，将返回"AbCdE”。|1766291|基本上，它所做的是将两个char*变量转换为小写后，并对其使用strcmp函数。|1766292|例如，如果我们为"AbCdE“和"ABCDE”的值调用strcmp函数，它将首先返回两个小写的值("abcde")，然后对它们执行strcmp函数。|1766293|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|P|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|F|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|F|7|T|8|@]|9|@]|A|$]]]|N|$]]

<pre><code>int strcmpInsensitive(char* a, char* b)
{
 return strcmp(lowerCaseWord(a), lowerCaseWord(b));
}

char* lowerCaseWord(char* a)
{
 char *b=new char[strlen(a)];
 for (int i = 0; i &lt; strlen(a); i++)
 {
 b[i] = tolower(a[i]); 
 }
 return b;
}
</code></pre>

good luck

Edit-lowerCaseWord function get a char* variable with, and return the lower case value of this char*. For example "AbCdE" for value of char*, will return "abcde".

Basically what it does is to take the two char* variables, after being transferred to lower case, and make use the strcmp function on them.

For example- if we call the strcmpInsensitive function for values of "AbCdE", and "ABCDE", it will first return both values in lower case ("abcde"), and then do strcmp function on them.

blocks|key|1501412|text|static+int+ignoreCaseComp+(const+char+*str1,+const+char+*str2,+int+length)
{
++++int+k;
++++for+(k+=+0;+k+<+length;+k%2B%2B)
++++{

++++++++if+((str1[k]+%7C+32)+!=+(str2[k]+%7C+32))
++++++++++++break;
++++}

++++if+(k+!=+length)
++++++++return+1;
++++return+0;
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1501413|Reference|unstyled|offset|length|1501414|entityMap|0|LINK|mutability|MUTABLE|url|https://chromium.googlesource.com/external/liblouis/%2B/master/liblouis/wrappers.c^0|0|0|9|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|R|8|@]|9|@$G|S|H|T|1|U]]|A|$]]|$1|I|3|-4|5|F|7|V|8|@]|9|@]|A|$]]]|J|$K|$5|L|M|N|A|$O|P]]]]

<pre><code>static int ignoreCaseComp (const char *str1, const char *str2, int length)
{
 int k;
 for (k = 0; k &lt; length; k++)
 {

 if ((str1[k] | 32) != (str2[k] | 32))
 break;
 }

 if (k != length)
 return 1;
 return 0;
}
</code></pre>

<a href="https://chromium.googlesource.com/external/liblouis/+/master/liblouis/wrappers.c" rel="nofollow">Reference</a>

blocks|key|5294007|text|执行不区分大小写的比较时需要注意的其他陷阱：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5294008|5294009|比较是小写还是大写？(足够常见的问题)|offset|length|style|BOLD|5294010|使用strcicmpL("A",+"a")和strcicmpU("A",+"a")，下面的两个函数都将返回0。|CODE|5294011|然而，strcicmpL("A",+"_")和strcicmpU("A",+"_")可以返回不同的带符号结果，因为'_'通常在大小写字母之间。|5294012|这会影响与qsort(...,+...,+...,+strcicmp)一起使用时的排序顺序。非标准库C函数，如常用的stricmp()或strcasecmp()，往往定义良好，并倾向于通过小写进行比较。然而，变化是存在的。|5294013|int+strcicmpL(char+const+*a,+char+const+*b)+{
++while+(*b)+{
++++int+d+=+tolower(*a)+-+tolower(*b);
++++if+(d)+{
++++++++return+d;
++++}+
++++a%2B%2B;
++++b%2B%2B;
++}+
++return+tolower(*a);
}

int+strcicmpU(char+const+*a,+char+const+*b)+{
++while+(*b)+{
++++int+d+=+toupper(*a)+-+toupper(*b);
++++if+(d)+{
++++++++return+d;
++++}+
++++a%2B%2B;
++++b%2B%2B;
++}+
++return+toupper(*a);
}|code-block|syntax|javascript|5294014|5294015|char+可以为负值。(并非罕见)|5294016|为unsigned+char值和负EOF指定touppper(int)和tolower(int)。此外，strcmp()会返回结果，就好像每个char都被转换为unsigned+char一样，而不管char是有符号的还是无符号的。|5294017|tolower(*a);+//+Potential+UB
tolower((unsigned+char)+*a);+//+Correct+(Almost+-+see+following)|5294018|5294019|char+可以是负值，但不能是2的补码。(罕见)|5294020|由于位模式应解释为unsigned+char，因此上述代码不能正确处理-0或其他负值。要正确处理所有整数编码，请首先更改指针类型。|5294021|//+tolower((unsigned+char)+*a);
tolower(*(const+unsigned+char+*)a);+//+Correct|5294022|5294023|语言环境(不太常见)|5294024|尽管使用ASCII代码(0-127)的字符集非常普遍，但其余的代码往往存在特定于区域设置的问题。因此，strcasecmp("\xE4",+"a")可能在一个系统上返回0，而在另一个系统上返回非零值。|5294025|5294026|Unicode+(未来之路)|5294027|如果解决方案需要处理的不仅仅是ASCII，那么可以考虑使用unicode_strcicmp()。由于C+lib不提供这样的函数，因此推荐使用某个备用库中的预编码函数。编写自己的unicode_strcicmp()是一项艰巨的任务。|5294028|5294029|所有的字母都是从下到上映射的吗？(书面语)|5294030|a-z与A-Z一对一映射，而不同的语言环境将各种小写字符映射为一个大写字符，反之亦然。此外，一些大写字符可能缺少小写等效项，反之亦然。|5294031|这使得代码必须同时通过tolower()和tolower()进行转换。|5294032|int+d+=+tolower(toupper(*a))+-+tolower(toupper(*b));|5294033|同样，如果代码tolower(toupper(*a))与toupper(tolower(*a))相比，可能会产生不同的排序结果。|5294034|5294035|可移植性|5294036|@B.+Nadolson建议避免使用自己的strcicmp()，这是合理的，除非代码需要高度等效的可移植功能。|5294037|下面是一种甚至比某些系统提供的功能执行得更快的方法。它通过使用与'\0'不同的两个不同的表，对每个循环执行一次比较，而不是两次比较。您的结果可能有所不同。|5294038|static+unsigned+char+low1[UCHAR_MAX+%2B+1]+=+{
++0,+1,+2,+3,+...
++'@',+'a',+'b',+'c',+...+'z',+`[`,+...++//+@ABC...+Z[...
++'`',+'a',+'b',+'c',+...+'z',+`{`,+...++//+`abc...+z{...
}
static+unsigned+char+low2[UCHAR_MAX+%2B+1]+=+{
//+v---+Not+zero,+but+A+which+matches+none+in+`low1[]`
++'A',+1,+2,+3,+...
++'@',+'a',+'b',+'c',+...+'z',+`[`,+...
++'`',+'a',+'b',+'c',+...+'z',+`{`,+...
}

