vis(3)

NAME

vis - visually encode characters

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <vis.h>
char *
vis(char *dst, int c, int flag, int nextc);
int
strvis(char *dst, const char *src, int flag);
int
strvisx(char *dst, const char *src, size_t len, int flag);

DESCRIPTION

The vis() function copies into dst a string which represents

the character 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 in

cluding 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 char

acters 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 be

low).

The strvis() and strvisx() functions copy into dst a visual

representation of the string src. The strvis() function encodes char

acters from
src up to the first NUL. The strvisx() function encodes ex

actly 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 per

form 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 back

slash character
`' to introduce a special sequence; two backslashes are used

to represent 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 repre

sent control
characters see (iscntrl(3)). The following

formats are
used:

C Represents the control character `C'.

Spans char

acters ` 00' through ` 37', 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^?').

40 Represents ASCII space.

240 Represents Meta-space.

VIS_CSTYLE Use C-style backslash sequences to represent

standard non

printable characters. The following se

quences are used to
represent the indicated characters:

BEL (007)

BS (010)

NP (014)
NCR((015)

SP (040)

HT (011)

VT (013)

NUL (000)

When using this format, the nextc argument is

looked at to
determine if a NUL character can be encoded

as ` '
instead of ` 00'. If nextc is an octal dig

it, 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 `dd'

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 ALSO

unvis(1), unvis(3)

R. Fielding, Relative Uniform Resource Locators, RFC1808.

HISTORY

These functions first appeared in 4.4BSD.

BUGS

The vis family of functions do not recognize multibyte char

acters, and
thus may consider them to be non-printable when they are in

fact printable (and vice versa.)

BSD April 9, 2006

BSD