关于realloc()使用的问题

Question

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

typedef struct HString {
    char *ch=NULL;  //需要初始化
    int length=0;
}HString;

void hsAssign(HString &T,char s[]) {
    if (T.ch)
        free(T.ch);
    T.ch = (char*)malloc(sizeof(char));
    if (!T.ch) {
        printf("Fail to create the string.");
        return;
    }
    T.length = strlen(s);
    for (int i = 0; i < strlen(s); i++)
        T.ch[i] = s[i];
}

int hsCompare(HString T, HString S) {
    //若S>T，则返回值>0
    for (int i = 0; i < T.length&&i < S.length; i++)
        if (T.ch[i] != S.ch[i]) 
            return S.ch[i] - T.ch[i];  //返回的是ASCII码的差
    return S.length - T.length;
}

int hsClear(HString &T) {
    free(T.ch);
    T.ch = NULL;
    T.length = 0;
    return 1;//确认清空串操作是否成功
}

void hsConcat(HString &con, HString T, HString S) {
    if (con.ch)
        free(con.ch);
    con.ch = (char*)malloc(sizeof(char));//确保con被分配了新的存储空间
    for (int i = 0; i < T.length; i++)
        con.ch[i] = T.ch[i];
    for (int i = T.length; i < T.length +  S.length; i++)
        con.ch[i] = S.ch[i - T.length];
    con.length = T.length + S.length;
}

void hsSub(HString &sub, HString T, int pos, int len){
    if (pos -1> T.length || pos <= 0 || pos + len -1> T.length || len < 0) {
        printf("Wrong input.");
        return;
    }
    int count = 0;
    if (sub.ch)
        free(sub.ch);
    sub.ch = (char*)malloc(sizeof(char));
    for (int i = pos - 1; i <=pos + len - 2; i++)
        sub.ch[count++] = T.ch[i];
    sub.length = len;
}

void hsInsert(HString &S, int pos, HString T) {
    int count = 0;
    if (pos <= 0 || pos - 1 > S.length) {
        printf("Wrong input");
        return;
    }
    for (int i = S.length - 1; i >= pos - 1; i--)
        S.ch[i + T.length] = S.ch[i];
    for (int i = pos - 1; i <= pos + T.length - 2; i++)
        S.ch[i] = T.ch[count++];
    S.length += T.length;
}

void hsPrint(HString T) {
    for(int i=0;i<T.length;i++)
        printf("%c", T.ch[i]);
}

int main() {
    HString T, S1,S2;
    char s[20];
    scanf_s("%s", s, 20);
    hsAssign(S1, s);
    scanf_s("%s", s, 20);
    hsAssign(S2, s);
    hsInsert(S1, 2, S2);
    hsPrint(S1);
    return 0;
} 

在hsInsert()函数中如果使用了S.ch=(char*)realloc(S.ch,(T.length+S.length)*sizoef(char));
结果是tTEST
而如果不用realloc()函数 结果是tTESTest
请问这是为什么？

Answer 1

https://blog.csdn.net/antiquedwr/article/details/50521623