这里用strlen(str2) - strlen(str1)的值来判断字符串长度大小。明明str2只有三个字符，但是结果显示的是str2>str1。这里就是因为strlen返回的值都是size_t(无符号整形)，两个无符号整形相减得到的结果也是无符号整形，结果本应该是-3，却因为是无符号整形改变了它的值，变成一个无穷大的数。这就是为什么输出结果是str2 > str1.简单的解决方法是在前面强制类型转换成int即可。

模拟实现

strcpy

char* strcpy(char * destination, const char * source)

Copies the C string pointed by source into the array pointed by destination, including the terminating null character(and stopping at that point)

源字符串必须以'\0'结束。

会将源字符串中的'\0'拷贝到目标空间。

目标空间必须足够大，以确保能存放源字符串。

目标空间必须可变。

模拟实现

strcat

char * strcat ( char * destination, const char * source)

Appends a copy of the source string to the destination string. The terminating null character in destination is overwritten by the first character of source, and a null-character is included at the end of the new string formed by the concatenation of both in destination.

源字符串必须以'\0'结束。

目标空间必须足够大，能容纳下源字符串的内容。

目标空间必须可修改。

模拟实现

strcmp

int strcmp( const char * str1,const char * str2)

This function starts comparing the first character of each string. If they r equal to each other, it continues with the following pairs until the characters differ or until a terminating null-character is reached.

标准规定：

第一个字符串大于达尔戈字符串，则返回大于0的数字。

第一个字符串等于第二个字符串，则返回0

第一个字符串小于第二个字符串，则返回小于0的数字

模拟实现

strncpy

char * strncpy( char * destination, const char * source, size_t num)

Copies the first num characters of source to destination. If the end of the source C string (which is signaled by a null-character) is found before num characters have been copied, destination is padded with zeros until a total of num characters have been written to it.

拷贝num个字符从源字符串到目标空间

如果源字符串的长度小于num，则拷贝完源字符串之后，在目标后面追加‘\0’，直到num个。

strncat

char * strncat( char * destination, const char * source, size_t num)

Appends the first num characters of source to destination, plus a terminating null-character.

追加完毕后后在末尾添加'\0'

If the length of the C string in source is less than num, only the content up to the terminating null-character is copied.

如果num大于source string长度，不会额外追加。

strncmp

int strncmp( const char *string1, const char *string2, size_t count)

比较到出现另个字符不一样或者一个字符串结束或者num个字符全部比较完。

大体与strcmp相似，也是在末尾添加一个num，限制了比较数量。

strstr

char * strstr( const char * string, const char *strCharSet)

Return Value

Each of these functions returns a pointer to the first occurrence of strCharSet in string, or NULL if strCharSet does not appear in string.If strCharSet points to a string of zero length, the function returns string.

返回的是在string中首次出现strCharSet字符串的地址。如果没有在string中找到strCharSet字符串，返回NULL。或者strCharSet字符串的长度为0，函数返回string。

模拟实现

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