Use and understanding of string functions (2)

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Use and understanding of string functions （2）

Here comes the note
After last time, there are ：
String function with limited length ： 1,strncpy 2,strncat 3,strncmp;
Memory manipulation function ：1,memcpy 2,memmove 3,memset 4,memcmp;
String search ：1,strstr 2,strtok;
Error message report ： strerror
1、 String function with limited length
strncpy The function prototype is ：

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

Glance at , Why ~, Than before strcpy The function prototype has more parameters , Not bad , In fact, relative and no length constraints , Here, you only need to set the number you want to control （ Company ： byte ）.
Let's give you an example ：

  char * my_strncpy(char *dest, const char *str,size_t count)
{
    
	assert(dest && str);  // Ensure the validity of the pointer ( This is with B Brother Peng of the station learned )
	char* ret = dest;
	while (count && (*dest++ = *str++))
	count--;
	if (count)
	while (--count)
	*dest++ = '\0';
	return ret;
}
int main()
{
    
	char arr1[10] = "hello";
	char arr2[] = "world";
// strncpy(arr1, arr2,4); // The third parameter is the number of bytes ,arr1 Is the source string ; When arr2 The specified number is much larger than the string "world" a long time , Make up automatically ‘\0’
// If the length of the source string is less than num, After copying the source string , Append... After the target 0, until num individual 
	my_strncpy(arr1, arr2,4); // Custom implementation function 
	printf("%s\n", arr1);
	return 0;
});

Compared with the previous one , In fact, they are roughly the same format , It's just , When operating a string, it is almost the same as what is implied behind the string ‘\0’ Dealing with .
I won't say much about the last two , Direct example ：
strncat String append prototype ：

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

Custom implementation function ：

char * my_strncat(char *dest, const char *str,size_t count)
{
    
 assert(dest && str);
 char* ret = dest;
 while (*dest++)
  ;
 dest--;
 while (count--)
 if(!(*dest++ = *str++))
 return ret;
 *dest = '\0';
 return ret;
}
int main()
{
    
 char arr1[20] = "hello\0abcd"; 
 char arr2[] = "world";
 //strncat(arr1, arr2,4); // The third parameter is the number of strings appended ,arr1 Is the source string ;arr2 Additional 4 A string , Not more than  "world" Length of string , Only in arr1 Of \0 After add 4 The first string is ："helloworl",
 // If the number of appendages exceeds the string arr2 The number of , Only all strings "world" Add the past （ belt \0）, No more superfluous \0
  my_strncat(arr1, arr2,4); // Custom function 
 printf("%s\n", arr1);
 return 0;
}

strncmp String comparison ,ASCII Value size comparison ;
The prototype function is

 int strncmp ( const char * str1, const char * str2, size_t num );

Find for string ：

------------------------strstr Find string -------------------------------------
// Prototype ：const char * strstr ( const char * str1, const char * str2 );
            char * strstr(char * str1, const char * str2);
// This example is in the str Mid search “simple” Substring and replace it with “sample”.
int main ()
{
    
char str[] = "This is a simple string";
char * pch;
pch = strstr(str, "simple");
if (pch != NULL)
strncpy(pch, "sample", 6);
printf("%s\n",str);   // Output  This is a sample string
return 0;
}

About memory operation functions ：

----------------------memcpy Memory copy --------------------------------
// Prototype ：void * memcpy ( void * destination, const void * source, size_t num );  The third parameter is to set how many bytes to copy 
// Implement a memory copy function by yourself 
struct s
{
    
	char name[20];
	int age;
};
void * my_memcpy(void * destination, const void * source, size_t num)
{
    
	assert(destination && source);
	char* ret = destination;
	while (num--)
	{
    
		*(char*)destination = *(char*)source;
		++(char*)destination; // First line data conversion , Again ++
		++(char*)source;
	}
	return ret;
}
int main()
{
    
	struct s arr1[] = {
     {
    "zhangsan",20},{
    "lisi",30} };
	struct s arr2[4] = {
     0 };
	my_memcpy(arr2, arr1, sizeof(arr1)); // take arr1 The contents are completely copied to arr2 In the middle .
	return 0;
}

It should be noted that ：C Language policy memcpy As long as the memory is not overlapped ,
memmove Function to handle the copy of memory overlap .
Define your own memmove function ：

void * my_memcpy(void * destination, const void * source, size_t num)
{
    
	char* ret = destination;
	assert(destination && source);
	if (destination < source)
	{
    
		// From before to after 
		while (num--)
		{
    
			*(char*)destination = *(char*)source;
			++(char*)destination; // First line data conversion , Again ++
			++(char*)source;
		}
	}
	else
	{
    
    // From back to front 
		while (num--)
			*((char*)destination +num) = *((char*)source+num);
	}
	return ret;
}

memset// Memory settings , This function It is used to fill a given value in a block of memory . Because it can only fill in one value , So the initialization of this function is the original initialization , Unable to initialize variable to data needed in program . use memset After initialization , Later, the program will store the required data in the memory space . The unit of change is bytes
example :

 int main()
{
    
	int arr[10] = {
     0 };
	memset(arr,'#',10);// take arr Array of 10 Change bytes into characters '#'
	int arr1[10] = {
     0 };
	memset(arr1, 1, 10); // This is easy to make mistakes , the reason being that int An array of types , And this sentence only changed the former 10 Bytes , But the whole array has 40 Bytes , In fact, it is only changed to ：
	//01 01 01 01 01 01 01 01 01 00 00 ( The first two int type ).....00
	return 0;
}

For these functions , In fact, I just want to know how to use it , Just find the rules for using functions , There are a lot of them on the Internet . But I want to write a custom function by myself , Then we need to fully understand this function , And certain thinking ability , Of course , There are also lower versions VS compiler , The source code is provided , however VS17 It's hard to find the source code .
Refer to the following website for these functions ~：

http://www.cplusplus.com/