Node: Open Modes, Next: Operations on C Streams, Previous: Shared Files, Up: The Implementation of Standard I/O
Open and Create calls result in a call to fopen
using the mode shown in Table 6.1
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.