Opening File Ports

The open-input-file and open-output-file procedures accept an optional flag argument after the filename that specifies a mode for the file:

• 'binary -- characters are returned from the port exactly as they are read from the file. Binary mode is the default mode.
• 'text -- return and linefeed characters written and read from the file are filtered by the port in a platform specific manner:
  • Unix: no filtering occurs.
  • Windows reading: a return-linefeed combination from a file is returned by the port as a single linefeed; no filtering occurs for return characters that are not followed by a linefeed, or for a linefeed that is not preceded by a return.
  • Windows writing: a linefeed written to the port is translated into a return-linefeed combination in the file; no filtering occurs for returns.
  • MacOS reading: a return character read from the file is returned as a linefeed by the port; no filtering occurs for linefeeds.
  • MacOS writing: a return character written to the port is translated into a linefeed in the file; no filtering occurs for linefeeds.

The open-output-file procedure can also take a flag argument that specifies how to proceed when a file with the specified name already exists:

• 'error -- raise an exception (this is the default)
• 'replace -- remove the old file and write a new one
• 'truncate -- overwrite the old data
• 'truncate/replace -- try 'truncate; if it fails, try 'replace
• 'append -- append to the end of the file

Extra flag arguments are passed to open-output-file in any order. Appropriate flag arguments can also be passed as the last argument(s) to call-with-input-file, with-input-from-file, call-with-output-file, and with-output-to-file. When conflicting flag arguments (e.g., both 'error and 'replace) are provided to open-output-file, with-output-to-file, or call-with-output-file, the exn:application:mode-conflict exception is raised.

When an input or output file port is created, it is placed into the management of the current custodian (see section 9.5).