Transfer character strings to and from connections, without assuming they are null-terminated on the connection.
readChar(con, nchars, useBytes = FALSE) writeChar(object, con, nchars = nchar(object, type = "chars"), eos = "", useBytes = FALSE)
- A connection object, or a character string naming a file, or a raw vector.
- integer vector, giving the lengths in characters of (unterminated) character strings to be read or written. Elements must be >= 0 and not
- logical: For
ncharsbe regarded as a number of bytes not characters in a multi-byte locale? For
- A character vector to be written to the connection, at least as long as
- ‘end of string’: character string . The terminator to be written after each string, followed by an ASCII
NULLfor no terminator at all.
These functions complement
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
writeChar, a raw vector (if
con is a raw vector) or invisibly
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
## test fixed-length strings zz <- file("testchar", "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("testchar", "rb") readChar(zz, nc) readChar(zz, nchar(x)+3) # need to read the terminator explicitly close(zz) unlink("testchar")
Documentation reproduced from R 2.15.3. License: GPL-2.