Character function and string function (2)

String function with limited length ：
strncpy
strnzat
strncmp

1.5 strncpy

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

• Copies the first num characters of source to destination. If the end of the source C string (which is signaled by a null-character) is found before num characters have been copied, destination is padded with zeros until a total of num characters have been written to it.
• Copy num Characters from the source string to the target space
• If the length of the source string is less than num, After copying the source string , Add... After the target 0, until num individual .

#include <stdio.h>
int main()
{
    
	char arr1[20] = "abcdef";
	char arr2[] = "hello";
	strncpy(arr1, arr2, 5);
	// The transfer parameters are ： Objective function , Source function , Number of parameters passed 
	char arr3[] = "xyz";
	strncpy(arr1, arr3, 5);
	// When the number of passed parameters is greater than the number of elements in the source function , Append 0
	printf("%s", arr1);
	return 0;
}

1.6 strncat

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

• Appends the first num characters of source to destination, plus a terminating null-character.
• If the length of the C string in source is less than num, only the content up to the terminating null-character is copied.

#include <stdio.h>
int main()
{
    
	char arr1[20] = "abcdef";
	char arr2[] = "hello";
	strncat(arr1, arr2, 5);
	//strncat The function will automatically add one after the addition '\0'
	char arr3[] = "xyz";
	strncat(arr1, arr3, 5);
	// When the parameters of the source function are less than the number of passed parameters , There is only one in the back '\0'
	printf("%s\n", arr1);
	return 0;
}

1.7 strncmp

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

• Compare to two different characters or the end of a string or num Compare all the characters

The standard stipulates ：

• The first string is larger than the second string , Return greater than 0 The number of
• The first string is equal to the second string , Then return to 0
• The first string is less than the second string , Then return less than 0 The number of

#include <stdio.h>
int main()
{
    
	char arr1[20] = "abcdef";
	char arr2[] = "abc";
	int ret = strncmp(arr1, arr2, 3);
	if (ret == 0)
	{
    
		printf("==\n");
	}
	else if (ret > 0)
	{
    
		printf(">\n");
	}
	else
	{
    
		printf("<\n");
	}
	return 0;
}

1.8 strstr（ Function to find substring ）

const char * strstr ( const char * str1, const char * str2 ); char * strstr ( 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.
• The matching process does not include the terminating null-characters, but it stops there.

#include<stdio.h>
int main()
{
    
	char arr1[] = "hello world";
	char arr2[] = "world";
	char* ret = strstr(arr1, arr2);
	if (ret == NULL)
	{
    
		printf(" Substring does not exist \n");
	}
	else
	{
    
		printf("%s\n", ret);
     // If the source string exists in the destination string , Will return the found address 
	}
	return 0;
}

Simulation Implementation strstr function ：

#include <stdio.h>
#include <assert.h>
char* my_strstr(const char* str1, const char* str2)
{
    
	assert(str1 && str2);
	const char* s1 = str1;
	const char* p = str1;
	const char* s2 = str2;
	while (*p)
	{
    
		s1 = p;
		s2 = str2;
		while (*s1 != '\0' && *s2 != '\0' && *s1 == *s2)
		{
    
			s1++;
			s2++;
		}
		if (*s2 == '\0')
		{
    
			return (char*)p;
		}
		p++;
	}
	return NULL;
}
int main()
{
    
	char arr1[] = "abbbcedf";
	char arr2[] = "bbc";
	char* ret = my_strstr(arr1, arr2);
	if (ret == NULL)
	{
    
		printf(" Substring does not exist \n");
	}
	else
	{
    
		printf("%s\n", ret);
	}
	return 0;
}

1.9 strtok

char * strtok ( char * str, const char * sep );

• 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 mark separated by one or more separators in a string
• 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 cut by the function is usually a temporary copy and can be modified .）
• strtok The first argument of the function is not NULL, Function will find str The first mark in ,strtok Function will hold its position in the string .
• strtok The first argument to the function is NULL. The function will start at the same position in the string that is saved , Find next tag .
• If there are no more tags in the string , Then return to NULL The pointer .

