fopen can't read a file(file always equal to NULL)

Question

I'm programming in code::blocks and I got problem reading file here is the code:

            char fileName[] ="";
            fflush(stdin);
            E11: printf("\n\tEnter the @ Mac (X:X:X:X:X:X) in Hex :");
            gets(MAC);
            E12: printf("\n\tEnter file name:");
            gets(fileName);
            FILE* fichier;
            fichier = fopen(fileName,"r");
            if (fichier == NULL)
            {
                printf("Error! Try Again");
                goto E12;
            }
            else
            {
                printf("sending Data....");
                //TODO..
            }

Doing `fflush(stdin)` is explicitly marked as undefined behavior in the C specification. Some standard libraries implement it, but if you want to be portable or standards-compliant you should not do that. — Some programmer dude, Dec 27 '15 at 14:05
Also, don't use labels and `goto` as loops, use actual loops. C might have started out as a high-level assembly kind of language, but it's not anymore. — Some programmer dude, Dec 27 '15 at 14:06
i forget to mention that i used 'scanf()' and 'getch()' before this code. — Konnichiwa sad, Dec 27 '15 at 16:34
i tried this too **'fichier = fopen("file.txt","r");'** but nothing new. — Konnichiwa sad, Dec 27 '15 at 16:38

score 3 · Answer 1 · answered Dec 27 '15 at 14:03

3

First of all, don't use gets(), use fgets() instead.

gets() seriously suffers from buffer overrun issue, which can be avoided by using fgets().

That said, coming to the main issue in your code, by saying

char fileName[] ="";

you're allocating a 1-char array, which is not enough to hold the input, anyway.

You should change that to

char fileName[64] = {0};

or something similar.

Also, fflush(stdin); is against the C standard, it invokes undefined behavior. Remove that statement.

answered Dec 27 '15 at 14:03

Sourav Ghosh

133,132
16
183
261

Doing `char fileName[64] = "";` would `0`-out all of `fileName` as well. – alk Dec 27 '15 at 14:08
but 'fgets()' reads from file ins't it wich is not my case here. – Konnichiwa sad Dec 27 '15 at 14:14
@alk Absolutely, matter of choice, sir. :) – Sourav Ghosh Dec 27 '15 at 14:14
1

@Konnichiwasad use `stdin` as input file. :) – Sourav Ghosh Dec 27 '15 at 14:14

score 0 · Answer 2 · answered Dec 27 '15 at 14:04

0

This program exhibits undefined behavior. filename is a char array with only one element. Try changing the declaration to char fileName[256] = "";

answered Dec 27 '15 at 14:04

owacoder

4,815
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score 0 · Answer 3 · edited May 23 '17 at 12:15

fflush(stdin) is undefined behavior.
gets is bad. From man gets:

Never use gets(). Because it is impossible to tell without knowing the data in advance how many characters gets() will read, and because gets() will continue to store characters past the end of the buffer, it is extremely dangerous to use. It has been used to break computer security. Use fgets() instead.

(Emphasis and code formatters added.)
The definition
```
char fileName[] = "";
```
only allocates one byte of memory because sizeof("") == 1. You need a more flexible approach, which is another argument for fgets. Just allocate some arbitrary number of bytes probably capable of storing the whole input and pass that size to fgets:
```
char fileName[32];

...

fgets(fileName, sizeof(fileName), stdin);
```
goto is bad.

Right, but there is another **major** issue in the code. :) – Sourav Ghosh Dec 27 '15 at 14:06 — Sourav Ghosh, Dec 27 '15 at 14:06
@SouravGosh Working on it. – cadaniluk Dec 27 '15 at 14:07 — cadaniluk, Dec 27 '15 at 14:07

score 0 · Answer 4 · answered Dec 27 '15 at 18:36

0

thank you all:

after trying and trying if found that the problem is the file that i'm trying to read which is lorem ipsum that i copied from: here. i can't belive it, it is so strange..

thanks for the answers.

answered Dec 27 '15 at 18:36

Konnichiwa sad

13
3

fopen can't read a file(file always equal to NULL)

4 Answers4