2.5. Standard I/O Streams
When a file is opened with append mode, the file position indicator is initially positioned at the beginning of the file.
The characteristics of unbuffered and fully buffered streams are supported.
Fully buffered streams use all bytes in the allocated buffer.
Line buffered streams are supported for all file types.
2.5.1. Interaction of File Descriptors and Standard I/O Streams
Input is seen exactly once provided the application follows the rules specified.