|I||Open for input (reading) only. The file must already exist. Other processes may read from the file (but not write to it) while it's open with this method.|
|O||Open for output (writing) only. If the file already exists, all of its current contents will be destroyed. You have exclusive access to the file (no other process can read from it nor write to it) while it's open with the "O" method. Does NOT support the "resource fork" open.|
|R||Open for "random access." You can either read from or write to the file. The "file mark" (which indicates where the next read or write operation will occur) is placed initially at the beginning of the file. If you write to the file, you only replace those bytes which you're writing; the rest of the file's contents are unaffected. You have exclusive access to the file.|
|A||Open for "append." This is just like method "R", except the file mark is placed initially at the end of the file. This method is normally used when you want to add data to the end of an existing file.|
|N||Open for non-exclusive random access. This is just like method "R", except that other processes may read from the file while you have it open. Your process is the only one allowed to write to the file and your code is responsible for assuring file readers don't process old or partial data. OPEN code only provides access; it does not provide any data integrity protection when there are concurrent readers and writers of the same file.
If a file is already open in "N" mode, a second attempt to open in 'N' mode by the current or other process automatically provides read-only access ( essentially "I" mode ) to the file. If the process is given read-only mode, the FB runtime sends a "Permission denied" ( _EAccess/13 ) error code which can used by the caller ( must be trapped with 'on error' and "error" ). If a process is completely denied access ( such as when another app has the file open in some exclusive mode ), the error handling will report a _EAgain/35 error.