fopen gives no error, but file stays empty

Question

I'm having trouble writing in a file, but the problem is that weird, I dont't even know for what I should exactly ask for.

At first: I use C, I am forced to keep it concurring to the 89/90-standard an to compile with gcc. My OS is Windows 7 Home Premium, 64Bit.

What I want to do, is to open three filestreams like this:

FILE *A=fopen("A.txt", "r");
FILE *B=fopen("B.txt", "w");
FILE *D=fopen("C.txt", "w");

The first one only to read, the two others to write. During the program, I write with

fprintf(B, "%c", letter);
fprintf(D, "%c", letter);

integers, interpreted as ASCII-charachters into the files. I did this twenty times before, it always worked. Now, the "D" file stays empty. If I change the order of the streams to:

FILE *A=fopen("A.txt", "r");
FILE *D=fopen("C.txt", "w");
FILE *B=fopen("B.txt", "w");

my file "B" stays empty! So always the last one opened does not work. But why?! I can't see any difference to my other programms, which are working, and this program also works, except in the case of this third filestream. I'm compiling with -Wall and -pedantic, the Compiler is not complaining, the Programm ist not crashing, everything works but the third stream!

Has anyone any idea, or even better, experience with a problem like this? I tried about one hour without getting any clue.

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Edit: Thanks for alle the comments! I'm programming with C for about two months till now, so there are many things, I'm not really sure of.

@Mhd.Tahawi: Yes, now I did, but no difference.

@Kninnug: The files get opened succesfully.

@Bit Fiddling Code Monkey: Yes, as far as I know, there is a limit somewhere. But I worked with 6 filestreams at the same time a week ago and everything was fine.

@Pandrei: Yes, the Output in the other file was fine.

@Martin James: I'm not sure what you mean. If you mean fclose or something similar: Since so many of you asked me for this, I tried it, but there was no difference.

@hmjd: 'letter' is an integer that gets its value by fgetc(A);

@squeamish ossifrage: I'm printing sign by sign into the file an get '1' for each, seems to be ok.

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Edit2: Now, that's nice, I'm facing some kind of Heisenbug. I was going to check the return values, as so many of you told me to.

So I initalized a new int retval=0; and wrote:

letter=fgtec(A);
while (letter!=EOF)
{
    retval=fprintf(B, "%c", letter);
    printf("%d", retval); /*Want to see the return values on stdout*/
    if (retval<0) /*If writing fails, the program stops*/
    {
        printf("Error while writing in file");
        return (-1);
    }
    letter=fgetc(A);
}

And this one works! New file gets filled, everything is fine. Except for the fact, that I have absolutely no idea, why the old one:

letter=fgtec(A);
while (letter!=EOF)
{
    fprintf(B, "%c", letter);
    letter=fgetc(A);
}

that worked tenthousand times before, didn't do anything at all.

Has anyone any further ideas? However, thanks so far for your ideas!

Answer 1

I think, I found the source of the problem, so I thought it might be helpful to share, that it seems to be compiler-related. As written above, I use gcc as part of the MinGW-package. I asked a friend of mine, who uses Pelles C, to compile the code himself and voila! For him, it worked fine! So it seems like gcc has, under certain conditions, possibly problems with filestreams. But I can't really say, which conditions.

However, if anyone else has a problem like this, try it with another compiler!

Answer 2

Unless you called fclose() your standard library will buffer the output data and you won't see it on disk until you either write enough, or flush it via fflush() or close the file via fclose() (or disable buffering via setbuf()/setvbuf()).

Obviously, disabling buffering or flushing after each I/O operation obviates the whole purpose of buffered I/O making performance suboptimal.