怎么读了文本就不能写入文本了

#include <stdio.h>
#include <stdlib.h>
int main()
{ char a,c='1';
FILE *q;
q=fopen("top.txt","r+");
fscanf(q,"%c",&c);
fprintf(q,"%c",c);
fputc(c,q);
fclose(q);
system("pause");
return 0;
}