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.Dd June 9, 1993
.Dt VIS 3
.Os
.Sh NAME
.Nm vis ,
.Nm strvis ,
.Nm strnvis ,
.Nm strvisx
.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" "const char *src" "int flag"
.Ft int
.Fn strnvis "char *dst" "const char *src" "size_t size" "int flag"
.Ft int
.Fn strvisx "char *dst" "const char *src" "size_t 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 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
.Fa 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 ,
.Fn strnvis
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 NUL.
The
.Fn strnvis
function encodes characters from
.Fa src
up to the first NUL or the end of
.Fa dst ,
as indicated by
.Fa size .
The
.Fn strvisx
function encodes exactly
.Fa len
characters from
.Fa src
(this
is useful for encoding a block of data that may contain NULs).
All three forms NUL terminate
.Fa dst ,
except for
.Fn strnvis
when
.Fa size
is zero, in which case
.Fa dst
is not touched.
For
.Fn strvis
and
.Fn strvisx ,
the size of
.Fa dst
must be four times the number
of characters encoded from
.Fa src
(plus one for the NUL).
.Fn strvis
and
.Fn strvisx
return the number of characters in
.Fa dst
(not including the trailing NUL).
.Fn strnvis
returns the length that
.Fa dst
would become if it were of unlimited size (similar to
.Xr snprintf 3
or
.Xr strlcpy 3 ) .
This can be used to detect truncation but it also means that
the return value of
.Fn strnvis
must not be used without checking it against
.Fa size .
.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
.Dq unsafe
characters.
These are 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
.Ql \e
character 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 a caret
.Ql ^
to represent control characters (see
.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
.Fa nextc
parameter is looked at to determine
if a 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
.Ar 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 unvis 1 ,
.Xr vis 1 ,
.Xr snprintf 3 ,
.Xr strlcpy 3 ,
.Xr unvis 3
.Sh HISTORY
The
.Fn vis ,
.Fn strvis
and
.Fn strvisx
functions first appeared in
.Bx 4.4 .
.br
The
.Fn strnvis
function first appeared in
.Ox 2.9 .