seek {base}R Documentation

Functions to Reposition Connections

Description

Functions to re-position connections.

Usage

seek(con, ...)
## S3 method for class 'connection':
seek(con, where = NA, origin = "start", rw = "", ...)

isSeekable(con)

truncate(con, ...)

Arguments

con a connection.
where integer. A file position (relative to the origin specified by origin), or NA.
rw character. Empty or "read" or "write", partial matches allowed.
origin character. One of "start", "current", "end".
... further arguments passed to or from other methods.

Details

seek with where = NA returns the current byte offset of a connection (from the beginning), and with a non-missing where argument the connection is re-positioned (if possible) to the specified position. isSeekable returns whether the connection in principle supports seek: currently only (possibly gz-compressed) file connections do. gzfile connections do not support origin = "end", and the file position they use is that of the uncompressed file.

The value returned by seek(where=NA) appears to be unreliable on Windows systems, at least for text files. Clipboard connections can seek too.

File connections can be open for both writing/appending, in which case R keeps separate positions for reading and writing. Which seek refers to can be set by its rw argument: the default is the last mode (reading or writing) which was used. Most files are only opened for reading or writing and so default to that state. If a file is open for reading and writing but has not been used, the default is to give the reading position (0).

The initial file position for reading is always at the beginning. The initial position for writing is at the beginning of the file for modes "r+" and "r+b", otherwise at the end of the file. Some platforms only allow writing at the end of the file in the append modes.

truncate truncates a file opened for writing at its current position. It works only for file connections, and is not implemented on all platforms.

Value

seek returns the current position (before any move), as a byte offset, if relevant, or 0 if not.
truncate returns NULL: it stops with an error if it fails (or is not implemented).
isSeekable returns a logical value, whether the connection is support seek.

See Also

connections


[Package base version 2.1.0 Index]