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Preface

This article goes on to learn about character and string functions , And simulated with code . The main learning contents include ：

4、 String search

• strstr
• strtok

5、 Error message report

• strerror

6、 Character manipulation functions

7、 Memory manipulation function

• memcpy
• memmove
• memset
• memcmp

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4、 String search

4.1 Library function strstr

char * strstr ( const char *str1, const char * str2);

• Return to point str1 The first one to appear str2 The pointer to , If str2 No str1 Part of , Return null pointer

int main()
{
    
	char a1[] = "abbbcdef";
	char a2[] = "bbc";
	char* ret = strstr(a1, a2);

	if (NULL==ret)
	{
    
		printf(" Could not find substring \n");
	}
	else
	{
    
		printf("%s\n", ret);
	}
}

4.2 Simulate the implementation of library functions strstr

char* my_strstr(const char* p1, const char* p2)
{
    
	assert(p1&&p2);
	const char* s1 = p1;// Give the pointer the initial position 
	const char* s2 = p2;// Give the pointer the initial position 
	const char* cur = p1;// Record the current pointer position of the string 

	while (*cur)
	{
    
		s1 = cur;
		s2 = p2;// Move to the initial position 
		while (*s1 && *s2 && (*s1==*s2))
		{
    
			s1++;// When equal , Pointer backward , Until two are not equal or one is 0
			s2++;
		}
		if (*s2=='\0')
		{
    
			return (char*)cur;
		}
		cur++;// It's not equal , Current pointer plus 1
	}
	// Jump out of , It's just a string 1 Move to the end 
	return NULL;
}
int main()
{
    
	char a1[] = "abbbcdef";
	char a2[] = "bbc";
	char* ret = my_strstr(a1, a2);
	if (NULL==ret)
	{
    
		printf(" Could not find substring \n");
	}
	else
	{
    
		printf("%s\n", ret);
	}
}

The results are as follows ：

4.3 Library function strtok

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

• sep The parameter is a string , Defines the set of characters used as delimiters. 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
• 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

int main()
{
    
	char a[] = "[email protected]@hello world";
	char a2[50] = {
     0 };
	strcpy(a2, a);
	printf("%s\n", a2);

	const char* sep = "@. ";// Three separators 
	printf("%s\n", strtok(a2, sep));//learnC
	printf("%s\n", strtok(NULL, sep));//day
	printf("%s\n", strtok(NULL, sep));//day
	return 0;
}

The results are as follows ：

int main()
{
    
	char a[] = "[email protected]@hello world";
	char a2[50] = {
     0 };
	strcpy(a2, a);
	printf("%s\n", a2);

	const char* sep = "@. ";// Three separators 
	char* str = NULL;
	// Find multiple strings separated by circular separators , Not empty , Just find the next one , Until the end 
	for (str =strtok(a2,sep); str != NULL; str=strtok(NULL,sep))
	{
    
		printf("%s\n", str);
	}
	return 0;

}

The results are as follows ：

5、 Error message report

5.1 Library function strerror

char * strerror ( int errnum );

• Return error code , The corresponding error message

int main()
{
    
	int* p = (int*)malloc(INT_MAX);// Apply for memory in heap 
	if (p==NULL)
	{
    
		printf("%s\n", strerror(errno));// Print error message 
		return 1;
	}
	return 0;
}

The results are as follows ：

6、 Character manipulation functions

function	If his parameters meet the following conditions, it returns true
iscntrl	Any control character
isspace	Blank character ： Space ‘ ’, Change the page ‘\f’, Line break ’\n’, enter ‘\r’, tabs ’\t’ Or vertical tabs ’\v’
isdigit	Decimal number 0~9
isxdigit	Hexadecimal number , Include all decimal digits , Lowercase letters a-f, Capital A~ F
islower	Lowercase letters a~z
isupper	Capital A~Z
isalpha	Letter a-z or A~Z
isalnum	Letters or numbers ,a-z, A-Z, 0~9
ispunct	Punctuation , Any graphic character that is not a number or letter （ Printable ）
isgraph	Any graphic character
isprint	Any printable character , Including graphic characters and white space characters

