R: Transfer Character Strings To and From Connections

readChar {base} | R Documentation

## Transfer Character Strings To and From Connections

### Description

Transfer character strings to and from connections, without assuming they are null-terminated on the connection.

### Usage

    readChar(con, nchars, useBytes = FALSE)

    writeChar(object, con, nchars = nchar(object, type = "chars"),
              eos = "", useBytes = FALSE)

### Arguments

`con` | A connection object, or a character string naming a file, or a raw vector.
`nchars` | integer vector, giving the lengths in characters of (unterminated) character strings to be read or written. Elements must be >= 0 and not `NA`.
`useBytes` | logical: For `readChar`, should `nchars` be regarded as a number of bytes not characters in a multi-byte locale? For `writeChar`, see `writeLines`.
`object` | A character vector to be written to the connection, at least as long as `nchars`.
`eos` | ‘end of string’: character string. The terminator to be written after each string, followed by an ASCII `nul`; use `NULL` for no terminator at all.

### Details

These functions complement `readBin` and `writeBin` which read and write C-style zero-terminated character strings. They are for strings of known length, and can optionally write an end-of-string mark. They are intended only for character strings valid in the current locale.

These functions are intended to be used with binary-mode connections. If `con` is a character string, the functions call `file` to obtain a binary-mode file connection which is opened for the duration of the function call.

If the connection is open it is read/written from its current position. If it is not open, it is opened for the duration of the call in an appropriate mode (binary read or write) and then closed again. An open connection must be in binary mode.

If `readChar` is called with `con` a raw vector, the data in the vector is used as input. If `writeChar` is called with `con` a raw vector, it is just an indication that a raw vector should be returned.

Character strings containing ASCII `nul`(s) will be read correctly by `readChar` but truncated at the first `nul` with a warning.

If the character length requested for `readChar` is longer than the data available on the connection, what is available is returned. For `writeChar` if too many characters are requested the output is zero-padded, with a warning.

Missing strings are written as `NA`.

### Value

For `readChar`, a character vector of length the number of items read (which might be less than `length(nchars)`).

For `writeChar`, a raw vector (if `con` is a raw vector) or invisibly `NULL`.

### Note

Earlier versions of R allowed embedded nul bytes within character strings, but not R >= 2.8.0. `readChar` was commonly used to read fixed-size zero-padded byte fields for which `readBin` was unsuitable. `readChar` can still be used for such fields if there are no embedded nuls: otherwise `readBin(what = "raw")` provides an alternative.

`nchars` will be interpreted in bytes not characters in a non-UTF-8 multi-byte locale, with a warning.

There is little validity checking of UTF-8 reads.

Using these functions on a text-mode connection may work but should not be mixed with text-mode access to the connection, especially if the connection was opened with an `encoding` argument.

### See Also

The ‘R Data Import/Export’ manual.

`connections`, `readLines`, `writeLines`, `readBin`

### Examples

    ## test fixed-length strings
    zzfil <- tempfile("testchar")
    zz <- file(zzfil, "wb")
    x <- c("a", "this will be truncated", "abc")
    nc <- c(3, 10, 3)
    writeChar(x, zz, nc, eos = NULL)
    writeChar(x, zz, eos = "\r\n")
    close(zz)

    zz <- file(zzfil, "rb")
    readChar(zz, nc)
    readChar(zz, nchar(x)+3) # need to read the terminator explicitly
    close(zz)
    unlink(zzfil)

[Package base version 3.5.1 Index]