c language : fwrite doesnt do anything

Question

{
    FILE* f1 = fopen("C:\\num1.bin", "wb+");//it will create a new file 
    int A[] = { 1,3,6,28 }; //int arr
    fwrite(A, sizeof(A), 1, f1); //should insert the A array to the file
}

I do see the file but even after the fwrite, the file remains empty (0 bytes), does anyone know why?

What is the value of `f1`? Why does your code not check it? And why don't you `close()` the file? From https://linux.die.net/man/3/fopen "Upon successful completion fopen(), fdopen() and freopen() return a FILE pointer. Otherwise, NULL is returned and errno is set to indicate the error." See https://stackoverflow.com/questions/503878/how-to-know-what-the-errno-means — Mawg says reinstate Monica, Feb 01 '18 at 19:22

score 7 · Answer 1 · answered Feb 01 '18 at 19:22

7

You need to close the file with fclose

Otherwise the write buffer will not (necessarily) force the file contents to be written to disk

answered Feb 01 '18 at 19:22

Grantly

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Or `fflush(f1);` – Thomas Padron-McCarthy Feb 01 '18 at 19:27
2

If the program does a normal exit, the file streams should be closed, so the buffers should be flushed ([C11 §7.22.4.4 The `exit` function ¶4](https://port70.net/~nsz/c/c11/n1570.html#7.22.4.4p4) and [C11 §5.1.2.2.3 Program Termination](https://port70.net/~nsz/c/c11/n1570.html#5.1.2.2.3)). – Jonathan Leffler Feb 02 '18 at 05:55

Govind Parmar · Answer 2 · 2023-01-27T02:30:52.580

A couple of things:

As @Grantly correctly noted above, you are missing a call to fclose or fflush after writing to the file. Without this any cached/pending writes will not necessarily be actually written to the open file.
You do not check the return value of fopen. If fopen fails for any reason it will return a NULL pointer and not a valid file pointer. Since you're writing directly to the root of the drive C:\ on a Windows platform, that's something you definitely do want to be checking for (not that you shouldn't in other cases too, but run under a regular user account that location is often write protected).

score 2 · Answer 3 · answered Feb 01 '18 at 19:29

Result of fwrite is not required to appear in the fille immediately after it returns. That is because file operations usually work in a buffered manner, i.e. they are cached and then flushed to speed things up and improve the performance.

The content of the file will be updated after you call fclose:

fclose()

(...) Any unwritten buffered data are flushed to the OS. Any unread buffered data are discarded.

You may also explicitly flush the internal buffer without closing the file using fflush:

fflush()

For output streams (and for update streams on which the last operation was output), writes any unwritten data from the stream's buffer to the associated output device.

c language : fwrite doesnt do anything

3 Answers3