7、 Memory manipulation function

7.1 Library function memcpy

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

• function memcpy from source The position of begins to be copied back 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

int main()
{
    
	int a1[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	int a2[5] = {
     0 };
	memcpy(a2, a1, 20);
	int sz = sizeof(a1) / sizeof(a1[0]);
	for (int i = 0; i < sz; i++)
	{
    
		printf("%d\n", a2[i]);
	}
	return 0;
}

7.2 Simulate the implementation of library functions memcpy

// Simulation Implementation 
void* my_memcpy(void* dest, const void* src, size_t cnt)
{
    
	assert(dest&&src);
	void* ret = dest;// Receive any type of data 
	while (cnt--)
	{
    
		*(char*)dest = *(char*)src;// Swap in byte order 
		dest = (char*)dest + 1;
		src = (char*)src + 1;
	}
	return ret;
}

int main()
{
    
	int a1[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	int a2[5] = {
     0 };
	
	my_memcpy(a2, a1, 20);
	int sz = sizeof(a1) / sizeof(a1[0]);
	for (int i = 0; i < sz; i++)
	{
    
		printf("%d\n", a2[i]);
	}
	return 0;
}

The results are as follows ：

7.3 Library function memmove

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

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

int main()
{
    
	int a1[10] = {
     1,2,3,4,5,6,7,8,9,10 };	
	memmove(a1+2, a1, 20);	
	int sz = sizeof(a1) / sizeof(a1[0]);
	for (int i = 0; i < sz; i++)
	{
    
		printf("%d\n", a1[i]);
	}
	return 0;
}

7.4 Simulate the implementation of library functions memmove

// Simulation Implementation 
void* my_memmove(void* dest, const void* src, size_t cnt)
{
    
	assert(dest&&src);
	void* ret = dest;
	if (dest<src)
	{
    // Copy from left to right 
		while (cnt--)
		{
    
			*(char*)dest = *(char*)src;
			dest = (char*)dest + 1;
			src = (char*)src + 1;
		}
	}
	else
	{
    // From right to left 
		while (cnt--)
		{
    
			*((char*)dest + cnt) = *((char*)src + cnt);
		}
	}
	return ret;
}

int main()
{
    
	int a1[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	my_memmove(a1+2, a1, 20);
	int sz = sizeof(a1) / sizeof(a1[0]);
	for (int i = 0; i < sz; i++)
	{
    
		printf("%d\n", a1[i]);
	}
	return 0;
}

The results are as follows ：

7.5 Library function memcmp

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

• Compare from ptr1 and ptr2 The pointer starts with num Bytes
• Return value less than 0, The data of the preceding byte is smaller than the data of the following corresponding byte
• The return value is equal to 0, The data of the preceding byte is equal to the data of the following corresponding byte
• Return value greater than 0, The data of the preceding byte is larger than the data of the following corresponding byte

int main()
{
    
	int a1[] = {
     1,2,3,4,5 };
	int a2[] = {
     1,2,3,4,0x11223305 };
	int ret = memcmp(a1, a2, 18);
	printf("%d\n", ret);
	return 0;
}

The results are as follows ：

7.6 Library function memset

void *memset( void *dest, int c, size_t count );

• memset Function will Target space dest Of in total count The contents of bytes are set to characters c.

int main()
{
    
	int a[] = {
     0x11111111,0x22222222,3,4,5 };
	memset(a, 6, 20);
	return 0;
}

The results are as follows ：

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summary

The learning of string library function is basically over . It's a little confusing after learning , This content can be used as a dictionary , Check again when necessary .

It feels a little boring to learn string library functions alone , You can understand and master the usage of these functions only after you brush the questions later .

Library functions to achieve learning dazzling , There's one more thing I haven't written completely , Add content later .

The next article updates the content related to structure .