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Preface

This article begins to learn the use of character functions and string library functions , And use the knowledge learned before to simulate and realize these library functions .

Library function recommendation query tool official website ：

• MSDN(Microsoft Developer Network)
• www.cplusplus.com
• http://en.cppreference.com（ English version ）
• http://en.cppreference.com（ Chinese version ）

C Handling characters and strings is common in languages ,C The language itself has no string type , Strings are usually placed in Constant string Medium or A character array in . String constant For string functions that do not modify it .

The main content of this article includes ：

1、 Find the string length

• strlen

2、 Unlimited length string function

• strcpy
• strcat
• strcmp

3、 Introduction to string functions with limited length

• strncpy
• strncat
• strncmp

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1、 Function to find the length of string

In the previous article, I learned to use the general method 、 recursive 、 Pointer to simulate the implementation of library functions strlen To read the length of the string

• 【C Language foundation 4—— function （2）】7.4 practice 2
• 【C Language foundation 11—— The pointer （2）】4.2 The pointer - The pointer
• 【 Library function for reading string length strlen And its simulation implementation 】

1.1 Library function strlen

size_t strlen ( const char * str );

• The string has ‘\0’ As an end sign ,strlen Function returns in a string ‘\0’ The number of characters that appear before （ It doesn't contain ‘\0’ )
• The string that the argument points to must be in ‘\0’ end
• Note that the return value of the function is size_t, It's unsigned

 Use library functions ：
int main() 
{
    
	const char* str1 = "abcdef";
	const char* str2 = "bbb";
	if (strlen(str2)-strlen(str1)>0)// Compare string length 
	{
    
		printf("str2>str1\n");
	}
	else
	{
    
		printf("str1>str2\n");
	}
	return 0;
}

1.2 Simulate the implementation of library functions strlen

int my_strlen(const char* str)// Constant pointer , Limit strings , Can not be modified in the process of transmission 
{
    
	assert(str != NULL);// When str When null pointer , Application error 
	char* p = str;// Record the location of the first element address 
	while (*p != '\0')
	{
    
		p++;
	}
	return p - str;// Tail address - The first address , That's the length of the string 
}

int main()
{
    
	//int len = strlen("abcdef");
	int len = my_strlen("abcdef");

	printf("%d\n", len);
	return 0;
}

The output results are shown in the figure below ：

2、 Unlimited length string function

2.1 Library function strcpy

char* strcpy(char * destination, const char * source );

• The source string must be in ‘\0’ end
• In the source string ‘\0’ Copy to target space
• The target space has to be large enough , To ensure that the source string can be stored
• The target space has to be variable

int main()
{
    
	char  a[10] = "x";
	//char* a1 = "qwertyuop";// Constant string cannot be modified 
	char a2[] = "abcdef";

	strcpy(a, a2);
	printf("%s\n", a);
	return 0;
}

2.2 Simulate the implementation of library functions strcpy

char* my_strcpy(char* dest, const char* src)
{
    
	//assert(dest != NULL);
	//assert(src != NULL);
	assert(dest && src);// The same as the two above 
	char* ret = dest;// Record the starting position of the pointer 
	while (*dest = *src)
	{
    
		dest++;
		src++;
	}
	/*while ( *src !='\0')// It's not simple enough  { *dest = *src; dest++; src++ } *dest = '\0';*/
	return ret;
}
int main()
{
    
	char  a[10] = "x";
	//char* a1 = "qwertyuop";// Constant string cannot be modified 
	char a2[] = "abcdef";

	printf("%s\n", my_strcpy(a, a2));// The chain 
	return 0;
}

The output results are shown in the figure below ：

2.3 Library function strcat

char * strcat ( char * destination, const char * source );

• The source string must be in ‘\0’ end
• The target space must be large enough , It can hold the contents of the source string
• The target space must be modifiable

int main()
{
    
	char a1[20] = "hello ";
	char a2[] = "world";
	strcat(a1, a2);
	printf("%s\n", a1);
	return 0;
}

2.4 Simulate the implementation of library functions strcat

char* my_strcat(char* dest, const char* src)
{
    
	assert(src&&dest);
	char* ret = dest;
	// Find the end of string flag  '\0'
	while (*dest)
	{
    
		dest++;
	}
	// Start copying strings 
	while (*dest++ == *src++)
	{
    
		;
	}
	return ret;
}

int main()
{
    
	char a1[20] = "hello ";
	char a2[] = "world";

	printf("%s\n", my_strcat(a1, a2));
	return 0;
}

The output results are shown in the figure below ：

2.5 Library function strcmp

int strcmp ( const char * str1, const char * str2 );

