Make sure fscanf returns 1:
#include<stdio.h>
int main(){
FILE *p;
int b=0, a=0;
p=fopen("text.txt", "a+");
while((b=fscanf(p,"%d",&a)) == 1)
printf("%d\n",a);
// no need to seek, or flush
fprintf(p, " %d %d",1,6);
fclose(p);
return 0;
}