Question

I am trying to open a file for writing and reading simultaneously, in windows.
I have one program which writes (every one second) to the file and one that reads from it. In unix it works prefectly but it doesn’t work in windows (I can’t open an already opened file).
I open the file with fopen().

How can I solve this problem?

EDIT2:

check out _fsopen it uses FILE *, and set the share flag accordingly.

EDIT:

First of all, some code: this is how I used to open the file

   FILE* f = NULL;
        int res = fopen_s(&f, "c:\\temp\\File1.txt", "w");
        if (res != 0) return;

        while (true) {
            Sleep(1000);
            fprintf_s(f , "Some data");
        }
        fclose(f); 

The read was in other applicaiton, but it did fscanf instead.

The fixed code:

char d[] = "data";


HANDLE h = CreateFile("c:\\temp\\f.txt", GENERIC_READ | GENERIC_WRITE,
    FILE_SHARE_WRITE | FILE_SHARE_READ, NULL, CREATE_ALWAYS, /*FILE_ATTRIBUTE_NORMAL*/ FILE_FLAG_WRITE_THROUGH, NULL);

if (h == INVALID_HANDLE_VALUE) return 0;

DWORD bytesW;
while(true) {
    Sleep(100);
        WriteFile(h, d, strlen(d), &bytesW, NULL);
}

CloseHandle(h);
return 0; 

Answer 1

Both Windows and Linux have a default way of opening a file, which fopen uses by default.

In Windows, that means blocking (only one process can open a file at a time).

In Linux, it means non-blocking.

fopen is a high-level API. To choose yourself the blocking policy on the file, for Windows you should use OpenFile from WinAPI. In particular, have a look at the OF_SHARE_* flags.