Stop program from running twice?

I like bean66's suggestion, but after googleing a little I feel it's a bit above my level. Therefore I'll try to stick with keeping the file open throughout the program. If I can't get that to work then I guess I'll have to look into locking the file...

I am performing both reading and writing operations on the file frequently, but I need to keep the file open to restrict its access. Am I right in thinking that freopen() would do the job perfectly?

Originally Posted by bean66

For file based locking:

1. use a seperate file to control locking! Not the same file used for I/O.
2. See fcntl (fd, F_SETLK, flock)
3. use type = F_WRLCK

Windows has fcntl()?

EDIT: Also, if you're going to lock a secondary file, what's wrong with the old school method of simply opening in O_RDWR | O_EXCL | O_CREAT, then unlinking it when done?

The C function fopen will deny read/write access by default.

Originally Posted by Elysia

The C function fopen will deny read/write access by default.

Yes I know that. But since I don't want the file to be accessible at all while the program is running, I should keep it open (whether reading or writing mode) and switch between the modes by using freopen(). something like this:

Code:

fopen(file, "r"); // first time
... // read from file and keep file open
freopen(NULL, "w", file); // switch to write mode
... // write to file
fclose();

Would this work (I already started implementing it, but I'm still not sure it'll work).

It's possible to open for read & write using "r+" or "w+".