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

1、 Unlimited length string function

1.1、strlen function

size_t strlen(const char* str);

1、sizeof — The operator — Calculate the size of ,sizeof The type returned is size_t, and size_t Essentially, unsigned int type

// Simulation Implementation strlen
#include <stdio.h>
#include <string.h>
#include <assert.h>
int my_strlen(const char* str)
{
    
	assert(str);// Assertion str It's not a null pointer 
	int count = 0;
	while (*str != '\0')
	{
    
		count++;
		str++;
	}
	return count;
}

int main()
{
    
	int len = my_strlen("abcdef");
	printf("%d\n", len);
	return 0;
}

2、 String to ‘\0’ As an end sign ,strlen Function returns in a string ‘\0’ The number of characters that appear before , It doesn't contain ’\0’

3、 The string pointed to by the parameter must be in ‘\0’ end

1.2、strcpy function

1、 The source string must be in ‘\0’ end （ The source string is the copied string ）

2、strcpy The function will the... In the source string ‘\0’ Copy to target space

3、 The target space has to be large enough , To ensure that the source string can be stored

4、 The target space has to be variable （ Rebuttal ）

// Wrong example , Code can't be written like this ！！！
#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr1[] = "abcdef";
	const char* p = "xxxxxxxx";
	// At this time, the pointer variable p By const modification ,p The content of the point cannot be changed , So it's wrong 
	strcpy(p, arr1);
	printf("%s\n", p);
	return 0;
}

5、 Simulation Implementation strcpy function

#include <assert.h>
#include <stdio.h>
// Simulation Implementation strcpy function 
char* my_strcpy(char* dest,const char* src)
{
    
	char* ret = dest;
	assert(dest && src);// Assert that neither is a null pointer 
	while (*dest++ = *src++)
	{
    
		;
	}
	return ret;
}

int main()
{
    
	char arr1[] = {
     'a','b','c','d','e','f','\0' };
	char arr2[20] = "xxxxxxxxxx";
	my_strcpy(arr2, arr1);
	printf("%s\n", arr2);
	return 0;
}

1.3、strcat function

1、 Append the contents of the string to Destination string Back
2、 The target space has to be large enough , It can hold the contents of the source string
3、 The appended string must be followed by ’\0’
4、 The target string must be followed by ’\0’
5、 The target space must be modifiable
6、 Simulation Implementation strcat function

1.4、strcmp function

1、 String comparison （ Compare the character size at the corresponding position ）
2、 The comparison is the corresponding character position ASCII Code value
3、 Simulation Implementation strcmp function

2、 String function with limited length

2.1、strncpy function

Examples of use

#include <stdio.h>
int main()
{
    
	char arr1[] = "xxxxxxxxxxxxxxxx";
	char arr2[] = "hello world";

	strncpy(arr1, arr2, 5);
	printf("%s\n", arr1);
	return 0;
}

2.2、strncat function

Use cases

int main()
{
    
	char arr1[20] = "hello";
	char arr2[] = "world";
	strncat(arr1, arr2, 5);
	printf("%s\n", arr1);
	return 0;
}

2.3、strncmp function

Use cases

int main()
{
    
	char arr1[] = "abcdef";
	char arr2[] = "abcqqqqq";
	int ret = strncmp(arr1, arr2, 4);
	printf("%d\n", ret);
	return 0;
}

3、strstr function

1、 The function is to find the target string in the string , Here are the use cases

#include <assert.h>
#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr1[] = "abcdefabcdef";
	char arr2[] = "bcd";

	char* ret = strstr(arr1, arr2);
	if (NULL == ret)
	{
    
		printf(" Did not find \n");
	}
	else
	{
    
		printf("%s\n", ret);
	}
	return 0;
}

2、 Simulation Implementation strstr function

Little knowledge

1、 When the pointer doesn't know what value to assign , Just give it to NULL
2、 After the pointer is used , assignment NULL
3、 Chained access （ give an example ）
4、 Two strings cannot be compared directly with an equal sign , It can't be subtracted , Because when using two strings , What is produced is the first address of two strings

int main()
{
    
   int len = strlen("abcdef");
   printf("%d\n",strlen("abcdef"));
   return 0;
}

Wrong example

// Error code , Do not imitate 
int* test()
{
    
   int a = 10;
   return &a;
}

int main()
{
    
   int* p = test();
   *p = 20;// Form illegal access 
   // Because the space requested by the temporary variable in the function 
   // It's destroyed when it comes out of the function 
   // Therefore, illegal access will be formed 
   return 0;
}