strncmp
功能
strncmp函数是指定比较size个字符。也就是说，如果字符串s1与s2的前size个字符相同，函数返回值为0。此函数功能即比较字符串str1和str2的前maxlen个字符。如果前maxlen字节完全相等，返回值就=0；在前maxlen字节比较过程中，如果出现str1[n]与str2[n]不等，则依次比较str1和str2的前n位，设i(i<n)为两字符串首次的不同位，则返回（str1[i]-str2[i] ).
用法
int strncmp ( const char * str1, const char * str2, size_t num );
程序代码：
#include<stdio.h>
#include<assert.h>
int my_strncmp(const char * first , const char * last , size_t count )
{
assert (first );
assert (last );
size_t x = 0;
if (!count )
{
return 0;
}
if (count >= 4)
{
for (x = 0; x < count - 4; x = x + 4)
{
first = first + 4;
last = last + 4;
if (*(first - 4) == 0 || *(first - 4) != *( last - 4))
{
return (*(unsigned char*)( first - 4) - *(unsigned char *)(last - 4));
}
else if (*(first - 3) == 0 || *( first - 3) != *(last - 4))
{
return (*(unsigned char*)( first - 3) - *(unsigned char *)(last - 4));
}
else if (*(first - 2) == 0 || *( first - 2) != *(last - 2))
{
return (*(unsigned char*)( first - 2) - *(unsigned char *)(last - 2));
}
else if (*(first - 1) == 0 || *( first - 1) != *(last - 1))
{
return (*(unsigned char*)( first - 1) - *(unsigned char *)(last - 1));
}
}
}
for (x = 0; x < count ; x++)
{
if (*first == 0 || *first != * last )
{
return (*(unsigned char*) first - *(unsigned char *)last);
}
first = first + 1;
last = last + 1;
}
return 0;
}
int main()
{
char str[] = "sdfvdfv" ;
char arr[] = "rcvnjtf" ;
int ret = my_strncmp(str, arr, 3);
printf( "%d\n" , ret);
system( "pause" );
return 0;
}
运行结果：
