seek {base}R Documentation

Functions to Reposition Connections


Functions to re-position connections.


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


truncate(con, ...)


con a connection.
where numeric. 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": see Details.
... further arguments passed to or from other methods.


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"; the file position they use is that of the uncompressed file.

where is stored as a real but should represent an integer: non-integer values are likely to be truncated. Note that the possible values can exceed the largest representable number in an R integer on 64-bit OSes, and on some 32-bit OSes.

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. (The reported write position for a file opened in an append mode will typically be unreliable until the file has been written to.)

truncate truncates a file opened for writing at its current position. It works only for file connections, and is not implemented on all platforms: on others (including Windows) it will not work for large (> 2Gb) files.


seek returns the current position (before any move), as a (numeric) byte offset from the origin, if relevant, or 0 if not. Note that the position can exceed the largest representable number in an R integer on 64-bit OSes, and on some 32-bit OSes.
truncate returns NULL: it stops with an error if it fails (or is not implemented).
isSeekable returns a logical value, whether the connection supports seek.

See Also


[Package base version 2.5.0 Index]