用C语言实现字符串替换函数.字符串真纠结,该怎么解决

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

char *replace(char *src, char *sub, char *dst)
{
    int pos = 0;
    int offset = 0;
    int srcLen, subLen, dstLen;
    char *pRet = NULL;

    srcLen = strlen(src);
    subLen = strlen(sub);
    dstLen = strlen(dst);
    pRet = (char *)malloc(srcLen + dstLen - subLen + 1);//(外部是否该空间)
    if (NULL != pRet)
    {
        pos = strstr(src, sub) - src;
        memcpy(pRet, src, pos);
        offset += pos;
        memcpy(pRet + offset, dst, dstLen);
        offset += dstLen;
        memcpy(pRet + offset, src + pos + subLen, srcLen - pos - subLen);
        offset += srcLen - pos - subLen;
        *(pRet + offset) = '\0';
    }
    return pRet;
}

------解决方案--------------------

C/C++ code

#include <stdio.h>
#include <string.h>
char s[256];
char *p;
int r,n,i;
int main() {
    while (1) {
        printf("请输入一行文字(空行结束),\"%%20\"将替换为\" \",\"你懂得\"将替换为\"XXXXXX\":\n");
        fgets(s,256,stdin);
        if ('\n'==s[0]) break;
        p=s;
        while (1) {
            p=strstr(p,"%20");
            if (p) {
                memmove(p+1,p+3,strlen(p)-3+1);
                p[0]=' ';
            } else break;
        }
        p=s;
        while (1) {
            p=strstr(p,"你懂得");
            if (p) {
                memmove(p+6,p+6,strlen(p)-6+1);
                for (i=0;i<6;i++) p[i]='X';
            } else break;
        }
        printf("%s",s);
    }
    return 0;
}
//请输入一行文字(空行结束),"%20"将替换为" ","你懂得"将替换为"XXXXXX":
//abcdefg%20helloworld%20something.pdf
//abcdefg helloworld something.pdf
//请输入一行文字(空行结束),"%20"将替换为" ","你懂得"将替换为"XXXXXX":
//这是测试文字你懂得，在这个你懂的地方，就得做你懂得的事
//这是测试文字XXXXX，在这个你懂的地方，就得做XXXXX的事
//请输入一行文字(空行结束),"%20"将替换为" ","你懂得"将替换为"XXXXXX":
//