一点关于putc();的问题

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

int main(int argc,char *argv[])
{
    int ch;
    FILE *fp;
    FILE *f;
    long count=0;

    if(argc!=2)
    {
        printf("Usage: %s filename\n",argv[0]);
        exit(1);
    }
    if((fp=fopen(argv[1],"r"))==NULL)
    {
        printf("Can't open %s\n",argv[1]);
        exit(1);
    }
    f=fopen(argv[2],"a+")

    while((ch=getc(fp))!=EOF)
    {
        putc(ch,f);
        count++;
    }
    fclose(fp);

    printf("\nFile %s has %ld charactters\n",argv[1],count);
    return 0;
}

为什么不可以将FILE指针fp指向的文件的内容追加到FILE指针f指向的文件中？?