关于#c语言#的问题，请各位专家解答！

#define MAXLEN 25
typedef struct{
    char ch[MAXLEN];
    int length;
}SString;
 Substring(SString &Sub,SString S,int pos,int len){
    if(pos+len-1>S.length)return false;
    for(int i=pos;iSub.ch[i-pos]=S.ch[i];
    Sub.length=len;}
    printf("%s",Sub.ch);
    
    
}
int main(){
   SString Sub;
   SString S;
   
   for(int i=0;i"%c",&S.ch[i]);
  }
   Substring(Sub,S,1,4);
}

修改如下，改动处见注释，供参考：

#include <stdio.h>
#define MAXLEN 25
typedef struct {
    char ch[MAXLEN];
    int length;
}SString;
void Substring(SString& Sub, SString S, int pos, int len) { //修改
    int i;      //修改
    if (pos + len > S.length || pos < 0) return; // false; 修改
    for (i = pos; i < pos + len; i++)
    {
        Sub.ch[i - pos] = S.ch[i];
    }
    Sub.ch[i - pos] = '\0'; //修改
    Sub.length = len;       //修改
    
    printf("%s", Sub.ch);
}
int main() {
    int  i;       //修改
    SString Sub;
    SString S;
    scanf("%s", S.ch);  //修改
    for (i = 0; S.ch[i]; i++); //修改
    S.length = i;      //修改  
    //{
          //scanf("%c", &S.ch[i]);
    //}
    Substring(Sub, S, 1, 4);
    return 0;
}

Substring 函数定义有问题，没有返回值，要么你就定义 void

#include<stdlib.h>
#include<stdio.h>
#include<math.h>
#define MAXLEN 10
typedef struct{
    char ch[MAXLEN];
    int length;
}SString;


void Substring(SString &Sub,SString S,int pos,int len){
    if(pos+len-1 > S.length)
        return;
    int i;
    for(i=pos;i<pos+len;i++)   
    {
        Sub.ch[i-pos]=S.ch[i];
        Sub.length=len;
    }
    Sub.ch[i-pos] = '\0';
}

int main(){
    SString Sub;
    SString S;
   
    scanf("%s", S.ch);
    S.length = sizeof(S.ch) - 1;

    Substring(Sub,S,1,4);

    printf("%s", Sub.ch);
}