C语言函数编写并调用

这个代码用来计算文件长度有什么问题啊为什么会是输出-1呢？

#include 
long filesize( FILE *fp)
{
    long cur_pos,length;
    cur_pos = ftell(fp);
    fseek(fp,0,SEEK_END);
    length=ftell(fp);
    fseek(fp,cur_pos,SEEK_SET);
    return length;
}
void main(int argc, char *argv[])
{
    FILE *fp;
    char   Name[20];
    fp=fopen("test.txt","rb");
    fgets(Name,20,fp);
    printf("file length is %ld.\n",filesize(fp));
    fgets(Name,20,fp);
    fclose(fp);
}

这个应该是默认的吧，像find函数，我改了一下

 
#include <stdlib.h>
#include <stdio.h>
long filesize( FILE *fp)
{
    long cur_pos,length;
    cur_pos = ftell(fp);
    fseek(fp,0,SEEK_END);
    length=ftell(fp);
    fseek(fp,cur_pos,SEEK_SET);
    return length;
}
int main(int argc, char *argv[])
{
    FILE *fp;
    char   Name[20];
    fp=fopen("test.txt","r");
    fgets(Name,20,fp);
    long long n=filesize(fp);
    if(n==-1)
    {
        n=0;
    }
    printf("file length is %ld.",n);
    fgets(Name,20,fp);
    fclose(fp);
}

文件打开成功了吗？