Find the length of the string
1.strlen
size_t strlen（const char* str）
The string ends with '\0', and the strlen function returns the number of characters that appear before '\0' in the string (not including
Contains '\0' )
The string pointed to by the parameter must end with '\0'
Note that the return value of the function is size_t, which is unsigned
Below is the code directly (simulating the strlen function):
In this arr array, there are mainly four elements of bit \0, so we let our * str move backwards in the my_strlen function until the end of \0. This is what we are simulating the strlen function, which ends with \0 flag, count the number of elements before \0
2.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)
The source string must be terminated with '\0'.
Will copy '\0' in the source string to the destination space.
The destination space must be large enough to hold the source string
The target space must be mutable.
Here we explain: strcpy (purpose function, source function)
So in the strcpy function, we can know that \0 can be copied in the strcpy function
Error demonstration: (the target space must be large enough)
In this array, arr2 can only store 3 elements, and we have elements such as abcdefghi \0 in arr1, so the elements of arr1 cannot be placed in arr2, resulting in an error
3. broken
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。
目标空间必须有足够的大，能容纳下源字符串的内容。
目标空间必须可修改
这里我们解释一下：在这个函数中我们可以把arr2中的元素存放在arr1元素的后面，而在这个过程中，我们arr1的数组必须足够大，而且是以\0为标志才能存放到源字符串的后面
4.strcmp
int strcmp ( const char * str1, const char * str2 )
This function starts comparing the first character of each string. If they are 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：
第一个字符串大于第二个字符串，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
strcmp函数中，我们是依次比较元素的ASCII码值的大小，如果arr1中ASCII码值大于arr2中ASCII码值则打印 > 号，反之，如果相等则打印0
这里我们是a与a比较，b与b比较，z与q比较
5.strstr
char * strstr ( const char *str1, const char * str2)
Returns a pointer to the first occurrence of str2 in str1, or a null pointer if str2 is not part of
str1
strstr函数中，我们是在arr1中元素找arr2的元素，而这些元素中，我们是找第一次出现的元素，从而打印出arr1中元素的地址
以上就是我们字符函数的上篇，明天后天我会更新出下篇，喜欢的话请三连留下您的关注哦！！！