data being printed out twice when using fread

Question

I am reading from a file using Binary File IO function fread When i run the program final entry is printed out twice

FILE *fp;
struct entry temp;
if (fp = fopen("recs.bin","rb"))
{
    while (!feof(fp))
    {
        fread(&temp,sizeof (struct entry),1,fp);
        printf("%s  %s  %s \n", temp.name ,temp.design ,temp.contact);
    }
    fclose(fp);
else 
{
      printf("\error opening file\n");
}

When i run the code the output is as following

    Pfor.ABC    Professor    9867552
    Sir Blah    lecturar     9237434
    Miss etc    Asst. Porfessor     03847363      
    Miss etc    Asst. Porfessor     03847363

I always get the last entry twice I tried by printing beofre reading like below

while (!feof(fp))
{
        printf("%s  %s  %s \n", temp.name ,temp.design ,temp.contact);
        fread(&temp,sizeof (struct entry),1,fp);
}

now output is

    #gd^&!d     $!du(!      #$@%@22        //Some garbage values
    Pfor.ABC    Professor    9867552
    Sir Blah    lecturar     9237434
    Miss etc    Asst. Porfessor     03847363

Now the printing twice is solved but a garbage value is being printed I think its the problem in feof(fp) but cant figure it out

`printing beofre reading`.. how does that sound to yourself?? doensn't it sound `//Some garbage values` ?? — Sourav Ghosh, Oct 31 '14 at 06:47
Check return value of `fread`. It fails to read last entry, but `temp` still holds old data. — keltar, Oct 31 '14 at 06:48
@Sourav i know but i was trying to figure out if it was being printed twice or read twice printing before reading made me clear that there is a problem in reading — Sayam Qazi, Oct 31 '14 at 06:49
@keltar then why !feof() not stopping it read further when file is ended — Sayam Qazi, Oct 31 '14 at 06:50
@SayamQazi because it checked after (failed) fread and printf. — keltar, Oct 31 '14 at 06:58

score 3 · Accepted Answer · edited May 23 '17 at 10:33

The double handling of the last entry comes from using feof, which is usually not a good way to control file Input loops.

Instead, you should check the return value of the reading functions fgetc, fgets and, in your case, fread. fread returns the number of items, i.e. the number of blocks of size bytes, read. If that is smaller than the number of items to read, you have an incomplete read. So:

while (fread(&temp, sizeof(struct entry), 1, fp) == 1) {
    printf("%s  %s  %s \n", temp.name, temp.design, temp.contact);
}

As long as you have a complete read, print out the data.

Others have already pointed out that printing the (uninitialised) data before reading it leads to your garbage values. If you must use such a set-up, you should at least place one read before the loop.

data being printed out twice when using fread

1 Answers1