C语言提取字符串

在只给<stdio.h>和<string>头文件的情况下，从下文中提取UserId=123456

https://www.cnblogs.com/ysjd UserId=123456 passwd=513920

代码如下：

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

int myatoi(char str[])
{
    int len=strlen(str);
    char *p=str+len-1;
    int sum=0,i=0,num;
    while(p>=str)
    {
        num=*p-48;
        for(int j=0;j<i;j++)
        {
            num*=10;
        }
        i++;
        sum+=num;
        p--;
    }
    return sum;
}

int getUserId(char *p,int *n)
{
    char *q=strstr(p,"UserId=");
    if(q==NULL)
        return 0;
    q+=7;
    char str[20];
    int i=0;
    while(*q>='0' && *q<='9')
    {
        str[i]=*q;
        i++;
        q++;
    }
    str[i]='\0';
    *n=myatoi(str);
    return 1;
}
int main()
{
    char str[]="https://www.cnblogs.com/ysjd UserId=123456 passwd=513920";
    int n;
    int ret=getUserId(str,&n);
    if(ret==0)
    {
        printf("not found\n");
49     }
    else
    {
        printf("UserId=%d\n",n);
    }
}