int+strcicmp_ch(char+const+*a,+char+const+*b)+{
++//+compare+using+tables+that+differ+slightly.
++while+(low1[*(const+unsigned+char+*)a]+==+low2[*(const+unsigned+char+*)b])+{
++++a%2B%2B;
++++b%2B%2B;
++}
++//+Either+strings+differ+or+null+character+detected.
++//+Perform+subtraction+using+same+table.
++return+(low1[*(const+unsigned+char+*)a]+-+low1[*(const+unsigned+char+*)b]);
}|5294039|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/5820810/case-insensitive-string-comp-in-c#comment45276351_5820991^0|0|0|0|J|0|2|J|M|J|0|3|J|N|J|1L|3|0|5|U|1M|9|1W|C|0|0|0|0|4|5|C|0|4|0|1|D|H|3|M|D|10|C|1G|8|1Z|4|28|D|2R|4|0|0|0|0|4|5|J|0|4|0|9|D|Z|2|0|0|0|0|A|0|1F|N|0|0|0|E|0|T|I|2G|I|0|0|0|L|0|0|B|9|L|9|0|0|7|K|S|K|0|0|0|4|0|L|A|0|C|0|0|W|4|0|0^^$0|@$1|2|3|4|5|6|7|2A|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|2B|8|@]|9|@]|A|$]]|$1|C|3|D|5|6|7|2C|8|@$E|2D|F|2E|G|H]]|9|@]|A|$]]|$1|I|3|J|5|6|7|2F|8|@$E|2G|F|2H|G|K]|$E|2I|F|2J|G|K]]|9|@]|A|$]]|$1|L|3|M|5|6|7|2K|8|@$E|2L|F|2M|G|K]|$E|2N|F|2O|G|K]|$E|2P|F|2Q|G|K]]|9|@]|A|$]]|$1|N|3|O|5|6|7|2R|8|@$E|2S|F|2T|G|K]|$E|2U|F|2V|G|K]|$E|2W|F|2X|G|K]]|9|@]|A|$]]|$1|P|3|Q|5|R|7|2Y|8|@]|9|@]|A|$S|T]]|$1|U|3|-4|5|6|7|2Z|8|@]|9|@]|A|$]]|$1|V|3|W|5|6|7|30|8|@$E|31|F|32|G|H]|$E|33|F|34|G|H]|$E|35|F|36|G|K]]|9|@]|A|$]]|$1|X|3|Y|5|6|7|37|8|@$E|38|F|39|G|K]|$E|3A|F|3B|G|K]|$E|3C|F|3D|G|K]|$E|3E|F|3F|G|K]|$E|3G|F|3H|G|K]|$E|3I|F|3J|G|K]|$E|3K|F|3L|G|K]|$E|3M|F|3N|G|K]]|9|@]|A|$]]|$1|Z|3|10|5|R|7|3O|8|@]|9|@]|A|$S|T]]|$1|11|3|-4|5|6|7|3P|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|3Q|8|@$E|3R|F|3S|G|H]|$E|3T|F|3U|G|H]|$E|3V|F|3W|G|K]]|9|@]|A|$]]|$1|14|3|15|5|6|7|3X|8|@$E|3Y|F|3Z|G|K]|$E|40|F|41|G|K]]|9|@]|A|$]]|$1|16|3|17|5|R|7|42|8|@]|9|@]|A|$S|T]]|$1|18|3|-4|5|6|7|43|8|@]|9|@]|A|$]]|$1|19|3|1A|5|6|7|44|8|@$E|45|F|46|G|H]]|9|@]|A|$]]|$1|1B|3|1C|5|6|7|47|8|@$E|48|F|49|G|K]]|9|@]|A|$]]|$1|1D|3|-4|5|6|7|4A|8|@]|9|@]|A|$]]|$1|1E|3|1F|5|6|7|4B|8|@$E|4C|F|4D|G|H]]|9|@]|A|$]]|$1|1G|3|1H|5|6|7|4E|8|@$E|4F|F|4G|G|K]|$E|4H|F|4I|G|K]]|9|@]|A|$]]|$1|1I|3|-4|5|6|7|4J|8|@]|9|@]|A|$]]|$1|1J|3|1K|5|6|7|4K|8|@$E|4L|F|4M|G|H]]|9|@]|A|$]]|$1|1L|3|1M|5|6|7|4N|8|@]|9|@]|A|$]]|$1|1N|3|1O|5|6|7|4O|8|@$E|4P|F|4Q|G|K]|$E|4R|F|4S|G|K]]|9|@]|A|$]]|$1|1P|3|1Q|5|R|7|4T|8|@]|9|@]|A|$S|T]]|$1|1R|3|1S|5|6|7|4U|8|@$E|4V|F|4W|G|K]|$E|4X|F|4Y|G|K]]|9|@]|A|$]]|$1|1T|3|-4|5|6|7|4Z|8|@]|9|@]|A|$]]|$1|1U|3|1V|5|6|7|50|8|@$E|51|F|52|G|H]]|9|@]|A|$]]|$1|1W|3|1X|5|6|7|53|8|@$E|54|F|55|G|K]]|9|@$E|56|F|57|1|58]]|A|$]]|$1|1Y|3|1Z|5|6|7|59|8|@$E|5A|F|5B|G|K]]|9|@]|A|$]]|$1|20|3|21|5|R|7|5C|8|@]|9|@]|A|$S|T]]|$1|22|3|-4|5|6|7|5D|8|@]|9|@]|A|$]]]|23|$24|$5|25|26|27|A|$28|29]]]]

<h1>Additional pitfalls to watch out for when doing case insensitive compares:</h1>
<hr />
Comparing as lower or as upper case? (common enough issue)
Both below will return 0 with <code>strcicmpL(&quot;A&quot;, &quot;a&quot;)</code> and <code>strcicmpU(&quot;A&quot;, &quot;a&quot;)</code>. 
Yet <code>strcicmpL(&quot;A&quot;, &quot;_&quot;)</code> and <code>strcicmpU(&quot;A&quot;, &quot;_&quot;)</code> can return different signed results as <code>'_'</code> is often between the upper and lower case letters.
This affects the sort order when used with <code>qsort(..., ..., ..., strcicmp)</code>. Non-standard library C functions like the commonly available <code>stricmp()</code> or <code>strcasecmp()</code> tend to be well defined and favor comparing via lowercase. Yet variations exist.
<pre><code>int strcicmpL(char const *a, char const *b) {
 while (*b) {
 int d = tolower(*a) - tolower(*b);
 if (d) {
 return d;
 } 
 a++;
 b++;
 } 
 return tolower(*a);
}

