C语言如何实现取字符串的一部分？

可以用strncpy()函数

#include <string.h>

char sub[20];

strncpy(sub,string+12,13)
/*将string中从string[12]开始的13个数字复制到sub中*/

#include <stdio.h>

int main()
{
int i, j, length;
char a[] ="hello world";
int from, num;

scanf("%d",&from); //截取开始的字符下标
scanf("%d",&num); //共截取的字符数
char b[num];

for (length = 0; a[length]!= '\0'; length++){}
if (from>=0)
if (from+num <= length ){
i = from; j=0;
while ( j<num){
b[j] = a[i];
printf("%c",b[j]);
j++; i++;}
}return 0;
}
#include <stdio.h>
int main(){
int i, j, length;
char a[] ="hello world";
int from, num;
scanf("%d",&from); //截取开始的字符下标
scanf("%d",&num); //共截取的字符数
char b[num];

for (length = 0; a[length]!=