Open and Create calls result in a call to fopen
using the mode shown in the following table:
Open and Create Call Modes OPEN CREATE
Append_File "r+" "w+"
In_File "r" "w+"
Out_File (Direct_IO) "r+" "w"
Out_File (all other cases) "w" "w"
Inout_File "r+" "w+"
If text file translation is required, then either ‘b’ or ‘t’ is added to the mode, depending on the setting of Text. Text file translation refers to the mapping of CR/LF sequences in an external file to LF characters internally. This mapping only occurs in DOS and DOS-like systems, and is not relevant to other systems.
A special case occurs with Stream_IO. As shown in the above table, the
file is initially opened in ‘r’ or ‘w’ mode for the
In_File and Out_File cases. If a Set_Mode operation
subsequently requires switching from reading to writing or vice-versa,
then the file is reopened in ‘r+’ mode to permit the required operation.