Is it necessary to close files after reading (only) in any programming language?

Question

I read that a program should close files after writing to them in case there is still data in the write buffer not yet physically written to it. I also read that some langua

Answer 1

Not closing a file will result in unnecessary resources being taken from the system (File Descriptors on Unix and Handles on windows). Especially when a bug happens in some sort of loop or a system is never turned off, this gets important. Some languages manage unclosed files themselves when they for example run out of scope, others don't or only at some random time when it is checked (like the garbage collector in Java).

Imagine you have some sort of system that needs to run forever. For example a server. Then unclosed files can consume more and more resources, till ultimately all space is used by unclosed files.

In order to read a file you have to open it. So independent of what you do with a file, space will be reserved for the file. So far I tried to explain the importance of closing a file for resources, it's also important that you as a programmer know when an object (file) could be closed since no further use will be required. I think it's bad practice to not be at-least aware of unclosed files, and it's good practice to close files if no further use is required.

Some applications also require only access to a file, so require no other applications to have the file open. For example when you're trying to empty your recycle bin or move a file which you still have open on windows. (This is referred to as file locking). When you still have the file open windows won't let you throw away or move the files. This is just an example of when it would be annoying that a file is open while it should (rather) not (be). The example happens to me daily.

Answer 2

Yes, it's better to close file after reading is completed.

That's necessary because the other software might request exclusive access to that file. If file is still opened then such request will fail.

Answer 3

In general, you should always close a file after you are done using it.

Reason number 1: There are not unlimited available File Descriptors (or in windows, the conceptually similar HANDLES). Every time you access a file ressource, even for reading, you are reducing the number of handles (or FD's) available to other processes. every time you close a handle, you release it and makes it available for other processes.

Now consider the consequences of a loop that opens a file, reads it, but doesn't close it...

http://en.wikipedia.org/wiki/File_descriptor

https://msdn.microsoft.com/en-us/library/windows/desktop/aa364225%28v=vs.85%29.aspx

Reason number 2: If you are doing anything else than reading a file, there are problems with race conditions, if multiple processes or threads accesses the same file.. To avoid this, you may find file locks in place. http://en.wikipedia.org/wiki/File_locking

if you are reading a file, and not closing it afterward, other applications, that could try to obtain a file lock are denied access.

oh - and the file can't be deleted by anyone that doesn't have rights to kill your process..

Reason number 3: There is absolutely no reason to leave a file unclosed. In any language, which is why Python helps the lazy programmers, and automatically closes a handle that drops out of scope, in case the programmer forgot.