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.Dd June 9, 1993
.Dt VIS 3
.Os
.Sh NAME
.Nm vis
.Nd visually encode characters
.Sh SYNOPSIS
.Fd #include <vis.h>
.Ft char *
.Fn vis "char *dst" "char c" "int flag" "char nextc"
.Ft int 
.Fn strvis "char *dst" "char *src" "int flag"
.Ft int 
.Fn strvisx "char *dst" "char *src" "int len" "int flag"
.Sh DESCRIPTION
The
.Fn vis
function
copies into
.Fa dst
a string which represents the character
.Fa c .
If
.Fa 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
.Dv 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
.Dv NUL . 
The flag parameter is used for altering the default range of
characters considered for encoding and for altering the visual
representation.
The additional character,
.Fa nextc ,
is only used when selecting the
.Dv VIS_CSTYLE
encoding format (explained below).
.Pp
The
.Fn strvis
and
.Fn strvisx
functions copy into
.Fa dst
a visual representation of
the string
.Fa src .
The
.Fn strvis
function encodes characters from
.Fa src
up to the
first
.Dv NUL . 
The
.Fn strvisx
function encodes exactly
.Fa len
characters from
.Fa src
(this
is useful for encoding a block of data that may contain
.Dv NUL Ns 's).
Both forms
.Dv NUL
terminate
.Fa dst .
The size of
.Fa dst
must be four times the number
of characters encoded from
.Fa src
(plus one for the
.Dv NUL ) . 
Both
forms return the number of characters in dst (not including
the trailing
.Dv NUL ) . 
.Pp
The encoding is a unique, invertible representation composed entirely of
graphic characters; it can be decoded back into the original form using 
the
.Xr unvis 3
or
.Xr strunvis 3
functions.
.Pp
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
.Xr isgraph 3 . )
The following flags
alter this:
.Bl -tag -width VIS_WHITEX
.It Dv VIS_SP
Also encode space.
.It Dv VIS_TAB		
Also encode tab.
.It Dv VIS_NL
Also encode newline.
.It Dv VIS_WHITE	
Synonym for
.Dv VIS_SP
\&|
.Dv VIS_TAB
\&|
.Dv VIS_NL .
.It Dv VIS_SAFE	
Only encode "unsafe" characters.  Unsafe means control
characters 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. 
.El
.Pp
There are three forms of encoding.
All forms use the backslash character
.Ql \e
to introduce a special
sequence; two backslashes are used to represent a real backslash.
These are the visual formats:
.Bl -tag -width VIS_CSTYLE
.It (default)
Use an
.Ql M
to represent meta characters (characters with the 8th
bit set), and use caret
.Ql ^
to represent control characters see
.Pf ( Xr iscntrl 3 ) .
The following formats are used:
.Bl -tag -width xxxxx
.It Dv \e^C
Represents the control character
.Ql C .
Spans characters
.Ql \e000
through
.Ql \e037 ,
and
.Ql \e177
(as
.Ql \e^? ) .
.It Dv \eM-C
Represents character
.Ql C
with the 8th bit set.
Spans characters
.Ql \e241
through
.Ql \e376 .
.It Dv \eM^C
Represents control character
.Ql C
with the 8th bit set.
Spans characters
.Ql \e200
through
.Ql \e237 ,
and
.Ql \e377
(as
.Ql \eM^? ) .
.It Dv \e040
Represents
.Tn ASCII
space.
.It Dv \e240
Represents Meta-space.
.El
.Pp
.It Dv VIS_CSTYLE
Use C-style backslash sequences to represent standard non-printable
characters.
The following sequences are used to represent the indicated characters:
.Bd -unfilled -offset indent
.Li \ea Tn  - BEL No (007)
.Li \eb Tn  - BS No (010)
.Li \ef Tn  - NP No (014)
.Li \en Tn  - NL No (012)
.Li \er Tn  - CR No (015)
.Li \et Tn  - HT No (011)
.Li \ev Tn  - VT No (013)
.Li \e0 Tn  - NUL No (000)
.Ed
.Pp
When using this format, the nextc parameter is looked at to determine
if a
.Dv NUL
character can be encoded as
.Ql \e0
instead of
.Ql \e000 .
If
.Fa nextc
is an octal digit, the latter representation is used to
avoid ambiguity.
.It Dv VIS_OCTAL
Use a three digit octal sequence.  The form is
.Ql \eddd
where
.Em d
represents an octal digit.
.El
.Pp
There is one additional flag,
.Dv VIS_NOSLASH ,
which inhibits the
doubling of backslashes and the backslash before the default
format (that is, control characters are represented by
.Ql ^C
and
meta characters as
.Ql M-C ) .
With this flag set, the encoding is
ambiguous and non-invertible.
.Sh SEE ALSO
.Xr vis 1 ,
.Xr unvis 1 ,
.Xr unvis 3
.Sh HISTORY
These functions first appeared in
.Bx 4.4 .