#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[] = "https://www.baidu.com";
	char cp[30] = {
     0 };
	const char* sep = ":.";
	strcpy(cp, arr);// Make a copy of the data , Handle cp Contents of array 

	char* ret = strtok(cp, sep);
	if (ret != NULL);
	printf("%s\n", ret);

	ret = strtok(NULL, sep);
	if (ret != NULL);
	printf("%s\n", ret);

	ret = strtok(NULL, sep);
	if (ret != NULL);
	printf("%s\n", ret);

	ret = strtok(NULL, sep);
	if (ret != NULL);
	printf("%s\n", ret);
	return 0;
}

// After simplification and modification ：
#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[] = "https://www.baidu.com";
	char cp[30] = {
     0 };
	const char* sep = ":.";
	strcpy(cp, arr);

	for (char* ret = strtok(cp, sep); ret != NULL; ret = strtok(NULL, sep))
	{
    
		printf("%s\n", ret);
	}
	return 0;
}

1.10 strerror

char * strerror ( int errnum );

Return the error message corresponding to the error code .
C When the library function of language fails to execute , Will set the error code

#include <stdio.h>
int main()
{
    
	printf("%s\n", strerror(0));
	printf("%s\n", strerror(1));
	printf("%s\n", strerror(2));
	printf("%s\n", strerror(3));
	printf("%s\n", strerror(4));
	printf("%s\n", strerror(5));
	printf("%s\n", strerror(6));
	return 0;
}

//errno - C A global error code storage variable of language settings 
#include <stdio.h>
#include <errno.h>
int main()
{
    
	FILE* pf = fopen("test.txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 1;
	}
	else
	{
    
		//
	}
	return 0;
}

Character classification function ：

#include <stdio.h>
#include <ctype.h>
int main()
{
    
	//isspace
	int a = isspace(' ');
	printf("%d\n", a);

	//isdigit
	int b = isdigit('x');
	printf("%d\n", b);

	//tolower
	printf("%c\n", tolower('w'));
}

Character conversion ：

int tolower ( int c );
int toupper ( int c );

1.11 memcpy

void * memcpy ( void * destination, const void * source, size_t num );

• function memcpy from source The position of the starts to be assigned backwards num Bytes of data to destination Memory location for .
• This function is encountering '\0' It doesn't stop .
• If source and destination There is any overlap , The results of replication are undefined （memcpy Be responsible for copying data in two independent spaces ）.

//memcpy
#include <stdio.h>
#include <string.h>
int main()
{
    
	int arr1[] = {
     1,2,3,4,5,6,7 };
	int arr2[10] = {
     0 };
	memcpy(arr2, arr1, 28);

	float s1[] = {
     1.0,2.0,3.0,4.0,5.0 };
	float s2[20] = {
     0 };
	memcpy(s2, s1, 20);
	return 0;
}

Simulation Implementation memcpy function ：

// Simulation Implementation memcpy function 
#include <stdio.h>
#include <assert.h>
void* my_memcpy(void* dest, const void* src, size_t num)
{
    
	assert(dest && src);
	void* ret = dest;
	while (num--)
	{
    
		*(char*)dest = *(char*)src;
		dest = (char*)dest + 1;
		src = (char*)src + 1;
	}
	return ret;
}
int main()
{
    
	int arr1[] = {
     1,2,3,4,5,6,7 };
	int arr2[10] = {
     0 };
	my_memcpy(arr2, arr1, 28);
	int i = 0;
	for(i = 0; i < 5; i++)
	{
    
		printf("%d ", arr2[i]);
	}

	return 0;
}

1.12 memmove

void * memmove ( void * destination, const void * source, size_t num );

• and memcpy The difference is that memmove The source memory blocks processed by the function can be overlapped .
• If the source space and the target space overlap , You have to use memove Function processing .

Simulation Implementation memmove function ：

// Simulation Implementation memmove function 
#include <stdio.h>
#include <assert.h>
void* my_memmove(void* dest, const void* src, size_t num)
{
    
	assert(dest && src);
	void* ret = dest;
	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;
}
int main()
{
    
	int arr1[] = {
     1,2,3,4,5,6,7,8,9,10 };
	my_memmove(arr1, arr1+2, 20);
	int i = 0;
	for (i = 0; i < 10; i++)
	{
    
		printf("%d ", arr1[i]);
	}

	return 0;
}

1.13 memcmp

int memcmp ( const void * ptr1, const void * ptr2, size_t num );

• Compare from ptr1 and ptr2 The pointer starts with num Bytes
• The return value is as follows ：

Example ：

//memcmp
#include <stdio.h>
#include <string.h>
int main()
{
    
	int arr1[] = {
     1,2,3,4,5 };
	int arr2[] = {
     1,3,2 };
	int ret = memcmp(arr1, arr2, 12);
	printf("%d\n", ret);
	return 0;
}

1.14 memset

void * memset ( void * ptr, int value, size_t num );

• take ptr Of the memory block pointed to num Set the number of bytes to the specified value

Example ：

//memset
#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[] = "hello world";
	memset(arr, 'x', 5);
	memset(arr + 6, 'x', 3);
	printf("%s\n", arr);
	int arr[10] = {
     0 };
	// hold arr Initialize to full 1（ This method is not feasible ）
	memset(arr, 1, 40);
	int i = 0;
	for (i = 0; i < 10; i++)
	{
    
		printf("%d\n", arr[i]);
	}
	return 0;
}