[advanced C language] string and memory function (II)

1、strtok function

1、strtok The string will be separated according to the separator , Split into markers , And change the separator in the divided mark into ‘\0’, At the same time, a pointer to this tag will be returned

2、 Be careful ：strtok Function changes the string being manipulated , So use strtok Function , It is generally used in the content copied at zero time

3、strtok When the function finds the first tag , The first argument of the function is not NULL, The position of the first mark will be strtok Function save

4、strtok When the function finds a non first tag , The first argument to the function is NULL, The segmentation will continue at the save location of the first mark

5、 When strtok After the function cannot find the tag , Will return a NULL

6、 Examples of function usage

2、strerror function

1、 Return the error message corresponding to the error code

3、 Character classification function

1、iscntrl function ： Any control character

2、isdigit：10 Hexadecimal Numbers 0~9

3、isxdigit： Hexadecimal number , Contains all decimal digits , Lowercase letters a~f, Capitalization A ~F

4、isslower: Determine if it's lowercase a~z

5、isspace: Determine whether it is a blank character ： Returns a non-zero value , Not just back 0

6、isupper: The judgment is that they are all capital letters A~F

7、isalpha： Judgment is all letters , Regardless of case

8、isalnum: Determine whether it is a letter or a number

9、tolower: Character conversion , Convert capital letters to lowercase

10、toupper： Character conversion , Convert lowercase letters to uppercase

The actual cases

4、 Memory function

4.1、memcpy function

1、 differ strcpy Functions can only copy strings ,memcpy Function can copy all types in memory

2、memcpy() Of 3 Parameters , The first is Destination space , The second is Source space , The third parameter is Number of bytes of copied content

3、 The actual cases

4、 Simulation Implementation memcpy function

#include <stdio.h>
#include <string.h>
#include <assert.h>
void* my_memcpy(void* dest, const void* src, size_t num)//num Unit is byte 
{
    
	void* ret = dest;
	assert(dest && src);

	while (num--)
	{
    
		*(char*)dest = *(char*)src;// The effect of cast is only valid in the current statement 
		// The cast effect is temporary 
		dest = (char*)dest + 1;// So not directly dest++, But in this form 
		src = (char*)src + 1;//src Empathy 
	}
	return ret;
}

int main()
{
    
	int arr1[] = {
     1,2,3,4,5,6,7,8,9,10 };
	int arr2[5] = {
     0 };
	my_memcpy(arr2, arr1, 5 * sizeof(arr1[0]));
	int i = 0;
	for (i = 0; i < 5; i++)
	{
    
		printf("%d ", arr2[i]);
	}
	return 0;
}

4.2、memmove function

1、memmove and memcpy The usage of is basically the same , The difference is that , If Destination space and Source space There is overlap ,memmove It can still be copied correctly , however memcpy You can't copy correctly （ Examples are attached below ）

2、C Language requirements ：
memcpy You can copy non overlapping memory space
memmove To handle those overlapping memory copies

3、 Simulation Implementation memmove function

#include <stdio.h>
#include <string.h>
#include <assert.h>
void* my_memmove(void* dest, const void* src, size_t num)
{
    
	void* ret = dest;
	assert(dest && src);

	if (dest < src)
	{
    
		// front  ->  after 
		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;
}

void test1()
{
    
	int arr1[] = {
     1,2,3,4,5,6,7,8,9,10 };
	my_memmove(arr1 + 2, arr1, 5 * sizeof(arr1[0]));
	int i = 0;
	for (i = 0; i < 10; i++)
	{
    
		printf("%d ", arr1[i]);
	}
}
int main()
{
    
	test1();
	return 0;
}

4.3、memcmp function

1、 Function declaration

int memcmp(const void *str1, const void *str2, size_t n)

2、 Return value

str1<str2               <0

str1=str2               =0

str1>str2               >0

3、 Use cases

// By entering the number of bytes n, Compare the two arrays starting from the first address n Bytes 
#include<stdio.h>
#include<string.h>
int main() {
    
	int arr1[5] = {
     1,2,3,4,5 };
	int arr2[5] = {
     1,2,3,4,0x11223305 };
	int ret = memcmp(arr1, arr2, 16);
	printf("%d", ret);
	return 0;
}

4.4、memset function

1、memset A function is an initialization function , You can initialize a continuous piece of memory to a certain value .
2、memset It is initialized in bytes
3、memset When initializing a function, you don't care what type of array you want to initialize , Are initialized in bytes