Deep parsing strings and memory functions

      Kangkang recently learned many string functions and memory functions , Among them, the basic function Kangkang doesn't say much . I want to tell you something strstr、strtok、strerror、memmove、memmset、memcmp.

   strstr Function is used to find substrings . It is used as follows ：

char*strstr(constchar*string,constchar*strCharSet); Its first argument points to a large string , The second parameter points to a small string （ The string being searched ） Its return value is the address of the large string . Now let's try it ！

    So how to simulate and realize its functions ？

char* my_strstr(const char* des, const char* src)
{

	assert(des && src);
	char* s1 = des;
	char* s2 = src;
	char* p = des;
	while (*p != '\0')
	{
		while (*s1 != *s2)
		{
			p++;
			s1 = p;
		}

		while (s1 != '\0' && s2 != '\0' && *s1 == *s2);
		{
			s1++;
			s2++;
		}
		if (*s2 == '\0')
		{
			return p;
		}
		p++;
		s2 = src;
	}
	return NULL;
}

strtok The use of functions is very strange , Its usage rules are as follows ：

char*strtok(char*str,constchar*sep );

1.sep The parameter is a string , Defines the set of characters used as separators
The first parameter specifies a string , It contains 0 One or more by sep A character separated by one or more separators in a string
remember .
2.strtok Function found str The next mark in , And use it \0 ending , Returns a pointer to the tag .（ notes ：
strtok Function changes the string being manipulated , So it's using strtok The string segmented by function is usually the content of temporary copy
And it can be modified .）
3.strtok The first argument of the function is not NULL , Function will find str The first mark in ,strtok The function will save it in the string
Position in .
strtok The first argument to the function is NULL , The function will start at the same position in the string that is saved , Find the next target
remember .
4. If there are no more tags in the string , Then return to NULL The pointer .

Now let me give you an example ：

 strerror Function is used to return the error information corresponding to the error code . The following functions are used ：

     char*strerror(interrnum);

Now let's run it ！！

  Next, let's talk about the use of character functions , First of all memmove.

String functions are only for strings , The memory function is for memory .

void*memmove(void*dest,constvoid*src,size_tcount);

Now let me give you an example ：

So how to simulate it ？ The code is as follows ：

 Simulation Implementation memmove
#include <stdio.h>

#include <assert.h>
void* my_memmove(void* dest, const void* src, size_t num)
{
	void* ret = dest;
	assert(dest && src);
	if (dest < src)
	{
		while (num--)
		{
			*(char*)dest = *(char*)src;
			dest = (char*)dest + 1;
			src = (char*)src + 1;
		}
	}
	else
	{
		while (num--)
		{
			*((char*)dest + num) = *((char*)src + num);
			
		}
	}
	return ret;

}

 memset How to use the function ？

memset Is the setting of function memory , The following functions are used ：

void*memset(void*dest,intc,size_tcount);

Here is how the code works ：

  This is the setting of memory data ！

The last function is memcmp, It's very easy to use .

It is a byte by byte comparison ,intmemcmp(constvoid*buf1,constvoid*buf2,size_tcount);

When buf1>buf2 When , Return a greater than 0 The number of .

When buf1=buf2 When , return 0.

When buf1<buf2 When , Returns a number less than zero . The following is an example for everyone ：

  These are my views on these functions , If there is any wrong , I hope you can correct me ！