int strcicmpU(char const *a, char const *b) {
 while (*b) {
 int d = toupper(*a) - toupper(*b);
 if (d) {
 return d;
 } 
 a++;
 b++;
 } 
 return toupper(*a);
}
</code></pre>
<hr />
<code>char</code> can have a negative value. (not rare)
<code>touppper(int)</code> and <code>tolower(int)</code> are specified for <code>unsigned char</code> values and the negative <code>EOF</code>. Further, <code>strcmp()</code> returns results as if each <code>char</code> was converted to <code>unsigned char</code>, regardless if <code>char</code> is signed or unsigned.
<pre><code>tolower(*a); // Potential UB
tolower((unsigned char) *a); // Correct (Almost - see following)
</code></pre>
<hr />
<code>char</code> can have a negative value and not 2's complement. (rare)
The above does not handle <code>-0</code> nor other negative values properly as the bit pattern should be interpreted as <code>unsigned char</code>. To properly handle all integer encodings, change the pointer type first.
<pre><code>// tolower((unsigned char) *a);
tolower(*(const unsigned char *)a); // Correct
</code></pre>
<hr />
Locale (less common)
Although character sets using ASCII code (0-127) are ubiquitous, the remainder codes tend to have locale specific issues. So <code>strcasecmp(&quot;\xE4&quot;, &quot;a&quot;)</code> might return a 0 on one system and non-zero on another.
<hr />
Unicode (the way of the future)
If a solution needs to handle more than ASCII consider a <code>unicode_strcicmp()</code>. As C lib does not provide such a function, a pre-coded function from some alternate library is recommended. Writing your own <code>unicode_strcicmp()</code> is a daunting task.
<hr />
Do all letters map one lower to one upper? (pedantic)
[A-Z] maps one-to-one with [a-z], yet various locales map various lower case chracters to one upper and visa-versa. Further, some uppercase characters may lack a lower case equivalent and again, visa-versa.
This obliges code to covert through both <code>tolower()</code> and <code>tolower()</code>.
<pre><code>int d = tolower(toupper(*a)) - tolower(toupper(*b));
</code></pre>
Again, potential different results for sorting if code did <code>tolower(toupper(*a))</code> vs. <code>toupper(tolower(*a))</code>.
<hr />
Portability
<a href="https://stackoverflow.com/questions/5820810/case-insensitive-string-comp-in-c#comment45276351_5820991">@B. Nadolson</a> recommends to avoid rolling your own <code>strcicmp()</code> and this is reasonable, except when code needs high equivalent portable functionality.
Below is an approach that even performed faster than some system provided functions. It does a single compare per loop rather than two by using 2 different tables that differ with <code>'\0'</code>. Your results may vary.
<pre><code>static unsigned char low1[UCHAR_MAX + 1] = {
 0, 1, 2, 3, ...
 '@', 'a', 'b', 'c', ... 'z', `[`, ... // @ABC... Z[...
 '`', 'a', 'b', 'c', ... 'z', `{`, ... // `abc... z{...
}
static unsigned char low2[UCHAR_MAX + 1] = {
// v--- Not zero, but A which matches none in `low1[]`
 'A', 1, 2, 3, ...
 '@', 'a', 'b', 'c', ... 'z', `[`, ...
 '`', 'a', 'b', 'c', ... 'z', `{`, ...
}

int strcicmp_ch(char const *a, char const *b) {
 // compare using tables that differ slightly.
 while (low1[*(const unsigned char *)a] == low2[*(const unsigned char *)b]) {
 a++;
 b++;
 }
 // Either strings differ or null character detected.
 // Perform subtraction using same table.
 return (low1[*(const unsigned char *)a] - low1[*(const unsigned char *)b]);
}
</code></pre>

blocks|key|1501465|text|我并不是most-upvoted+answer+here的粉丝(部分原因是它似乎不正确，因为如果它在任一字符串中读取空终止符--但不是同时读取两个字符串中的空终止符--它就应该执行continue操作--而且它不会这样做)，所以我编写了自己的代码。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1501466|这是对strncmp()的直接替代，并且已经用大量测试用例进行了测试，如下所示。|1501467|它与strncmp()相同，除了：|1501468|如果任一字符串为case-insensitive.|1501469|The，则|unordered-list-item|1501470|It+is+null行为不是未定义的(它是明确定义的)。如果任一字符串为null+ptr，则常规strncmp()具有未定义的行为(请参阅:如果任一输入字符串为NULL+ptr，则INT_MIN将作为特殊标记错误值返回。|ordered-list-item|1501471|1501472|1501473|限制:请注意，此代码适用于原始+(十进制值0到127，包括0和127)，而不适用于字符，例如unicode字符编码+(最常用)、和。|1501474|这里的只是代码(没有注释)：|BOLD|1501475|int+strncmpci(const+char+*+str1,+const+char+*+str2,+size_t+num)
{
++++int+ret_code+=+0;
++++size_t+chars_compared+=+0;

++++if+(!str1+%7C%7C+!str2)
++++{
++++++++ret_code+=+INT_MIN;
++++++++return+ret_code;
++++}

++++while+((chars_compared+<+num)+&&+(*str1+%7C%7C+*str2))
++++{
++++++++ret_code+=+tolower((int)(*str1))+-+tolower((int)(*str2));
++++++++if+(ret_code+!=+0)
++++++++{
++++++++++++break;
++++++++}
++++++++chars_compared%2B%2B;
++++++++str1%2B%2B;
++++++++str2%2B%2B;
++++}

++++return+ret_code;
}|code-block|syntax|javascript|1501476|全注释版本：|1501477|///+\brief++++++Perform+a+case-insensitive+string+compare+(`strncmp()`+case-insensitive)+to+see
///+++++++++++++if+two+C-strings+are+equal.
///+\note+++++++1.+Identical+to+`strncmp()`+except:
///+++++++++++++++1.+It+is+case-insensitive.
///+++++++++++++++2.+The+behavior+is+NOT+undefined+(it+is+well-defined)+if+either+string+is+a+null
///+++++++++++++++ptr.+Regular+`strncmp()`+has+undefined+behavior+if+either+string+is+a+null+ptr
///+++++++++++++++(see:+https://en.cppreference.com/w/cpp/string/byte/strncmp).
///+++++++++++++++3.+It+returns+`INT_MIN`+as+a+special+sentinel+value+for+certain+errors.
///+++++++++++++-+Posted+as+an+answer+here:+https://stackoverflow.com/a/55293507/4561887.
///+++++++++++++++-+Aided/inspired,+in+part,+by+`strcicmp()`+here:
///+++++++++++++++++https://stackoverflow.com/a/5820991/4561887.
///+\param[in]++str1++++++++C+string+1+to+be+compared.
///+\param[in]++str2++++++++C+string+2+to+be+compared.
///+\param[in]++num+++++++++max+number+of+chars+to+compare
///+\return+++++A+comparison+code+(identical+to+`strncmp()`,+except+with+the+addition
///+++++++++++++of+`INT_MIN`+as+a+special+sentinel+value):
///
///+++++++++++++INT_MIN+(usually+-2147483648+for+int32_t+integers)++Invalid+arguments+(one+or+both
///++++++++++++++++++++++of+the+input+strings+is+a+NULL+pointer).
///+++++++++++++<0+++++++The+first+character+that+does+not+match+has+a+lower+value+in+str1+than
///++++++++++++++++++++++in+str2.
///++++++++++++++0+++++++The+contents+of+both+strings+are+equal.
///+++++++++++++>0+++++++The+first+character+that+does+not+match+has+a+greater+value+in+str1+than
///++++++++++++++++++++++in+str2.
int+strncmpci(const+char+*+str1,+const+char+*+str2,+size_t+num)
{
++++int+ret_code+=+0;
++++size_t+chars_compared+=+0;

++++//+Check+for+NULL+pointers
++++if+(!str1+%7C%7C+!str2)
++++{
++++++++ret_code+=+INT_MIN;
++++++++return+ret_code;
++++}

