fscanf和feof的組合使用

http://stackoverflow.com/questions/15719360/using-fscanf-using-feof

靶子代碼：

#include<stdio.h>
void main(){
    FILE *fp;
    int a,b;
    fp=fopen("hello.txt","r");
    while(!feof(fp)){
      fscanf(fp,"%d %d",&a,&b);
      printf("%d %d\n",a,b);
    }
}

My hello.txt is

1 2

3 4

My Output is

1 2

3 4

4 4


解決方法

1. It means that the way the feof() function (and other functionality with regards to EOF in general) is used is often misunderstood and wrong. So is your code.

First, fscanf() doesn't always do what you think it does, and getting lines from a file is better performed using fgets(). However, if you're really inclined to use fscanf(), then check if it could read someting at all, else when it couldn't, you will print the variables one time more than needed. So what you should do is:

while (fscanf(fp,"%d %d",&a,&b) == 2)
{
    printf("%d %d\n",a,b);
}

 

2.The reason you're getting an extra line is that EOF isn't set until after fscanf tries to read a third time, so it fails, and you print the results anyway. This would do the sort of thing you've intended

while(1){
  fscanf(fp,"%d %d",&a,&b);
  if (feof(fp))
     break;
  printf("%d %d\n",a,b);
}