C言語に初めて触ってから20年のロートルですが今まで知りませんでした。man fopen より。 The fopen() function opens the file whose name is the string pointed to by filename and associates a stream with it. ``a'' Open for writing. The file is created if it does not exist. The stream is positioned at the end of the file. Subsequent writes to the file will always end up at the then current end of file, irrespective of any intervening fseek
