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.Dd $Mdocdate: July 30 2022 $
.Dt VIS 3
.Os
.Sh NAME
.Nm vis ,
.Nm strvis ,
.Nm strnvis ,
.Nm strvisx ,
.Nm stravis
.Nd visually encode characters
.Sh SYNOPSIS
.In stdlib.h
.In vis.h
.Ft char *
.Fn vis "char *dst" "int c" "int flag" "int nextc"
.Ft int
.Fn strvis "char *dst" "const char *src" "int flag"
.Ft int
.Fn strnvis "char *dst" "const char *src" "size_t dstsize" "int flag"
.Ft int
.Fn strvisx "char *dst" "const char *src" "size_t srclen" "int flag"
.Ft int
.Fn stravis "char **outp" "const char *src" "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.
.Fa dst
will be NUL-terminated and must be at least 5 bytes long
(maximum encoding requires 4 bytes plus the NUL).
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 .
.Pp
The
.Fn strvis
function encodes characters from
.Fa src
up to the first NUL, into a buffer
.Fa dst
(which must be at least 4 * strlen(src) + 1 long).
.Pp
The
.Fn strnvis
function encodes characters from
.Fa src
up to the first NUL or the end of the buffer
.Fa dst ,
as indicated by
.Fa dstsize .
.Pp
The
.Fn strvisx
function encodes exactly
.Fa srclen
characters from
.Fa src
into a buffer
.Fa dst
(which must be at least 4 * srclen + 1 long).
This
is useful for encoding a block of data that may contain NULs.
.Pp
The
.Fn stravis
function writes a visual representation of the string
.Fa src
into a newly allocated string
.Fa outp ;
it does not attempt to
.Xr realloc 3
.Fa outp .
.Fa outp
should be passed to
.Xr free 3
to release the allocated storage when it is no longer needed.
.Fn stravis
checks for integer overflow when allocating memory.
.Pp
All forms NUL-terminate
.Fa dst ,
except for
.Fn strnvis
when
.Fa dstsize
is zero, in which case
.Fa dst
is not touched.
.Pp
The
.Fa flag
parameter is used for altering the default range of
characters considered for encoding and for altering the visual
representation.
.Ss Encodings
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_ALL
Encode all characters, whether visible or not.
.It Dv VIS_DQ
Also encode double quote characters
.Pf ( Ql \&" ) .
.It Dv VIS_GLOB
Also encode magic characters recognized by
.Xr glob 3
.Pf ( Ql * ,
.Ql \&? ,
.Ql \&[ )
and
.Ql # .
.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 | VIS_TAB | 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 \(em in addition to all graphic characters \(em 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.
.It Dv \e-C
Represents character
.Ql C .
Only used with
.Dv VIS_ALL .
.El
.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 \es Tn  - SP No (040)
.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 RETURN VALUES
.Fn vis
returns a pointer to the terminating NUL character of the string
.Fa dst .
.Pp
.Fn strvis
and
.Fn strvisx
return the number of characters in
.Fa dst
(not including the trailing NUL).
.Pp
.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 dstsize .
.Pp
Upon successful completion,
.Fn stravis
returns the number of characters in
.Pf * Fa outp
(not including the trailing NUL).
Otherwise,
.Fn stravis
returns -1 and sets
.Va errno
to
.Er ENOMEM .
.Sh EXAMPLES
.Fn strvis
has unusual storage requirements that can lead to stack or heap corruption
if the destination is not carefully constructed.
A common mistake is to use the same size for the source and destination
when the destination actually needs up to 4 * strlen(source) + 1 bytes.
.Pp
If the length of a string to be encoded is not known at compile time, use
.Fn stravis :
.Bd -literal -offset indent
char *src, *dst;

\&...
if (stravis(&dst, src, VIS_OCTAL) == -1)
	err(1, "stravis");

\&...
free(dst);
.Ed
.Pp
To encode a fixed size buffer,
.Fn strnvis
can be used with a fixed size target buffer:
.Bd -literal -offset indent
char src[MAXPATHLEN];
char dst[4 * MAXPATHLEN + 1];

\&...
if (strnvis(dst, src, sizeof(dst), VIS_OCTAL) >= sizeof(dst))
	err(1, "strnvis");
.Ed
.Sh SEE ALSO
.Xr unvis 1 ,
.Xr vis 1 ,
.Xr free 3 ,
.Xr snprintf 3 ,
.Xr strlcpy 3 ,
.Xr unvis 3
.Sh HISTORY
The
.Fn vis ,
.Fn strvis
and
.Fn strvisx
functions first appeared in
.Bx 4.3 Reno ,
.Fn strnvis
in
.Ox 2.9
and
.Fn stravis
in
.Ox 5.7 .
.Pp
The
.Dv VIS_ALL
flag first appeared in
.Ox 4.9 .