++++//+Continue+doing+case-insensitive+comparisons,+one-character-at-a-time,+of+`str1`+to+`str2`,+so
++++//+long+as+1st:+we+have+not+yet+compared+the+requested+number+of+chars,+and+2nd:+the+next+char
++++//+of+at+least+*one*+of+the+strings+is+not+zero+(the+null+terminator+for+a+C-string),+meaning
++++//+that+string+still+has+more+characters+in+it.
++++//+Note:+you+MUST+check+`(chars_compared+<+num)`+FIRST+or+else+dereferencing+(reading)+`str1`+or
++++//+`str2`+via+`*str1`+and+`*str2`,+respectively,+is+undefined+behavior+if+you+are+reading+one+or
++++//+both+of+these+C-strings+outside+of+their+array+bounds.
++++while+((chars_compared+<+num)+&&+(*str1+%7C%7C+*str2))
++++{
++++++++ret_code+=+tolower((int)(*str1))+-+tolower((int)(*str2));
++++++++if+(ret_code+!=+0)
++++++++{
++++++++++++//+The+2+chars+just+compared+don't+match
++++++++++++break;
++++++++}
++++++++chars_compared%2B%2B;
++++++++str1%2B%2B;
++++++++str2%2B%2B;
++++}

++++return+ret_code;
}|1501478|测试代码：|1501479|从我的eRCaGuy_hello_world存储库下载包含单元测试的完整示例代码："strncmpci.c"|1501480|(这只是一小段)|1501481|int+main()
{
++++printf("-----------------------\n"
+++++++++++"String+Comparison+Tests\n"
+++++++++++"-----------------------\n\n");

++++int+num_failures_expected+=+0;

++++printf("INTENTIONAL+UNIT+TEST+FAILURE+to+show+what+a+unit+test+failure+looks+like!\n");
++++EXPECT_EQUALS(strncmpci("hey",+"HEY",+3),+'h'+-+'H');
++++num_failures_expected%2B%2B;
++++printf("------+beginning+------\n\n");


++++const+char+*+str1;
++++const+char+*+str2;
++++size_t+n;

++++//+NULL+ptr+checks
++++EXPECT_EQUALS(strncmpci(NULL,+"",+0),+INT_MIN);
++++EXPECT_EQUALS(strncmpci("",+NULL,+0),+INT_MIN);
++++EXPECT_EQUALS(strncmpci(NULL,+NULL,+0),+INT_MIN);
++++EXPECT_EQUALS(strncmpci(NULL,+"",+10),+INT_MIN);
++++EXPECT_EQUALS(strncmpci("",+NULL,+10),+INT_MIN);
++++EXPECT_EQUALS(strncmpci(NULL,+NULL,+10),+INT_MIN);

++++EXPECT_EQUALS(strncmpci("",+"",+0),+0);
++++EXPECT_EQUALS(strncmp("",+"",+0),+0);

++++str1+=+"";
++++str2+=+"";
++++n+=+0;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+0);
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+0);

++++str1+=+"hey";
++++str2+=+"HEY";
++++n+=+0;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+0);
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+0);

++++str1+=+"hey";
++++str2+=+"HEY";
++++n+=+3;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+0);
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+'h'+-+'H');

++++str1+=+"heY";
++++str2+=+"HeY";
++++n+=+3;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+0);
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+'h'+-+'H');

++++str1+=+"hey";
++++str2+=+"HEdY";
++++n+=+3;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+'y'+-+'d');
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+'h'+-+'H');

++++str1+=+"heY";
++++str2+=+"hEYd";
++++n+=+3;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+0);
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+'e'+-+'E');

++++str1+=+"heY";
++++str2+=+"heyd";
++++n+=+6;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+-'d');
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+'Y'+-+'y');

++++str1+=+"hey";
++++str2+=+"hey";
++++n+=+6;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+0);
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+0);

++++str1+=+"hey";
++++str2+=+"heyd";
++++n+=+6;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+-'d');
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+-'d');

++++str1+=+"hey";
++++str2+=+"heyd";
++++n+=+3;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+0);
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+0);

++++str1+=+"hEY";
++++str2+=+"heyYOU";
++++n+=+3;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+0);
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+'E'+-+'e');

++++str1+=+"hEY";
++++str2+=+"heyYOU";
++++n+=+10;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+-'y');
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+'E'+-+'e');

++++str1+=+"hEYHowAre";
++++str2+=+"heyYOU";
++++n+=+10;
++++EXPECT_EQUALS(strncmpci(str1,+str2,+n),+'h'+-+'y');
++++EXPECT_EQUALS(strncmp(str1,+str2,+n),+'E'+-+'e');

++++EXPECT_EQUALS(strncmpci("nice+to+meet+you.,;",+"NICE+TO+MEET+YOU.,;",+100),+0);
++++EXPECT_EQUALS(strncmp(++"nice+to+meet+you.,;",+"NICE+TO+MEET+YOU.,;",+100),+'n'+-+'N');
++++EXPECT_EQUALS(strncmp(++"nice+to+meet+you.,;",+"nice+to+meet+you.,;",+100),+0);

++++EXPECT_EQUALS(strncmpci("nice+to+meet+you.,;",+"NICE+TO+UEET+YOU.,;",+100),+'m'+-+'u');
++++EXPECT_EQUALS(strncmp(++"nice+to+meet+you.,;",+"nice+to+uEET+YOU.,;",+100),+'m'+-+'u');
++++EXPECT_EQUALS(strncmp(++"nice+to+meet+you.,;",+"nice+to+UEET+YOU.,;",+100),+'m'+-+'U');

++++EXPECT_EQUALS(strncmpci("nice+to+meet+you.,;",+"NICE+TO+MEET+YOU.,;",+5),+0);
++++EXPECT_EQUALS(strncmp(++"nice+to+meet+you.,;",+"NICE+TO+MEET+YOU.,;",+5),+'n'+-+'N');

++++EXPECT_EQUALS(strncmpci("nice+to+meet+you.,;",+"NICE+eo+UEET+YOU.,;",+5),+0);
++++EXPECT_EQUALS(strncmp(++"nice+to+meet+you.,;",+"nice+eo+uEET+YOU.,;",+5),+0);

++++EXPECT_EQUALS(strncmpci("nice+to+meet+you.,;",+"NICE+eo+UEET+YOU.,;",+100),+'t'+-+'e');
++++EXPECT_EQUALS(strncmp(++"nice+to+meet+you.,;",+"nice+eo+uEET+YOU.,;",+100),+'t'+-+'e');

++++EXPECT_EQUALS(strncmpci("nice+to+meet+you.,;",+"nice-eo+UEET+YOU.,;",+5),+'+'+-+'-');
++++EXPECT_EQUALS(strncmp(++"nice+to+meet+you.,;",+"nice-eo+UEET+YOU.,;",+5),+'+'+-+'-');


++++if+(globals.error_count+==+num_failures_expected)
++++{
++++++++printf(ANSI_COLOR_GRN+"All+unit+tests+passed!"+ANSI_COLOR_OFF+"\n");
++++}
++++else
++++{
++++++++printf(ANSI_COLOR_RED+"FAILED+UNIT+TESTS!+NUMBER+OF+UNEXPECTED+FAILURES+=+%25i"
++++++++++++ANSI_COLOR_OFF+"\n",+globals.error_count+-+num_failures_expected);
++++}

