From QB64.org wiki
CLOSE closes an open file or port using the number(s) assigned in an OPEN statement.
- CLOSE [fileNumber[, ...]]
- fileNumber indicates the file or list of file numbers to close. When not specified, all open files are closed.
- A file must be closed when changing to another file mode.
- CLOSE files when they are no longer needed, in order to save memory.
- Files cannot be opened in the same OPEN mode using another number until the first one is closed.
- Use holding variables for each file number returned by FREEFILE so that the file reference is known.
- Will not return an error if a filenumber is already closed or was never opened. It does not verify that a file was closed.
- CLEAR can be used to close all open files.
- CLOSE can also be used to close an open TCP/IP connection using a handle returned by QB64.