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C language string function summary

2022-07-06 10:33:00 【Banse】

<string.h> There are many commonly used string functions in the Library , Cute new bloggers have made some summaries here

Catalog

1.strlen

The function prototype :size_t strlen ( const char * str );

function : Calculate string length .

Parameters :str The string to evaluate

Return value : The length of the returned string does not include '\0'

Example :

int main()
{
	char str[] = "abcdef";
	printf("%d\n", strlen(str));
	// The result is 6
	return 0;
}

Simulation Implementation :

// Counter method 
int my_strlen1(const char *str)
{
	int count = 0;
	assert(str);
	while (*str++)
	{
		count++;
	}
	return count;
}

// Pointer minus pointer 
int my_strlen2(const char *str)
{
	char *p = str;
	assert(str);
	while (*str)
		str++;
	return str - p;
}

// recursive 
int my_strlen2(const char *str)
{
	assert(str);
	if (*str != '\0')
	{
		return 1 + my_strlen(str + 1);
	}
	else
	{
		return 0;
	}
}

2.strcpy

The function prototype :char* strcpy(char * destination, const char * source );

function : Copy a string to another place

Parameters :source The source of the string to be copied ; destination Where the string to be copied will be placed ;

Return value : Return the address of the copied string

Be careful : The space where the string will be placed should be larger than the string to be copied .

Example :

int main()
{
	char str1[10] = { 0 };
	char str2[] = "abcdef";
	printf("%s\n", strcpy(str1, str2));
	// The result is abcdef
	return 0;
}

Simulation Implementation :

char* my_strcpy(char *dest, const char *src)
{
	char *tmp = dest;
	assert(dest && src);
	while (*src)
	{
		*dest = *src;
		src++;
		dest++;
	}
	return tmp;
}

3.strcat

The function prototype :char * strcat ( char * destination, const char * source );

function : Copy one string to the end of another .

Parameters :source String to be copied ,destination Copy to the end of this string

Return value : Return the address after copying

Example :

int main()
{
	char str1[10] = "abc";
	char str2[] = "def";
	printf("%s\n", strcat(str1, str2));
	// result abcdef

	return 0;
}

Simulation Implementation :

char* my_strcat(char *dest, const char *src)
{
	char *tmp = dest;
	assert(dest && src);
	while (*dest)// Find the end 
		dest++;
	while (*src)
	{
		*dest++ = *src++;
	}
	return tmp;
}

4.strcmp

The function prototype :int strcmp ( const char * str1, const char * str2 );

function : String comparison , Press ASCII Code table comparison characters

Parameters :str1,str2 Two strings to compare

Return value :str1 Greater than str2 Return is greater than the 0 The number of :str1 be equal to str2 return 0;str1 Less than str2 Back to less than 0 The number of

Example :

int main()
{
	char str1[] = "abcd";
	char str2[] = "acde";
	int tmp = strcmp(str1, str2);
	if (tmp > 0)
	{
		printf("str1 > str2\n");
	}
	else if (tmp < 0)
	{
		printf("str1 < str2\n");
	}
	else
	{
		printf("str1 = str2\n");
	}
	// result str1 < str2
	return 0;
}

Simulation Implementation :

int my_strcmp(const char *dest, const char *src)
{
	assert(dest && src);

	while (*dest == *src && *dest)
	{
		dest++;
		src++;
	}
	return *dest - *src;
}

5.strncpy

The function prototype : char * strncpy ( char * destination, const char * source, size_t num );

function : Copy n Characters from the source string to the target space .

Parameters : destination Copy destination source Copy source num Number of copied characters

Return value : Return the address after copying

Example :

int main()
{
	char str1[] = "abcdefg";
	char str2[10] = { 0 };
	printf("%s\n", strncpy(str2, str1, 3));
	// result abc
	return 0;
}

6.strncat

The function prototype : char * strncat ( char * destination, const char * source, size_t num );

function : The source of n Copy characters to the end of the destination plus '\0'

Parameters : destination Copy destination , sourse Copy source , num Number of copies

Return value : Return the address after copying

Example :

int main()
{
	char str1[10] = "abc";
	char str2[] = "defgh";
	printf("%s\n", strncat(str1, str2, 3));
	// result abcdef
	return  0;
}

7.strncmp

The function prototype : int strncmp ( const char * str1, const char * str2, size_t num );

function : Compare to the occurrence of different characters or the end of a string or n Compare all the characters .

