R: Functions to Reposition Connections

seek {base} | R Documentation

## Functions to Reposition Connections

### Description

Functions to re-position connections.

### Usage

    seek(con, where = NA, origin = "start", rw = "", ...)
    truncate(con, ...)

    isSeekable(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 file connections do.

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`

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