++++assert(globals.error_count+==+num_failures_expected);
++++return+globals.error_count;
}|1501482|示例输出：|1501483|1501484|+$+-Wall+-Wextra+-Werror+-ggdb+-std=c11+-o+./bin/tmp+strncmpci.c+&&+./bin/tmp在函数main的第250行失败！strncmpci(“嘿”，“嘿”，3)+!=+'H‘-+'h’a:+strncmpci(“嘿”，“嘿”，3)+is+0+b：'H‘-+'h’is+32+-开始-所有单元测试通过！|blockquote|1501485|1501486|1501487|参考文献：|1501488|这个问题和这里的其他答案给了我灵感，并给出了一些洞察力和行为研究，我做了一些研究来修复上面的部分代码(参见下面的评论)：|1501489|Google+search+|1501490|+"c+undefined+behavior+reading+outside+array+bounds"|1501491|Is+accessing+a+global+array+outside+its+bound+undefined+behavior?|1501492|https://en.cppreference.com/w/cpp/language/ub+-也可以看到底部的许多非常棒的“外部链接”！|1501493|1/3：http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html|1501494|2/3：https://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html|1501495|3/3：https://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html|1501496|https://blog.regehr.org/archives/213|1501497|https://www.geeksforgeeks.org/accessing-array-bounds-ccpp/|1501498|1501499|需要进一步研究的课题|1501500|1501501|(注意:这是C%2B%2B，不是C)+OnlineGDB上的Lowercase+of+Unicode+character|1501502|tolower_tests.c：https://onlinegdb.com/HyZieXcew|1501503|1501504|TODO：|1501505|1501506|制作此代码的一个版本，它也适用于Unicode的UTF-8实现(字符编码)！|1501507|1501508|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/5820991/4561887|1|http://www.cplusplus.com/reference/cstring/strncmp/|2|https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world|3|https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world/blob/master/c/strncmpci.c|4|https://www.google.com/search?q=c%2Bundefined%2Bbehavior%2Breading%2Boutside%2Barray%2Bbounds&oq=c%2Bundefined%2Bbehavior%2Breading%2Boutside%2Barray%2Bbounds&aqs=chrome..69i57j69i64l3.10484j0j7&sourceid=chrome&ie=UTF-8|5|https://stackoverflow.com/questions/26426910/is-accessing-a-global-array-outside-its-bound-undefined-behavior|6|https://en.cppreference.com/w/cpp/language/ub|7|http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html|8|https://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html|9|https://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html|10|11|12|https://stackoverflow.com/questions/34433380/lowercase-of-unicode-character|13|https://onlinegdb.com/HyZieXcew|14|https://en.wikipedia.org/wiki/UTF-8^0|2I|8|4|O|0|0|3|9|3|9|1|0|2|9|0|0|0|1C|9|28|4|2I|7|0|0|0|0|3|B|0|0|0|6|0|0|0|3|J|2|16|C|3|0|0|0|0|0|0|0|0|0|0|0|1|1F|4|0|0|1T|5|0|0|19|6|0|4|1X|7|0|4|21|8|0|4|21|9|0|0|10|A|0|0|1M|B|0|0|0|0|Q|U|C|0|0|F|G|V|D|0|0|0|0|O|5|E|0|0^^$0|@$1|2|3|4|5|6|7|3M|8|@$9|3N|A|3O|B|C]]|D|@$9|3P|A|3Q|1|3R]]|E|$]]|$1|F|3|G|5|6|7|3S|8|@$9|3T|A|3U|B|C]]|D|@$9|3V|A|3W|1|3X]]|E|$]]|$1|H|3|I|5|6|7|3Y|8|@$9|3Z|A|40|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|41|8|@]|D|@]|E|$]]|$1|L|3|M|5|N|7|42|8|@]|D|@]|E|$]]|$1|O|3|P|5|Q|7|43|8|@$9|44|A|45|B|C]|$9|46|A|47|B|C]|$9|48|A|49|B|C]]|D|@]|E|$]]|$1|R|3|-4|5|6|7|4A|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|4B|8|@]|D|@]|E|$]]|$1|T|3|U|5|6|7|4C|8|@]|D|@]|E|$]]|$1|V|3|W|5|6|7|4D|8|@$9|4E|A|4F|B|X]]|D|@]|E|$]]|$1|Y|3|Z|5|10|7|4G|8|@]|D|@]|E|$11|12]]|$1|13|3|14|5|6|7|4H|8|@$9|4I|A|4J|B|X]]|D|@]|E|$]]|$1|15|3|16|5|10|7|4K|8|@]|D|@]|E|$11|12]]|$1|17|3|18|5|6|7|4L|8|@]|D|@]|E|$]]|$1|19|3|1A|5|6|7|4M|8|@]|D|@$9|4N|A|4O|1|4P]|$9|4Q|A|4R|1|4S]]|E|$]]|$1|1B|3|1C|5|6|7|4T|8|@]|D|@]|E|$]]|$1|1D|3|1E|5|10|7|4U|8|@]|D|@]|E|$11|12]]|$1|1F|3|1G|5|6|7|4V|8|@]|D|@]|E|$]]|$1|1H|3|-4|5|6|7|4W|8|@]|D|@]|E|$]]|$1|1I|3|1J|5|1K|7|4X|8|@]|D|@]|E|$]]|$1|1L|3|-4|5|6|7|4Y|8|@]|D|@]|E|$]]|$1|1M|3|-4|5|6|7|4Z|8|@]|D|@]|E|$]]|$1|1N|3|1O|5|6|7|50|8|@]|D|@]|E|$]]|$1|1P|3|1Q|5|6|7|51|8|@]|D|@]|E|$]]|$1|1R|3|1S|5|Q|7|52|8|@]|D|@]|E|$]]|$1|1T|3|1U|5|Q|7|53|8|@]|D|@$9|54|A|55|1|56]]|E|$]]|$1|1V|3|1W|5|Q|7|57|8|@]|D|@$9|58|A|59|1|5A]]|E|$]]|$1|1X|3|1Y|5|Q|7|5B|8|@]|D|@$9|5C|A|5D|1|5E]]|E|$]]|$1|1Z|3|20|5|Q|7|5F|8|@]|D|@$9|5G|A|5H|1|5I]]|E|$]]|$1|21|3|22|5|Q|7|5J|8|@]|D|@$9|5K|A|5L|1|5M]]|E|$]]|$1|23|3|24|5|Q|7|5N|8|@]|D|@$9|5O|A|5P|1|5Q]]|E|$]]|$1|25|3|26|5|Q|7|5R|8|@]|D|@$9|5S|A|5T|1|5U]]|E|$]]|$1|27|3|28|5|Q|7|5V|8|@]|D|@$9|5W|A|5X|1|5Y]]|E|$]]|$1|29|3|-4|5|6|7|5Z|8|@]|D|@]|E|$]]|$1|2A|3|2B|5|6|7|60|8|@]|D|@]|E|$]]|$1|2C|3|-4|5|6|7|61|8|@]|D|@]|E|$]]|$1|2D|3|2E|5|Q|7|62|8|@]|D|@$9|63|A|64|1|65]]|E|$]]|$1|2F|3|2G|5|Q|7|66|8|@$9|67|A|68|B|X]]|D|@$9|69|A|6A|1|6B]]|E|$]]|$1|2H|3|-4|5|6|7|6C|8|@]|D|@]|E|$]]|$1|2I|3|2J|5|6|7|6D|8|@]|D|@]|E|$]]|$1|2K|3|-4|5|6|7|6E|8|@]|D|@]|E|$]]|$1|2L|3|2M|5|Q|7|6F|8|@]|D|@$9|6G|A|6H|1|6I]]|E|$]]|$1|2N|3|-4|5|6|7|6J|8|@]|D|@]|E|$]]|$1|2O|3|-4|5|6|7|6K|8|@]|D|@]|E|$]]]|2P|$2Q|$5|2R|2S|2T|E|$2U|2V]]|2W|$5|2R|2S|2T|E|$2U|2X]]|2Y|$5|2R|2S|2T|E|$2U|2Z]]|30|$5|2R|2S|2T|E|$2U|31]]|32|$5|2R|2S|2T|E|$2U|33]]|34|$5|2R|2S|2T|E|$2U|35]]|36|$5|2R|2S|2T|E|$2U|37]]|38|$5|2R|2S|2T|E|$2U|39]]|3A|$5|2R|2S|2T|E|$2U|3B]]|3C|$5|2R|2S|2T|E|$2U|3D]]|3E|$5|2R|2S|2T|E|$2U|26]]|3F|$5|2R|2S|2T|E|$2U|28]]|3G|$5|2R|2S|2T|E|$2U|3H]]|3I|$5|2R|2S|2T|E|$2U|3J]]|3K|$5|2R|2S|2T|E|$2U|3L]]]]

