Overwrite executable file when the process is running

Question

When a program is still running (a UNIX server process), can I overwrite the executable file with a newer version?
also, can i overwrite a shared library file that is being used by a currently running process?
I don't think it is a good idea. But what happens?
This is my first posting. Sorry if this is not the right place.

Answer 1

Your question is about Unix. This forum is about C++ programming. You are more likely to get comprehensive answers in an OS forum. But I can tell you part of the answer.
Remember that in Unix, a directory entry is separate from the file it refers to. One file can be referred to (linked to) by more than one directory entry. The file system keeps a count of how many directory entries refer to a file, and how many processes currently have the file open. When the directory count and the process count both go to zero, the space used by the file is reclaimed.
The answer to your questions depend on exactly how you overwrite the files. A typical way to overwrite a file is to delete the orginal file and create a new file with the same name in the same directory. Most applications that seem to be changing the contents of a file actually delete the original and create a new file.
Suppose you create a new version of /mydir/mylib.so, which is currently in use by process1. The directory entry /mydir/mylib.so now points to a different file, but process1 continues to use the old file, which currently has no name, and no directory entries pointing to it. If you start a new process that depends on /mydir/mylib.so, it will use the new library version. When process1 exits, the last reference to the original version disappears, and the disk blocks used by the original version will be reclaimed.
If you used some other mechanism to update a file, such as a binary editor that changes disk blocks directly, you could create inconsistencies for processes that are using the file. Whether processes using the file see a change depends on whether they go back to refer to the file, or have finished reading it.