程序代码：

#include<stdio.h>

char *insert(char *s,char *p,int n)
{
    char *str,strcp[50];
    int i;
    str=strcp;
    for(i=0 ;*s != '\0' ;i++)
    {
        if(i == n-1)
            for( ;*p != '\0'; )
            {
                str[i]=*p;
                p++;
                i++;
            }
        str[i]=*s;
              s++;
    }

    str[i+1]='\0';
    return str;
}

void main()
{
    int n;
    char *a,*b;

    a="we are  friends";
    b="good";

    printf("please input the position\n");
    scanf("%d",&n);

    a=insert(a,b,n);
    printf("%s\n",a);
    getch();
}

程序代码：

#include <stdio.h>
#include <string.h>
#define SIZE 81

// to的空间不足或index越界运行结果不确定
char * str_insert(char * to, const char * from, int index) {
    int i, from_size = strlen(from);
    for(i = strlen(to) + from_size; i >= index + from_size; i--)
        to[i] = to[i - from_size];
    for(i = 0; i < from_size; i++)
        to[i + index] = from[i];
    return to;
}

int main(void) {
    char str1[SIZE] = "123456789";
    char str2[SIZE] = "123";
    str_insert(str1, str2, 9);
    printf("%s\n", str1);
    str_insert(str1, str2, 3);
    printf("%s\n", str1);
    return 0;
}
/* Output:
123456789123
123123456789123

Process returned 0 (0x0)   execution time : 0.016 s
Press any key to continue.
*/

程序代码：

char s1[200],s2[200];
   int j,k,n;
   printf("input string s1:\n");
   gets(s1);
   printf("input string s2:\n");
   gets(s2);
   printf("input the insert location:\n");
   scanf("%d",&n);
   if (n>strlen(s1))
     n=strlen(s1);
   for (j=n,k=strlen(s2);j<strlen(s1);j++,k++)
     s2[k]=s1[j];
   s2[k]='\0';
   s1[n]=0;
  strcat(s1,s2);
   printf("the after insert,s1=[%s]\n",s1);

程序代码：

void insert (char *a,char *b,int n) {
        char *c;
        int i=0;

        while(*(a+i)&&i<n) {
                *(c+i)=*(a+i);
                i++;
        }

        while(*b) *(c+(i++))=*b++;

        while(*(a+n)) *(c+(i++))=*(a+(n++));

        *(c+i)='\0';

        strcpy(a,c);
}