R: Functions to Reposition Connections

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` | 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.

### 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.

`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 builds, and on some 32-bit builds.

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 both 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.)

`gzfile` connections support `seek` with a number of limitations, using the file position of the uncompressed file. They do not support `origin = "end"`. When writing, seeking is only possible forwards: when reading seeking backwards is supported by rewinding the file and re-reading from its start.

If `seek` is called with a non-`NA` value of `where`, any pushback on a text-mode connection is discarded.

`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.

None of these should be expected to work on text-mode connections with re-encoding selected.

### Value

`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 builds, and on some 32-bit builds.

`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`.

### Warning

Use of `seek` on Windows is discouraged. We have found so many errors in the Windows implementation of file positioning that users are advised to use it only at their own risk, and asked not to waste the R developers' time with bug reports on Windows' deficiencies.

### See Also

`connections`

[Package base version 3.1.2 Index]