I'm not really a fan of the <a href="https://stackoverflow.com/a/5820991/4561887">most-upvoted answer here</a> (in part because it seems like it isn't correct since it should <code>continue</code> if it reads a null terminator in either string--but not both strings at once--and it doesn't do this), so I wrote my own.
<h2>This is a direct drop-in replacement for <a href="http://www.cplusplus.com/reference/cstring/strncmp/" rel="nofollow noreferrer"><code>strncmp()</code></a>, and has been tested with numerous test cases, as shown below.</h2>
It is identical to <code>strncmp()</code> except:
<ol>
<li>It is case-insensitive.</li>
<li>The behavior is NOT undefined (it is well-defined) if either string is a null ptr. Regular <code>strncmp()</code> has undefined behavior if either string is a null ptr (see: <a href="https://en.cppreference.com/w/cpp/string/byte/strncmp" rel="nofollow noreferrer">https://en.cppreference.com/w/cpp/string/byte/strncmp</a>).</li>
<li>It returns <code>INT_MIN</code> as a special sentinel error value if either input string is a <code>NULL</code> ptr.</li>
</ol>
LIMITATIONS: Note that this code works on the original <a href="https://en.wikipedia.org/wiki/ASCII#Printable_characters" rel="nofollow noreferrer">7-bit ASCII character set only</a> (decimal values 0 to 127, inclusive), NOT on <a href="https://en.wikipedia.org/wiki/Unicode" rel="nofollow noreferrer">unicode</a> characters, such as unicode character encodings <a href="https://en.wikipedia.org/wiki/UTF-8" rel="nofollow noreferrer">UTF-8</a> (the most popular), <a href="https://en.wikipedia.org/wiki/UTF-16" rel="nofollow noreferrer">UTF-16</a>, and <a href="https://en.wikipedia.org/wiki/UTF-32" rel="nofollow noreferrer">UTF-32</a>.
Here is the code only (no comments):
<pre><code>int strncmpci(const char * str1, const char * str2, size_t num)
{
 int ret_code = 0;
 size_t chars_compared = 0;

 if (!str1 || !str2)
 {
 ret_code = INT_MIN;
 return ret_code;
 }

 while ((chars_compared &lt; num) &amp;&amp; (*str1 || *str2))
 {
 ret_code = tolower((int)(*str1)) - tolower((int)(*str2));
 if (ret_code != 0)
 {
 break;
 }
 chars_compared++;
 str1++;
 str2++;
 }

 return ret_code;
}
</code></pre>
Fully-commented version:
<pre><code>/// \brief Perform a case-insensitive string compare (`strncmp()` case-insensitive) to see
/// if two C-strings are equal.
/// \note 1. Identical to `strncmp()` except:
/// 1. It is case-insensitive.
/// 2. The behavior is NOT undefined (it is well-defined) if either string is a null
/// ptr. Regular `strncmp()` has undefined behavior if either string is a null ptr
/// (see: https://en.cppreference.com/w/cpp/string/byte/strncmp).
/// 3. It returns `INT_MIN` as a special sentinel value for certain errors.
/// - Posted as an answer here: https://stackoverflow.com/a/55293507/4561887.
/// - Aided/inspired, in part, by `strcicmp()` here:
/// https://stackoverflow.com/a/5820991/4561887.
/// \param[in] str1 C string 1 to be compared.
/// \param[in] str2 C string 2 to be compared.
/// \param[in] num max number of chars to compare
/// \return A comparison code (identical to `strncmp()`, except with the addition
/// of `INT_MIN` as a special sentinel value):
///
/// INT_MIN (usually -2147483648 for int32_t integers) Invalid arguments (one or both
/// of the input strings is a NULL pointer).
/// &lt;0 The first character that does not match has a lower value in str1 than
/// in str2.
/// 0 The contents of both strings are equal.
/// &gt;0 The first character that does not match has a greater value in str1 than
/// in str2.
int strncmpci(const char * str1, const char * str2, size_t num)
{
 int ret_code = 0;
 size_t chars_compared = 0;

 // Check for NULL pointers
 if (!str1 || !str2)
 {
 ret_code = INT_MIN;
 return ret_code;
 }

 // Continue doing case-insensitive comparisons, one-character-at-a-time, of `str1` to `str2`, so
 // long as 1st: we have not yet compared the requested number of chars, and 2nd: the next char
 // of at least *one* of the strings is not zero (the null terminator for a C-string), meaning
 // that string still has more characters in it.
 // Note: you MUST check `(chars_compared &lt; num)` FIRST or else dereferencing (reading) `str1` or
 // `str2` via `*str1` and `*str2`, respectively, is undefined behavior if you are reading one or
 // both of these C-strings outside of their array bounds.
 while ((chars_compared &lt; num) &amp;&amp; (*str1 || *str2))
 {
 ret_code = tolower((int)(*str1)) - tolower((int)(*str2));
 if (ret_code != 0)
 {
 // The 2 chars just compared don't match
 break;
 }
 chars_compared++;
 str1++;
 str2++;
 }

 return ret_code;
}
</code></pre>
<h2>Test code:</h2>
Download the entire sample code, with unit tests, from my <a href="https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world" rel="nofollow noreferrer">eRCaGuy_hello_world</a> repository here: &quot;<a href="https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world/blob/master/c/strncmpci.c" rel="nofollow noreferrer">strncmpci.c&quot;</a>:
(this is just a snippet)
<pre><code>int main()
{
 printf(&quot;-----------------------\n&quot;
 &quot;String Comparison Tests\n&quot;
 &quot;-----------------------\n\n&quot;);

 int num_failures_expected = 0;

 printf(&quot;INTENTIONAL UNIT TEST FAILURE to show what a unit test failure looks like!\n&quot;);
 EXPECT_EQUALS(strncmpci(&quot;hey&quot;, &quot;HEY&quot;, 3), 'h' - 'H');
 num_failures_expected++;
 printf(&quot;------ beginning ------\n\n&quot;);


 const char * str1;
 const char * str2;
 size_t n;

