为啥我的len显示未声明标识符

Question

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

int mystrlen(char c[]){
    int i=0;
    while(c[i++]);
    return i-1;
}

int main() {
    char a[20];
    char c[20]="world";
    gets(a);
    puts(a);
    len=strlen(a);
    printf("len=%d\n",len);
    len=mystrlen(a);
    printf("mystrlen=%d\n",len);
    strcat(a,c);
    strcpy(c,a);
    puts(a);
    puts(c);
    printf("c?d %d\n",strcmp(c,"how"));
    return 0;
}

Answer 1

int len;