不知道为什么插入串2后输出会出现乱码，希望有人能教一下要怎么修改

程序目的是为将串2插入串1某一指定位置中
不知道为什么插入串2后输出会出现乱码，希望有人能教一下要怎么修改
插入函数在35行

#include
#include
#define MaxSize 100
#define false 0
#define true 1

typedef struct str
{
    char data[MaxSize];
    int length;
}SqString;

void InitSqString(SqString* str1, SqString* str2)
{
    str1->length = str2->length = 0;
}

void StrAssign(SqString* str1, SqString* str2,char *a,char *b)
{
    int i=0;
    while (a[i]!='\0')
    {
        str1->length++;
        str1->data[str1->length - 1] = a[i];
        i++;
    }
    i = 0;
    while (b[i] != '\0')
    {
        str2->length++;
        str2->data[str2->length - 1] = b[i];
        i++;
    }
}
int InsertStr(SqString* str1, SqString* str2, int post)
{
    if (post<0 || post-1>str1->length) {
        printf("error\n");
        return false;
    }
    int i;
    for (i = post - 1; i < str1->length; i++)
    {
        str1->data[post + str2->length] = str1->data[i];
    }
    for (i = 0; i < str2->length; i++)
    {
        str1->data[post - 1 + i] = str2->data[i];
    }
    str1->length += str2->length;
    return true;
}

void printStr(SqString *str1)
{
    int i;
    for (i = 0; i < str1->length; i++)
    {
        printf("%c", str1->data[i]);
    }
}

int main()
{
    SqString str1,str2;
    char a[10] = "abcde", b[10] = "fghij";
    int post;
    InitSqString(&str1, &str2);
    StrAssign(&str1, &str2, a, b);
    printf("please enetr postion of you want to insert\n");
    scanf_s("%d", &post);
    InsertStr(&str1, &str2, post);
    printStr(&str1);
}

你初始化的时候，需要将字符数组所有元素初始化为0。你的代码缺少字符串结束符
另外插入函数中，第一个for循环应该采用倒序移动，否则可能会产生字符覆盖现象，修改如下：

#include<stdio.h>
#include<stdlib.h>
#include <string.h>
#define MaxSize 100
#define false 0
#define true 1
 
typedef struct str
{
    char data[MaxSize];
    int length;
}SqString;
 
void InitSqString(SqString* str1, SqString* str2)
{
    str1->length = str2->length = 0;
    memset(str1->data,0,MaxSize);
    memset(str2->data,0,MaxSize);
}
 
void StrAssign(SqString* str1, SqString* str2,char *a,char *b)
{
    int i=0;
    while (a[i]!='\0')
    {
        str1->length++;
        str1->data[str1->length - 1] = a[i];
        i++;
    }
    i = 0;
    while (b[i] != '\0')
    {
        str2->length++;
        str2->data[str2->length - 1] = b[i];
        i++;
    }
}
int InsertStr(SqString* str1, SqString* str2, int post)
{
    if (post<0 || post-1>str1->length) {
        printf("error\n");
        return false;
    }
    int i;
    for(i=str2->length-1;i>=0;i--)
    {
        str1->data[i + str2->length] = str1->data[i];
    }
    for (i = 0; i < str2->length; i++)
    {
        str1->data[post - 1 + i] = str2->data[i];
    }
    str1->length += str2->length;
    return true;
}
 
void printStr(SqString *str1)
{
    int i;
    for (i = 0; i < str1->length; i++)
    {
        printf("%c", str1->data[i]);
    }
}
 
int main()
{
    SqString str1,str2;
    char a[10] = "abcde", b[10] = "fghij";
    int post;
    InitSqString(&str1, &str2);
    StrAssign(&str1, &str2, a, b);
    printf("please enetr postion of you want to insert\n");
    scanf_s("%d", &post);
    InsertStr(&str1, &str2, post);
    printStr(&str1);
}