Is it safe to recompile an executable while it's running?

Question

What happens if I recompile an executable while it\'s running? Does the operating system read all of the executable\'s contents into memory when it starts running it, so it

Answer 1

Under Linux, executables are demand paged into memory as needed. The executable on disk becomes the backing store for the application. This means you cannot modify the executable on disk or you will affect a running application. If you try to open(2) an in-use executable for writing, you will get an ETXTBSY (Text file busy) error (check the man page for open(2)).

As many others have said, you can remove the file from the filesystem (unlink(2)) and the kernel will maintain a reference to it and not delete it from disk until there are no more references (when the process exits, it will release its reference to the file). This means you can effectively "overwrite" an in-use executable by first removing it and then creating a new file with the same name as the old file.

So, it comes down to how the compiler creates the executable when "overwriting" an existing file. If it just opens the file for writing and truncates it (O_WRONLY|O_CREAT|O_TRUNC), the it will fail with an ETXTBSY error. If it first removes the existing output file and creates a new one, it will work without error.