CreateFile always override the specified file

Question

I'm trying to log the actions made by a service I wrote using the Windows-API & C-language, so I made a log file system.

The problem is that at each CreateFile call, the file is overridden instead of just opening it and write at the end of the file.

Here's the code of my WriteInLogfile function :

void WriteInLogFile(LPCTSTR log_string)
{
    HANDLE hFile;
    DWORD dBytesWritten;

    if ((hFile = CreateFile(LOG_FILE_PATH, GENERIC_WRITE, 0, NULL, 
                            OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL)) == INVALID_HANDLE_VALUE
        && (GetLastError() == ERROR_FILE_NOT_FOUND))
    {
        if ((hFile = CreateFile(LOG_FILE_PATH, GENERIC_WRITE, 0, NULL, 
                                CREATE_NEW, FILE_ATTRIBUTE_NORMAL, NULL)) != INVALID_HANDLE_VALUE)
        {
            if (!WriteFile(hFile, log_string, strlen(log_string), &dBytesWritten, NULL))
                aff_error("WriteInLogFile");
            CloseHandle(hFile);
        }
    }
    else
    {
        if (!WriteFile(hFile, log_string, strlen(log_string), &dBytesWritten, NULL))
            aff_error("WriteInLogFile");
        CloseHandle(hFile);
    }
}

Do someone know where the issue comes from ?

Thanks ;)

By the way, you can simplify this code by using `OPEN_ALWAYS`, which tells `CreateFile` to open the file if it exists or to create the file if it doesn't. Then you won't have to duplicate so much of your code. — Adrian McCarthy, Mar 27 '12 at 16:24

score 6 · Accepted Answer · answered Mar 27 '12 at 14:46

6

Even though you're opening the existing file you're not specifying that you want to append to it. Hence it opens as a generic write and you end up overwriting the contents. You need to pass the FILE_APPEND_DATA flag to the CreateFile method. This is best done by using the FILE_GENERIC_WRITE flag which includes FILE_APPEND_DATA

if ((hFile = CreateFile(LOG_FILE_PATH, FILE_GENERIC_WRITE, 0, NULL, 
                            OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL)) == INVALID_HANDLE_VALUE
        && (GetLastError() == ERROR_FILE_NOT_FOUND))
    {

answered Mar 27 '12 at 14:46

JaredPar

733,204
149
1,241
1,454

This works fine. Do you know if there's a way to keep newlines with this ? It seems that it skips mine... – BMN Mar 27 '12 at 14:49
@tsabz new lines shouldn't be changed if you use this flag. Are you having a problem writing them out? You may need to add them manually as `WriteFile` itself won't add them for you . – JaredPar Mar 27 '12 at 14:50
I make my call to the function like this : `WriteInLogFile("LOG: somethingtowrite\n");` and it doesn't keep the newline. – BMN Mar 27 '12 at 14:52
1

@tsabz Try `\r\n` instead of `\n` – JaredPar Mar 27 '12 at 14:58
Using `FILE_GENERIC_WRITE` didn't work for me. Directly using `FILE_APPEND_DATA` did! Wonder why. – CinCout Mar 01 '16 at 06:01

Jerry Coffin · Answer 2 · 2013-04-11T16:22:08.177

2

When you open a file, the pointer will always be positioned to the beginning of the file. To append, you need to explicitly seek to the end (SetFilePointer(hFile, 0, 0, FILE_END);).

Although it may not be causing your actual problem, I'd replace your current logic trying to use CreateFile with OPEN_EXSTING, then with CREATE_NEW if the first attempt fails. Instead, just pass the OPEN_ALWAYS flag, which pretty much automates that logic -- open an existing file if it exists, and create a new one if it doesn't.

edited Apr 11 '13 at 16:22

answered Mar 27 '12 at 14:50

Jerry Coffin

476,176
80
629
1,111

Thank you, I mixed your answer with JaredPar's one and all is going better now :) – BMN Mar 27 '12 at 15:02

score 1 · Answer 3 · answered Mar 27 '12 at 14:45

1

You need to set the file pointer to the end of the file before writing with SetFilePointer. See the MSDN example.

answered Mar 27 '12 at 14:45

Mike Kwan

24,123
12
63
96

score 1 · Answer 4 · answered Mar 27 '12 at 14:46

1

I couldn't see anything obvious about opening for Append in the CreateFile documentation, but you could use the SetFilePointer function to seek to the end of the file before writing.

answered Mar 27 '12 at 14:46

James

9,064
3
31
49

CreateFile always override the specified file

4 Answers4

Linked