#include<stdio.h>
#include<stdlib.h>
//#define NULL 0         
typedef struct node
{
    char ch;      
    struct node *next;
}STRNODE;                      //串的链式存储结构表示
            
int str_len(STRNODE *s)            //串的长度计算函数/
{
    return s->ch;                        
}

void str_input(STRNODE *s)          //串的输入函数/
 {
    int i=0;                  
    char ch;
    STRNODE *p,*q;
    p=s;
    q=s->next;
    s->ch=0;
    while((ch=getchar())!='\n')         //通过循环，从键盘输入一串字符串
    {
        q=(STRNODE *)malloc(sizeof(STRNODE));
        q->ch=ch;
        q->next=NULL;
        p->next=q;
        p=q;
        s->ch++;
    }   
}
STRNODE *str_sub(STRNODE *sub,STRNODE *s,int pos,int len)
{//将串s第pos个字符开始的长度为len的字符序列复制到串sub中
    int i=1;
    STRNODE *p,*q;
    p=s->next;
    q=sub->next;

    for(i=1;i<=pos;i++)
    {
      p=p->next;
    }
    for(i=1;i<=len;i++)
    {q=(STRNODE *)malloc(sizeof(STRNODE));
    q->ch=p->ch;
    q=q->next;
    p=p->next;
    printf("0\n");
    }
    sub->ch=len;
    return s;
    return sub;

}
void str_print(STRNODE *s)
{//字符串输出显示
    int i;
    STRNODE *p;
    p=s->next;
    for(i=0;i<s->ch;i++)
    {
        putchar(p->ch);
        p=p->next;
    }
    printf("\n");
}
void showmenu()
{         
    printf("    串操作\n");
    printf("\t4、求子串\n");
    printf("\t5、退出\n");   
}void main()
{
       int position,length,no;
    STRNODE str,str1,str2;   
    str.ch=0;
    str1.ch=0,str2.ch=0;
    while(1)
    {
        showmenu();
        printf("    请输入你的选择：");
        scanf("%d",&no);
        switch(no)
        {
            case 4:printf("请输入一串字符：");
                   fflush(stdin);
                   str_input(&str1);
                   printf("请输入取子串的位置和长度：");
                   scanf("%d%d",&position,&length);
                   str_sub(&str,&str1,position,length);
                   printf("子串为：");
                   str_print(&str1);
                   str_print(&str);
                   system("pause");
                   system("cls");
                   break;
               case 5:
                   return;
        }
    }        
}
我好像是做了这个操作但是会卡在输出串str那里..想不出来该怎么做了，请dalao们指点我一下

链串，看你说的应该不是块链串，相当于一个特殊的线性表
假设要截取从 start 开始后面的 length 个
基本思路就是定义一个计数变量，当计数变量等于 start 的时候，向后面走 length 个

char *sub(int start, int length) {
    char *substring = (char *)malloc(sizeof(char) * (length - start));
    int count = 0;
    node *n = s;
    while(count != start) {
        count++;
        if(count = start) {
            break;
        }
        n = s->next;
    }
    for(int i = 1, j = 0; i <= length; i++, j++) {
        substring[j] = n->ch;
    }
    return substring;
}