writing in file using threads (WINAPI) - c

Question

Just when I thought I finally understood threads, I got a problem.
In the main thread, I create a mutex using this:
Mutex = CreateMutex(NULL, FALSE, Mutex_Name);
Then, in one of the worker threads (the first one to reach), I have a function that returns a handle to a file by creating it like this:
file_h = CreateFile("The_File.txt",
GENERIC_WRITE | GENERIC_READ,
FILE_SHARE_WRITE | FILE_SHARE_READ | FILE_SHARE_DELETE,
NULL,
CREATE_NEW,
FILE_ATTRIBUTE_NORMAL,
NULL);
The other threads have functions that return a handle to the file by this:
file_h = CreateFile("The_File.txt",
GENERIC_WRITE | GENERIC_READ,
FILE_SHARE_WRITE | FILE_SHARE_READ | FILE_SHARE_DELETE,
NULL,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
NULL);
At the last part, both threads try to write to the file using this function:
int W_to_f(char* str_to_w, HANDLE file)
{
HANDLE mutex = NULL;
DWORD waitcode1;
DWORD dwBytesWrite;
char line[Line_Len];
mutex = OpenMutex(MUTEX_ALL_ACCESS, TRUE, LOCK);
if (mutex == NULL)
{
printf("Failed at opening LOCK in write file\n");
return -1;
}
waitcode1 = WaitForSingleObject(mutex, 15000);
if (WAIT_OBJECT_0 != waitcode1)
{
printf("Error when waiting for LOCK in file\n");
return -1;
}
strcpy(line, str_to_w);
while ((Line_Len - (strlen(line) + 1)) > 0)
{
strcat(line, "#");
}
SetFilePointer(file, 0, NULL, FILE_END); //problem?
WriteFile(file, line, strlen(line), &dwBytesWrite, NULL);
WriteFile(file, "\n", 2, &dwBytesWrite, NULL);
if(!ReleaseMutex(mutex))
{
printf("failed release mutex in write_file fun\n");
return -1;
}
return 0;
}
All I want them to do is to write to the file like this:
firststring############
secondstring###########
But, after checking with breakpoints, what I get is that the first one manages to write to file:
firststring###########
But then after the second one writes to the file, it becomes:
癡慩㩤㈱㐳䁀䁀䁀䁀䁀䁀䁀䁀䁀幀搀湩楤㩮㘵㠷䁀䁀䁀䁀䁀䁀䁀䁀䁀
Just one line, it doesn't even write the Chinese after the firststring####.
Should I somehow use SetFilePointer?
Should I CreateFile() in the function using OPEN_EXISTING for each thread?