VIS(3) BSD Library Functions Manual VIS(3)NAME
vis — visually encode characters
Standard C Library (libc, -lc)
vis(char *dst, int c, int flag, int nextc);
strvis(char *dst, const char *src, int flag);
strvisx(char *dst, const char *src, size_t len, int flag);
The vis() function copies into dst a string which represents the charac‐
ter c. If c needs no encoding, it is copied in unaltered. The string is
null terminated, and a pointer to the end of the string is returned. The
maximum length of any encoding is four characters (not including the
trailing NUL); thus, when encoding a set of characters into a buffer, the
size of the buffer should be four times the number of characters encoded,
plus one for the trailing NUL. The flag argument is used for altering
the default range of characters considered for encoding and for altering
the visual representation. The additional character, nextc, is only used
when selecting the VIS_CSTYLE encoding format (explained below).
The strvis() and strvisx() functions copy into dst a visual representa‐
tion of the string src. The strvis() function encodes characters from
src up to the first NUL. The strvisx() function encodes exactly len
characters from src (this is useful for encoding a block of data that may
contain NUL's). Both forms NUL terminate dst. The size of dst must be
four times the number of characters encoded from src (plus one for the
NUL). Both forms return the number of characters in dst (not including
the trailing NUL).
The encoding is a unique, invertible representation composed entirely of
graphic characters; it can be decoded back into the original form using
the unvis(3) or strunvis(3) functions.
There are two parameters that can be controlled: the range of characters
that are encoded, and the type of representation used. By default, all
non-graphic characters except space, tab, and newline are encoded. (See
isgraph(3).) The following flags alter this:
VIS_GLOB Also encode magic characters (‘*’, ‘?’, ‘[’ and ‘#’) recog‐
nized by glob(3).
VIS_SP Also encode space.
VIS_TAB Also encode tab.
VIS_NL Also encode newline.
VIS_WHITE Synonym for VIS_SP | VIS_TAB | VIS_NL.
VIS_SAFE Only encode "unsafe" characters. Unsafe means control char‐
acters which may cause common terminals to perform unexpected
functions. Currently this form allows space, tab, newline,
backspace, bell, and return - in addition to all graphic
characters - unencoded.
There are four forms of encoding. Most forms use the backslash character
‘\’ to introduce a special sequence; two backslashes are used to repre‐
sent a real backslash. These are the visual formats:
(default) Use an ‘M’ to represent meta characters (characters with
the 8th bit set), and use caret ‘^’ to represent control
characters see (iscntrl(3)). The following formats are
\^C Represents the control character ‘C’. Spans char‐
acters ‘\000’ through ‘\037’, and ‘\177’ (as
\M-C Represents character ‘C’ with the 8th bit set.
Spans characters ‘\241’ through ‘\376’.
\M^C Represents control character ‘C’ with the 8th bit
set. Spans characters ‘\200’ through ‘\237’, and
‘\377’ (as ‘\M^?’).
\040 Represents ASCII space.
\240 Represents Meta-space.
VIS_CSTYLE Use C-style backslash sequences to represent standard non-
printable characters. The following sequences are used to
represent the indicated characters:
\a BEL (007)
\b BS (010)
\f NP (014)
\n NL (012)
\r CR (015)
\s SP (040)
\t HT (011)
\v VT (013)
\0 NUL (000)
When using this format, the nextc argument is looked at to
determine if a NUL character can be encoded as ‘\0’
instead of ‘\000’. If nextc is an octal digit, the latter
representation is used to avoid ambiguity.
VIS_HTTPSTYLE Use URI encoding as described in RFC 1808. The form is
‘%dd’ where d represents a hexadecimal digit.
VIS_OCTAL Use a three digit octal sequence. The form is ‘\ddd’
where d represents an octal digit.
There is one additional flag, VIS_NOSLASH, which inhibits the doubling of
backslashes and the backslash before the default format (that is, control
characters are represented by ‘^C’ and meta characters as ‘M-C’). With
this flag set, the encoding is ambiguous and non-invertible.
SEE ALSOunvis(1), unvis(3)
R. Fielding, Relative Uniform Resource Locators, RFC1808.
These functions first appeared in 4.4BSD.
The vis family of functions do not recognize multibyte characters, and
thus may consider them to be non-printable when they are in fact print‐
able (and vice versa.)
BSD April 9, 2006 BSD