VIS(3) MachTen Programmer’s Manual VIS(3)

NAME
vis - visually encode characters

SYNOPSIS
#include <vis.h>

char *
vis(char *dst, char c, int flag, char nextc)

int
strvis(char *dst, char *src, int flag)

int
strvisx(char *dst, char *src, int len, int flag)

DESCRIPTION
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 NULL); thus, when encoding a set of characters into a buffer,
the size of the buffer should be four times the number of characters en-
coded, plus one for the trailing NULL. The flag parameter is used for al-
tering 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 representa-
tion of the string src. The strvis() function encodes characters from src
up to the first NULL. The strvisx() function encodes exactly len charac-
ters from src (this is useful for encoding a block of data that may con-
tain NULL’s). Both forms NULL terminate dst. The size of dst must be four
times the number of characters encoded from src (plus one for the NULL).
Both forms return the number of characters in dst (not including the
trailing NULL).

The encoding is a unique, invertible representation comprised 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_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 three forms of encoding. All 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 carat ‘^’ to represent control charac-
ters see (iscntrl(3)). The following formats are used:

C Represents the control character ‘C’. Spans characters
‘ 00’ through ‘ 37’, and ‘177’ (as ‘?’).

C Represents character ‘C’ with the 8th bit set. Spans
characters ‘241’ through ‘376’.

C Represents control character ‘C’ with the 8th bit set.
Spans characters ‘200’ through ‘237’, and ‘377’ (as
‘?’).

40 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:

- BEL (007) -
BS (010)
- NP (014)
- NL (012)
- CR (015)
- HT (011)
- VT (013)
- NUL (000)

When using this format, the nextc parameter is looked at to
determine if a NULL character can be encoded as ‘ ’ instead
of ‘ 00’. If nextc is an octal digit, the latter representa-
tion is used to avoid ambiguity.

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.