怎么把一个不定长的string字符串嵌入到另一个字符串中

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
main()
{
   char a[10],b[20],c[5]=" is ";
   char *p="this %1% a good idea";
   sscanf(p,"%s",a);
   sscanf(p,"%*s%*s %[^'/0']",b);
   strcat(a,c);
   strcat(a,b);
   puts(a);
}

------解决方案--------------------
用%s可以简单的实现：

C/C++ code

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

char* strpl(const char* str, const char* substr)
{
    char* p = (char*)malloc(strlen(str) * sizeof(char));
    if(p != NULL)
        sprintf(p, str, substr);
    return p;
}

int main(int argc, char* argv[])
{
    printf("%s\n", strpl("this %s a good idea", "is"));
    printf("%s\n", strpl("this %s a good idea", "is not"));

    return 0;
}