gno/usr.man/man3/vis.3

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.\"     From: @(#)vis.3	8.1 (Berkeley) 6/9/93
.\"	$Id: vis.3,v 1.1 1999/01/07 05:17:24 gdr-ftp Exp $
.\"
.TH VIS 3 "3 January 1999" GNO "Library Routines"
.SH NAME
.BR vis
\- visually encode characters
.SH SYNOPSIS
.BR "#include <vis.h>"
.sp 1
char *\fBvis\fR
.RI "(char *" dst ,
.RI "int " c ,
.RI "int " flag ,
.RI "int " nextc );
.br
int 
.BR strvis
.RI "(char *" dst ,
.RI "const char *" src ,
.RI "int " flag );
.br
int 
.BR strvisx
.RI "(char *" dst ,
.RI "const char *" src ,
.RI "size_t " len ,
.RI "int " flag );
.SH DESCRIPTION
The
.BR vis
function copies into
.IR dst
a string which represents the character
.IR c .
If
.IR 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 encoded, plus one for the trailing
NULL.
The flag parameter is used for altering the default range of
characters considered for encoding and for altering the visual
representation.
The additional character,
.IR nextc ,
is only used when selecting the
.IR VIS_CSTYLE
encoding format (explained below).
.PP
The
.BR strvis
and
.BR strvisx
functions copy into
.IR dst
a visual representation of
the string
.IR src .
The
.BR strvis
function encodes characters from
.IR src
up to the
first
NULL.
The
.BR strvisx
function encodes exactly
.IR len
characters from
.IR src
(this
is useful for encoding a block of data that may contain
NULL's).
Both forms
NULL
terminate
.IR dst .
The size of
.IR dst
must be four times the number
of characters encoded from
.IR src
(plus one for the
NULL). 
Both
forms return the number of characters in dst (not including
the trailing
NULL). 
.PP
The encoding is a unique, invertible representation comprised entirely of
graphic characters; it can be decoded back into the original form using 
the
.BR unvis (3)
or
.BR 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
.BR isgraph (3).)
The following flags
alter this:
.IP VIS_SP
Also encode space.
.IP VIS_TAB		
Also encode tab.
.IP VIS_NL
Also encode newline.
.IP VIS_WHITE	
Synonym for
VIS_SP
\&|
VIS_TAB
\&|
VIS_NL .
.IP 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. 
.PP
There are three forms of encoding.
All forms use the backslash character '\e' to introduce a special
sequence; two backslashes are used to represent a real backslash.
These are the visual formats:
.de zb
.br
.in +0.8i
..
.de ze
.sp 1
.in -0.8i
..
.IP (default)
Use an 'M'
to represent meta characters (characters with the 8th
bit set), and use carat
.BR ^
to represent control characters see
.BR iscntrl (3).
The following formats are used:
.sp 1
\e^C
.zb
Represents the control character
.BR C .
Spans characters '\e000' through '\e037',
and '\e177' (as '\e^?').
.ze
\eM-C
.zb
Represents character
.RB ' C '
with the 8th bit set.
Spans characters '\e241'
through '\e376'.
.ze
\eM^C
.zb
Represents control character
.BR C
with the 8th bit set.
Spans characters '\e200' through '\e237', and '\e377' (as '\eM^?').
.ze
\e040
.zb
Represents ASCII space.
.ze
\e240
.zb
Represents Meta-space.
.ze
.IP VIS_CSTYLE
Use C-style backslash sequences to represent standard non-printable
characters.
The following sequences are used to represent the indicated characters:
.nf

     \\a - BEL (007)
     \\b - BS (010)
     \\f - NP (014)
     \\n - NL (012)
     \\r - CR (015)
     \\t - HT (011)
     \\v - VT (013)
     \\0 - NULL (000)

.fi
.sp 1
When using this format, the nextc parameter is looked at to determine
if a NULL character can be encoded as '\\0' instead of '\\000'.  If
.IR nextc
is an octal digit, the latter representation is used to
avoid ambiguity.
.IP VIS_OCTAL
Use a three digit octal sequence.  The form is '\\ddd' where
.IR d
represents an octal digit.
.PP
There is one additional flag,
.IR 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.
.SH SEE ALSO
.BR unvis (1),
.BR strunvis (3),
.BR unvis (3)
.SH HISTORY
These functions first appeared in 4.4BSD.