 // NULL ptr checks
 EXPECT_EQUALS(strncmpci(NULL, &quot;&quot;, 0), INT_MIN);
 EXPECT_EQUALS(strncmpci(&quot;&quot;, NULL, 0), INT_MIN);
 EXPECT_EQUALS(strncmpci(NULL, NULL, 0), INT_MIN);
 EXPECT_EQUALS(strncmpci(NULL, &quot;&quot;, 10), INT_MIN);
 EXPECT_EQUALS(strncmpci(&quot;&quot;, NULL, 10), INT_MIN);
 EXPECT_EQUALS(strncmpci(NULL, NULL, 10), INT_MIN);

 EXPECT_EQUALS(strncmpci(&quot;&quot;, &quot;&quot;, 0), 0);
 EXPECT_EQUALS(strncmp(&quot;&quot;, &quot;&quot;, 0), 0);

 str1 = &quot;&quot;;
 str2 = &quot;&quot;;
 n = 0;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 0);
 EXPECT_EQUALS(strncmp(str1, str2, n), 0);

 str1 = &quot;hey&quot;;
 str2 = &quot;HEY&quot;;
 n = 0;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 0);
 EXPECT_EQUALS(strncmp(str1, str2, n), 0);

 str1 = &quot;hey&quot;;
 str2 = &quot;HEY&quot;;
 n = 3;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 0);
 EXPECT_EQUALS(strncmp(str1, str2, n), 'h' - 'H');

 str1 = &quot;heY&quot;;
 str2 = &quot;HeY&quot;;
 n = 3;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 0);
 EXPECT_EQUALS(strncmp(str1, str2, n), 'h' - 'H');

 str1 = &quot;hey&quot;;
 str2 = &quot;HEdY&quot;;
 n = 3;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 'y' - 'd');
 EXPECT_EQUALS(strncmp(str1, str2, n), 'h' - 'H');

 str1 = &quot;heY&quot;;
 str2 = &quot;hEYd&quot;;
 n = 3;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 0);
 EXPECT_EQUALS(strncmp(str1, str2, n), 'e' - 'E');

 str1 = &quot;heY&quot;;
 str2 = &quot;heyd&quot;;
 n = 6;
 EXPECT_EQUALS(strncmpci(str1, str2, n), -'d');
 EXPECT_EQUALS(strncmp(str1, str2, n), 'Y' - 'y');

 str1 = &quot;hey&quot;;
 str2 = &quot;hey&quot;;
 n = 6;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 0);
 EXPECT_EQUALS(strncmp(str1, str2, n), 0);

 str1 = &quot;hey&quot;;
 str2 = &quot;heyd&quot;;
 n = 6;
 EXPECT_EQUALS(strncmpci(str1, str2, n), -'d');
 EXPECT_EQUALS(strncmp(str1, str2, n), -'d');

 str1 = &quot;hey&quot;;
 str2 = &quot;heyd&quot;;
 n = 3;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 0);
 EXPECT_EQUALS(strncmp(str1, str2, n), 0);

 str1 = &quot;hEY&quot;;
 str2 = &quot;heyYOU&quot;;
 n = 3;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 0);
 EXPECT_EQUALS(strncmp(str1, str2, n), 'E' - 'e');

 str1 = &quot;hEY&quot;;
 str2 = &quot;heyYOU&quot;;
 n = 10;
 EXPECT_EQUALS(strncmpci(str1, str2, n), -'y');
 EXPECT_EQUALS(strncmp(str1, str2, n), 'E' - 'e');

 str1 = &quot;hEYHowAre&quot;;
 str2 = &quot;heyYOU&quot;;
 n = 10;
 EXPECT_EQUALS(strncmpci(str1, str2, n), 'h' - 'y');
 EXPECT_EQUALS(strncmp(str1, str2, n), 'E' - 'e');

 EXPECT_EQUALS(strncmpci(&quot;nice to meet you.,;&quot;, &quot;NICE TO MEET YOU.,;&quot;, 100), 0);
 EXPECT_EQUALS(strncmp( &quot;nice to meet you.,;&quot;, &quot;NICE TO MEET YOU.,;&quot;, 100), 'n' - 'N');
 EXPECT_EQUALS(strncmp( &quot;nice to meet you.,;&quot;, &quot;nice to meet you.,;&quot;, 100), 0);

 EXPECT_EQUALS(strncmpci(&quot;nice to meet you.,;&quot;, &quot;NICE TO UEET YOU.,;&quot;, 100), 'm' - 'u');
 EXPECT_EQUALS(strncmp( &quot;nice to meet you.,;&quot;, &quot;nice to uEET YOU.,;&quot;, 100), 'm' - 'u');
 EXPECT_EQUALS(strncmp( &quot;nice to meet you.,;&quot;, &quot;nice to UEET YOU.,;&quot;, 100), 'm' - 'U');

 EXPECT_EQUALS(strncmpci(&quot;nice to meet you.,;&quot;, &quot;NICE TO MEET YOU.,;&quot;, 5), 0);
 EXPECT_EQUALS(strncmp( &quot;nice to meet you.,;&quot;, &quot;NICE TO MEET YOU.,;&quot;, 5), 'n' - 'N');

 EXPECT_EQUALS(strncmpci(&quot;nice to meet you.,;&quot;, &quot;NICE eo UEET YOU.,;&quot;, 5), 0);
 EXPECT_EQUALS(strncmp( &quot;nice to meet you.,;&quot;, &quot;nice eo uEET YOU.,;&quot;, 5), 0);

 EXPECT_EQUALS(strncmpci(&quot;nice to meet you.,;&quot;, &quot;NICE eo UEET YOU.,;&quot;, 100), 't' - 'e');
 EXPECT_EQUALS(strncmp( &quot;nice to meet you.,;&quot;, &quot;nice eo uEET YOU.,;&quot;, 100), 't' - 'e');

 EXPECT_EQUALS(strncmpci(&quot;nice to meet you.,;&quot;, &quot;nice-eo UEET YOU.,;&quot;, 5), ' ' - '-');
 EXPECT_EQUALS(strncmp( &quot;nice to meet you.,;&quot;, &quot;nice-eo UEET YOU.,;&quot;, 5), ' ' - '-');


 if (globals.error_count == num_failures_expected)
 {
 printf(ANSI_COLOR_GRN &quot;All unit tests passed!&quot; ANSI_COLOR_OFF &quot;\n&quot;);
 }
 else
 {
 printf(ANSI_COLOR_RED &quot;FAILED UNIT TESTS! NUMBER OF UNEXPECTED FAILURES = %i&quot;
 ANSI_COLOR_OFF &quot;\n&quot;, globals.error_count - num_failures_expected);
 }

 assert(globals.error_count == num_failures_expected);
 return globals.error_count;
}
</code></pre>
<h2>Sample output:</h2>
<blockquote>
<pre><code>$ gcc -Wall -Wextra -Werror -ggdb -std=c11 -o ./bin/tmp strncmpci.c &amp;&amp; ./bin/tmp
-----------------------
String Comparison Tests
-----------------------

INTENTIONAL UNIT TEST FAILURE to show what a unit test failure looks like!
FAILED at line 250 in function main! strncmpci(&quot;hey&quot;, &quot;HEY&quot;, 3) != 'h' - 'H'
 a: strncmpci(&quot;hey&quot;, &quot;HEY&quot;, 3) is 0
 b: 'h' - 'H' is 32