Parameters : str1,str2 Two strings to compare num The number of characters to compare

Return value : str1 Greater than str2 Return is greater than the 0 The number of :str1 be equal to str2 return 0;str1 Less than str2 Back to less than 0 The number of

Example :

int main()
{
	char str[][5] = { "R2D2" , "C3PO" , "R2A6" };
	int n;
	puts("Looking for R2 astromech droids...");
	for (n = 0; n < 3; n++)
		if (strncmp(str[n], "R2xx", 2) == 0)
		{
			printf("found %s\n", str[n]);
		}
	// result :
	//found R2D2
	//found R2A6
	return 0;
}

8.strstr

The function prototype : char * strstr ( const char *str1, const char * str2);

function : String lookup function , In string str1 Find string in str2

Parameters : str1 will By String to find ,str2 The string to find

Return value : Return to the address where you found the same location

Example :

// Example 1
int main()
{
	char str1[] = "hello whord!";
	char str2[] = "ell";
	printf("%s", strstr(str1, str2));
	// result ello whord!
	return 0;
}
// Example 2
int main ()
{
  char str[] ="This is a simple string";
  char * pch;
  pch = strstr (str,"simple");
  strncpy (pch,"sample",6);
  puts (str);
  // result This is a simple string
  return 0;
}

9.strtok

The function prototype : char * strtok ( char * str, const char * sep );

function : Find the separator flag in the string , And then split

Parameters str The string to be searched ,sep The string of delimiters can be multiple delimiters

Return value : The first call returns the address of the first string , When str Parameter input NULL Return the address of the next string , Returns... At the end of the string NULL

Add :
1.sep The parameter is a string , Defines the set of characters used as separators
2. The first parameter specifies a string , It contains 0 One or more by sep A character separated by one or more separators in a string remember .
3.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 segmented by function is usually the content of temporary copy And it can be modified .）
4.strtok The first argument of the function is not NULL , Function will find str The first mark in ,strtok The function will save it in the string Position in .
5.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 .
6. If there are no more tags in the string , Then return to NULL The pointer .

Example :

int main()
{
	char str1[] = "hell#o wh$ord";
	char str2[] = "#$";
	char* pstr = NULL;
	for (pstr = strtok(str1, str2); pstr; pstr = strtok(NULL, str2))
	{
		printf("%s\n", pstr);
	}
	// result 
	//hell
	//o wh
	//ord
	return 0;
}

10.strerror

The function prototype : char * strerror ( int errnum );

function : Return error information according to the error code

Parameters : ermun Error code

Return value : The address of the error message

Add :strerror Function with global variables errno Use

errno The header file <errno.h>


	printf(" error message : %s", strerror(errno));

11.memcpy

The function prototype : void * memcpy ( void * destination, const void * source, size_t num );

function : Memory copy

Parameters destination The destination of the copy source Source of copy num Number of bytes copied

Return value : return void* Type of The address of the destination

Example

int main()
{
	int a[] = { 1, 5, 6, 848, 4, 11, 3, 51, 9 };
	int b[20] = { 0 };
	int i = 0;
	memcpy(b, a, sizeof(int) * 8);
	for (i = 0; i < 20; i++)
	{
		printf("%d ", b[i]);
	}
	// result 1 5 6 848 4 11 3 51 0 0 0 0 0 0 0 0 0 0 0 0
	return 0;
}

Simulation Implementation :

void* my_memcpy(void* dest, const void* src, size_t n)
{
	void* tmp = dest;
	assert(dest);
	assert(src);

	while (n--)
	{
		*(char*)dest = *(char*)src;
		(char*)dest += 1;
		(char*)src += 1;

	}

	return tmp;
}

12.memmove

The function prototype : void * memmove ( void * destination, const void * source, size_t num );

function : Function and memmove Same but can handle string overlap , Can replace memcpy

Parameters : destination The destination of the copy source Source of copy num Number of bytes copied

Return value : return void* Type of The address of the destination

Simulation Implementation :

void* my_memmove(void* dest, const void* src, size_t n)
{
	void* tmp = dest;
	assert(dest);
	assert(src);
	if (dest < src)// Copy from front to back 
	{
		while (n--)
		{
			*(char*)dest = *(char*)src;
			src = (char*)src + 1;
			(char*)dest += 1;
		}
	}
	else
	{
		while (n--)
		{
			*((char*)dest + n - 1) = *((char*)src + n - 1);
		}
	}
	return tmp;
}

13.memcmp

The function prototype : int memcmp ( const void * ptr1, const void * ptr2, size_t num );

function : Memory compare function , Compare by byte ,

Parameters : ptr1 ptr2 Two memory addresses to be compared num Number of bytes to compare

Return value : ptr1 Greater than ptr2 Return is greater than the 0 The number of :ptr1 be equal to ptr2 return 0;ptr1 Less than ptr2 Back to less than 0 The number of