Write chars into file C++

Question

I have a function that writes random numbers into a file while reading another file.

void writeFile() {
    FILE* file = fopen(source, "r");
    FILE* file2 = fopen(target, "w");
    srand (time(NULL));

    while (!feof(file)) {
        fgetc(file);
        fputc(0 + ( rand() % ( 50 - 0 + 1 ) ), file2);
    }


    fclose(file);
    fclose(file2);
}

The two files should have the same size. What happens is that the second file has more 1byte at the end compared with the first file. How can I avoid this?

[`while (!eof())` is wrong.](http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong) — chris, Dec 01 '13 at 22:24
checking for EOF doesn't magically happen between the two lines of your while block. — Lightness Races in Orbit, Dec 01 '13 at 22:25
If feof worked by predicting whether the next call to fgetc would return EOF then your code would be good. Unfortunately for you and the million other programmers who write code like this, it doesn't work like that. — john, Dec 01 '13 at 22:28

score 2 · Answer 1 · answered Dec 01 '13 at 22:25

2

As well as the EOF comments, you should also open both files in binary mode

FILE* file = fopen(source, "rb");
FILE* file2 = fopen(target, "wb");

In non-binary mode line endings may be translated (depending on platform). This potentially changes the number of characters read or written to a file.

answered Dec 01 '13 at 22:25

john

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Good spotting! =) Perhaps add a quick one-liner to explain why this matters. – paddy Dec 01 '13 at 22:26

score 1 · Accepted Answer · answered Dec 01 '13 at 22:24

1

Don't rely on feof to tell you that you've read all characters. Instead, check the return value from fgetc:

while (fgetc(file) != EOF) {
    fputc(0 + ( rand() % ( 50 - 0 + 1 ) ), file2);
}

answered Dec 01 '13 at 22:24

paddy

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score 1 · Answer 3 · answered Dec 01 '13 at 22:24

1

The very final fgetc(file) reads EOF, then your code writes a byte for it into the new file and only then it tests for feof in the while.

Don't use feof, use this instead:

if (fgetc(file) == EOF) break;

answered Dec 01 '13 at 22:24

Jongware

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Write chars into file C++

3 Answers3