• 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

int main()
{
    
	char a1[] = "abcd";
	char a2[] = "abdc";
	
	int ret= strcmp(a1, a2);
	if (ret>0)
	{
    
		printf(">\n");
	}
	else if (ret==0)
	{
    
		printf("==\n");
	}
	else
	{
    
		printf("<\n");
	}
	printf("%d\n", ret);

	return 0;
}

2.6 Simulate the implementation of library functions strcmp

int my_strcmp(const char* s1, const char* s2)
{
    
	assert(s1&&s2);
	while (*s1 == *s2)
	{
    
		if (*s1=='\0')// Consistent to the end , Represents the end of a string 
		{
    
			return 0;
		}
		s1++;
		s2++;
	}
	// Return difference 
	return *s1 - *s2;
}
int main()
{
    
	char a1[] = "abcd";
	char a2[] = "abdc";
	
	int ret = my_strcmp(a1, a2);
	if (ret>0)
	{
    
		printf(">\n");
	}
	else if (ret==0)
	{
    
		printf("==\n");
	}
	else
	{
    
		printf("<\n");
	}
	printf("%d\n", ret);

	return 0;
}

The output results are shown in the figure below ：

3、 String function with limited length

3.1 Library function strncpy

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

• 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

int main()
{
    
	char a1[] = "abcdef";
	char a2[] = "qwe";
	strncpy(a1, a2, 4);
	printf("%s\n", a1);
	return 0;
}

3.2 Simulate the implementation of library functions strncpy

char* my_strncpy(char* dest, const char* src, int cnt)
{
    
	assert(dest&&src);
	char *ret = dest;

	while (cnt && (*dest++ = *src++) != '\0')
	{
    
		cnt--;
	}
	// If the number of characters in the source string is less than cnt, After copying the source string , Followed by the complement character '\0', Until cnt individual 
	if (cnt)
	{
    
		while (--cnt)
		{
    
			*dest++ = '\0';
		}		
	}
	return ret;
}

int main()
{
    
	char a1[] = "abcdef";
	char a2[] = "qwe";
	strncpy(a1, a2, 4);
	printf("%s\n", a1);
	printf("%s\n", my_strncpy(a1, a2, 4));

	return 0;
}

The output results are shown in the figure below ：

3.3 Library function strncat

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

• The first of the source num Character plus terminated null Append character to target
• If the length of the string in the source code is less than num, Then only copy until the null character is terminated ‘\0’ The content of

int main()
{
    
	char a1[20] = "ab";
	char a2[] = "qwe";
	strncat(a1,a2,5);
	printf("%s\n", a1);
	return 0;
}

3.4 Simulate the implementation of library functions strncat

char* my_strncat(char* dest, const char* src,int cnt)
{
    
	assert(dest&&src);
	char* ret = dest;

	while (*dest)// Move the pointer to the end of the string '\0' It's about 
	{
    
		dest++;
	}	
	while (cnt--)// Number of copied characters 
	{
    
		*dest++ = *src++;
		if (*dest)// Here, if you copy to characters '\0', It's over 
		{
    
			return ret;
		}
	}
	// When the length of the source string is greater than  cnt when , Add one at the end '\0'
	*dest = '\0';
	return ret;
}

int main()
{
    
	char a1[10] = "ab";
	char a2[] = "qwe";
	printf("%s\n", my_strncat(a1, a2, 5));
	return 0;
}

The output results are shown in the figure below ：

3.5 Library function strncmp

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

• The character of the first string is larger than the character of the corresponding second string , Return greater than 0 The number of
• All characters of the string are the same , Then return to 0
• The character of the first string is smaller than the character of the corresponding second string , Then return less than 0 The number of

int main()
{
    
	char a1[] = "abcdef";
	char a2[] = "abcdq";
	int ret = strncmp(a1, a2, 4);
	if (ret > 0)
	{
    
		printf(">\n");
	}
	else if (ret == 0)
	{
    
		printf("==\n");
	}
	else
	{
    
		printf("<\n");
	}
	return 0;
}

3.6 Simulate the implementation of library functions strncmp

int my_strncmp(const char* s1, const char* s2, int cnt)
{
    
	assert(s1&&s2);

	while ((cnt-1) && (*s1 == *s2))// Here for cnt It's wrong. 
	{
    
		cnt--;
		if (*s1 == '\0')// Consistent to the end , Represents the end of a string 
		{
    
			return 0;
		}
		s1++;
		s2++;		
	}
	// Return difference 
	return *s1 - *s2;
}
//strncmp
int main()
{
    
	char a1[] = "abcdef";
	char a2[] = "abcde";
	int ret = my_strncmp(a1, a2, 6);

	if (ret > 0)
	{
    
		printf(">\n");
	}
	else if (ret == 0)
	{
    
		printf("==\n");
	}
	else
	{
    
		printf("<\n");
	}
	return 0;
}

The output results are shown in the figure below ：

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summary

This article has learned several library functions and their simulation code .

Continue to learn about string library functions in the next blog post .