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When a file is opened by using the "a" access type or the "a+" access type, all write operations occur at the end of the file. The file pointer can be repositioned by using fseek or rewind, but is always moved back to the end of the file before any write operation is carried out. Therefore, existing data cannot be overwritten.

The "a" mode does not remove the EOF marker before it appends to the file. After appending has occurred, the MS-DOS TYPE command only shows data up to the original EOF marker and not any data appended to the file. Before it appends to the file, the "a+" mode does remove the EOF marker. After appending, the MS-DOS TYPE command shows all data in the file. The "a+" mode is required for appending to a stream file that is terminated with the CTRL+Z EOF marker.