Catalog

String function with limited length

1.strncpy

2.strncat

3.strncmp

String function with limited length

  This article will introduce you to several string functions with limited length , Namely strncpy、strncat and strncmp. Why are these three functions string functions with limited length ？ Because when calling these functions , We need to pass one more unsigned integer data . Compared with string functions with unlimited length , String functions with limited length are also more secure . First of all, let's learn strncpy.

1.strncpy

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

  about strncpy function , We need to pay attention to a few points ： 

• 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 . 
• The target space has to be variable .
• Simulation Implementation strncpy function

Code example ：  

Simulation Implementation strncpy function ：

#include <stdio.h>
#include <string.h>
#include <assert.h>
char* my_strncpy(char* dest, char* source, int n)
{
	assert(dest && source);
	char* start = dest;
	while (n && ((*dest++ = *source++) != '\0'))
	{
		n--;
	}
	// Be careful source First give dest assignment , To determine dest Is it \0
	// So when source by \0 when , I will dest assignment , Push out the cycle if the condition is not met 
	if (n)
	{
		while (--n)
		{
			*dest++ = '\0';		
		}
	}
	return start;
}

int main()
{
	char arr1[30] = "remember who you are!";
	char arr2[] = "believe yourself!";
	my_strncpy(arr1, arr2, 30);
	printf("%s\n", arr1);
	return 0;
}

2.strncat

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

  about strncat function , We need to pay attention to a few points ：

• Append the characters of the source string to the destination space , as well as '\0' .
• If the length of the source string is less than num, Copy only '\0' Previous content .
• The target space has to be variable .
• Simulation Implementation strncat function .

Code example ：

Simulation Implementation strncat function

#include <stdio.h>
#include <string.h>
#include <assert.h>
char* my_strncat(char* dest, char* source,int n)
{
	assert(dest && source);
	char* start = dest;
	while (*dest != '\0')
	{
		dest++;
	}
	while (n)
	{
		*dest++ = *source++;
		n--;
		if (*source == '\0')
		{
			return start;
		}
	}
	*dest = '\0';
	return start;
}

int main()
{
	char arr[20] = "hello ";
	char arr2[] = "ww";
	my_strncat(arr, arr2, 2);
	printf("%s\n", arr);
	return 0;
}

3.strncmp

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

  about strncmp function , We need to pay attention ：  This function starts comparing the first character of each string . If they are equal , Then continue to compare backward , Until the characters are different or until '\0' Or the number of comparisons reaches num.strncmp The return value of is greater than 0、 be equal to 0 Or less than 0.

Code example

Simulation Implementation strncmp function

#include <stdio.h>
#include <string.h>
#include <assert.h>
int my_strncmp(const char* str1, const char* str2, int n)
{
	assert(str1 && str2);
	int count = 0;
	while (*str1 == *str2)
	{
		count++;
		if (*str1 == '\0')
			return 0;
		if (count == n)
			return 0;
		str1++;
		str2++;
	}
	if (*str1 > *str2)
		return 1;
	else
		return -1;
}
int main()
{
	char arr1[] = "hello ";
	char arr2[] = "hello sz";
	int ret;
	ret = my_strncmp(arr1, arr2, 6);
	printf("ret = %d\n", ret);
	ret = my_strncmp(arr1, arr2, 7);
	printf("ret = %d\n", ret);
	return 0;

}

  The above is the content of this blog , You can try to simulate and implement these functions , Let yourself understand more deeply . If you think you've got something , You can click a like to support it ！！