------ beginning ------

All unit tests passed!
</code></pre>
</blockquote>
<h2>References:</h2>
<ol>
<li>This question &amp; other answers here served as inspiration and gave some insight (<a href="https://stackoverflow.com/questions/5820810/case-insensitive-string-comp-in-c/">Case Insensitive String comp in C</a>)</li>
<li><a href="http://www.cplusplus.com/reference/cstring/strncmp/" rel="nofollow noreferrer">http://www.cplusplus.com/reference/cstring/strncmp/</a></li>
<li><a href="https://en.wikipedia.org/wiki/ASCII" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/ASCII</a></li>
<li><a href="https://en.cppreference.com/w/c/language/operator_precedence" rel="nofollow noreferrer">https://en.cppreference.com/w/c/language/operator_precedence</a></li>
<li>Undefined Behavior research I did to fix part of my code above (see comments below):
<ol>
<li>Google search for <a href="https://www.google.com/search?q=c+undefined+behavior+reading+outside+array+bounds&amp;oq=c+undefined+behavior+reading+outside+array+bounds&amp;aqs=chrome..69i57j69i64l3.10484j0j7&amp;sourceid=chrome&amp;ie=UTF-8" rel="nofollow noreferrer">&quot;c undefined behavior reading outside array bounds&quot;</a></li>
<li><a href="https://stackoverflow.com/questions/26426910/is-accessing-a-global-array-outside-its-bound-undefined-behavior">Is accessing a global array outside its bound undefined behavior?</a></li>
<li><a href="https://en.cppreference.com/w/cpp/language/ub" rel="nofollow noreferrer">https://en.cppreference.com/w/cpp/language/ub</a> - see also the many really great &quot;External links&quot; at the bottom!</li>
<li>1/3: <a href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html" rel="nofollow noreferrer">http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html</a></li>
<li>2/3: <a href="https://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html" rel="nofollow noreferrer">https://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html</a></li>
<li>3/3: <a href="https://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html" rel="nofollow noreferrer">https://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html</a></li>
<li><a href="https://blog.regehr.org/archives/213" rel="nofollow noreferrer">https://blog.regehr.org/archives/213</a></li>
<li><a href="https://www.geeksforgeeks.org/accessing-array-bounds-ccpp/" rel="nofollow noreferrer">https://www.geeksforgeeks.org/accessing-array-bounds-ccpp/</a></li>
</ol>
</li>
</ol>
<h2>Topics to further research</h2>
<ol>
<li>(Note: this is C++, not C) <a href="https://stackoverflow.com/questions/34433380/lowercase-of-unicode-character">Lowercase of Unicode character</a></li>
<li>tolower_tests.c on OnlineGDB: <a href="https://onlinegdb.com/HyZieXcew" rel="nofollow noreferrer">https://onlinegdb.com/HyZieXcew</a></li>
</ol>
<h2>TODO:</h2>
<ol>
<li>Make a version of this code which also works on Unicode's <a href="https://en.wikipedia.org/wiki/UTF-8" rel="nofollow noreferrer">UTF-8</a> implementation (character encoding)!</li>
</ol>

blocks|key|3877002|text|正如其他人所说，没有可移植的功能可以在所有系统上工作。您可以使用简单的ifdef部分地绕过这一点|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3877003|#include+<stdio.h>

#ifdef+_WIN32
#include+<string.h>
#define+strcasecmp+_stricmp
#else+//+assuming+POSIX+or+BSD+compliant+system
#include+<strings.h>
#endif

int+main()+{
++++printf("%25d",+strcasecmp("teSt",+"TEst"));
}|code-block|syntax|javascript|3877004|entityMap^0|Z|5|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

As others have stated, there is no portable function that works on all systems. You can partially circumvent this with simple <code>ifdef</code>:

<pre><code>#include &lt;stdio.h&gt;

#ifdef _WIN32
#include &lt;string.h&gt;
#define strcasecmp _stricmp
#else // assuming POSIX or BSD compliant system
#include &lt;strings.h&gt;
#endif

int main() {
 printf("%d", strcasecmp("teSt", "TEst"));
}
</code></pre>

blocks|key|3877019|text|简单的解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3877020|int+str_case_ins_cmp(const+char*+a,+const+char*+b)+{
++int+rc;

++while+(1)+{
++++rc+=+tolower((unsigned+char)*a)+-+tolower((unsigned+char)*b);
++++if+(rc+%7C%7C+!*a)+{
++++++break;
++++}

++++%2B%2Ba;
++++%2B%2Bb;
++}

++return+rc;
}|code-block|syntax|javascript|3877021|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Simple solution:

<pre><code>int str_case_ins_cmp(const char* a, const char* b) {
 int rc;

 while (1) {
 rc = tolower((unsigned char)*a) - tolower((unsigned char)*b);
 if (rc || !*a) {
 break;
 }

 ++a;
 ++b;
 }

 return rc;
}
</code></pre>

blocks|key|1766482|text|如果我们有一个null结尾的字符：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1766483|+++bool+striseq(const+char*+s1,const+char*+s2){+
+++++for(;*s1;){+
+++++++if(tolower(*s1%2B%2B)!=tolower(*s2%2B%2B))+
+++++++++return+false;+
++++++}+
++++++return+*s1+==+*s2;
++++}|code-block|syntax|javascript|1766484|或者使用这个使用位运算的版本：|1766485|++++int+striseq(const+char*+s1,const+char*+s2)
+++++++{for(;*s1;)+if((*s1%2B%2B%7C32)!=(*s2%2B%2B%7C32))+return+0;+return+*s1+==+*s2;}|1766486|我不确定这是否适用于符号，我还没有在那里测试过，但对字母很有效。|1766487|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

if we have a null terminated character:
<pre><code> bool striseq(const char* s1,const char* s2){ 
 for(;*s1;){ 
 if(tolower(*s1++)!=tolower(*s2++)) 
 return false; 
 } 
 return *s1 == *s2;
 }

</code></pre>
or with this version that uses bitwise operations:
<pre><code> int striseq(const char* s1,const char* s2)
 {for(;*s1;) if((*s1++|32)!=(*s2++|32)) return 0; return *s1 == *s2;}

</code></pre>
i'm not sure if this works with symbols, I haven't tested there, but works fine with letters.

I have two postcodes <code>char*</code> that I want to compare, ignoring case.
Is there a function to do this?
Or do I have to loop through each use the <code>tolower</code> function and then do the comparison?
Any idea how this function will react with numbers in the string
Thanks

Case Insensitive String Comparison in C

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

文章

问答

视频

学习中心

腾讯云实验室

直播

竞赛

腾讯云代码分析专区

腾讯iOA零信任安全管理系统专区

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋

腾讯云代码助手

云原生构建

TAPD 敏捷项目管理

Cloud Studio

SDK中心

API中心

命令行工具

涵盖代码开发、场景应用、自动测试全流程，助你从零构建专属AI助手

一站式MCP教程库，解锁AI应用新玩法

我有两个邮政编码char*，我想要比较，忽略大小写。有没有一个函数可以做到这一点？或者我必须遍历每个use tolower函数，然后进行比较？你知道这个函数将如何处理字符串中的数字吗？谢谢

问C中不区分大小写的字符串比较
EN

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问C中不区分大小写的字符串比较EN