fscanf() fclose() or reading file exit and ends program

Question

Below is a part of my code which is having issues with file handling. The file opens fine with fopen but when I try to read or just close the file my program exits without an error. I tried to run this code independently and it works fine. Would really appreciate if someone could help me out which pointing out what I am doing wrong.

    int ctrlSend(char *etherPort, uint8_t *inPayload, int payloadLen, int vlanID) 
    {
    char intName [10]; // Interface name from file
    int intVlan; // Interface VLAN from file
    printf("In ctrlSend\n");

    FILE * pFile; // File pointer
    pFile = fopen ("vlan.conf","r");

    while(!feof(pFile))
    {
      fscanf(pFile,"%s %d",intName,&intVlan)
      printf("In ctrlSend while loop");
    }


    fclose (pFile);
    return 0;
    }

UPDATE1: Updated above code

UPDATE2: Alternate code below which has same issue.

    int ctrlSend(char *etherPort, uint8_t *inPayload, int payloadLen, int vlanID) 
    {

    printf("In ctrlSend\n");

    char intName [10]; // Interface name from file
    int intVlan; // Interface VLAN from file
    FILE * pFile; // File pointer
    pFile = fopen ("vlan.conf","r");

    while (fscanf (pFile,"%s %d",intName,&intVlan) == 2)
    {
        printf("In ctrlSend while loop");
    }


    fclose (pFile);
    return 0;
    }

UPDATE3: Seems like the file is not opening, looking into it.

The while loop won't terminate for a non-empty file. Looks like an opposite problem from what you are describing. — Eugene Sh., Sep 20 '17 at 15:41
How do you know that the file "opens fine", since you do not test the return value of `fopen()` before using it? — John Bollinger, Sep 20 '17 at 15:42
Also see [Why is “while ( !feof (file) )” always wrong?](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — Andrew Henle, Sep 20 '17 at 15:44
@Jean-FrançoisFabre Is the EOF indicator updating on IO only? — Eugene Sh., Sep 20 '17 at 15:44
How do you know that added-by-a-recent-edit call to `fscanf()` works? You're are not checking *anything* for errors. — Andrew Henle, Sep 20 '17 at 15:48
@JohnBollinger Good question, I assumed its reading the file as it was working when I ran the code independently. — ankushkool, Sep 20 '17 at 15:54
Do not make assumptions about program behavior, @ankushkool. Always check functions' return values for error indicators, if indeed they provide any, except in the rare cases where you don't actually care whether they succeeded. — John Bollinger, Sep 20 '17 at 16:00
Do learn to use a debugger. These are very handy for figuring out details of how what your program is actually doing differs from what you think it ought to be doing. — John Bollinger, Sep 20 '17 at 16:01

score 2 · Answer 1 · answered Sep 20 '17 at 15:42

2

When you do while (!feof ...) you check each time if you have reach the end of the file. However, at no point your advance in the file (fread ?). That means this will never terminate.

answered Sep 20 '17 at 15:42

TDk

1,019
6
18

1

And `while (!feof ...)` [is always wrong anyway](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) because it returns true only *after* an attempt to read *past* the end of the file occurs. – Andrew Henle Sep 20 '17 at 15:45
1

Although this answer makes statements about the pre-edit question that appear to be true, it does not *answer* the question, which asks why the program exits silently and unexpectedly (exactly what this answer asserts it will not do). As the question original stood, this would have been better as a comment; now it is just wrong, and should be deleted. – John Bollinger Sep 20 '17 at 15:50

score 0 · Accepted Answer · answered Sep 20 '17 at 16:02

0

Check whether the file exists or not. You should always check whether File pointer is NULL or not after opening the file. I think your program is unable to open the file and you are trying to use the file pointer without checking which is causing undefined behavior.

answered Sep 20 '17 at 16:02

kadina

5,042
4
42
83

Thats a great advice, the file was not opening correctly. Thanks – ankushkool Sep 20 '17 at 16:04

fscanf() fclose() or reading file exit and ends program

2 Answers2