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VIS(3)			 BSD Library Functions Manual			VIS(3)

NAME
     vis — visually encode characters

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <vis.h>

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

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

     int
     strvisx(char *dst, const char *src, size_t 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 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 flag argument is used for altering
     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 below).

     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 NUL.  The strvisx() function encodes exactly len
     characters from src (this is useful for encoding a block of data that may
     contain NUL's).  Both forms NUL terminate dst.  The size of dst must be
     four times the number of characters encoded from src (plus one for the
     NUL).  Both forms return the number of characters in dst (not including
     the trailing NUL).

     The encoding is a unique, invertible representation composed 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_GLOB	 Also encode magic characters (‘*’, ‘?’, ‘[’ and ‘#’) recog‐
		 nized by glob(3).

     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 four forms of encoding.	Most 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 caret ‘^’ to represent control
		    characters see (iscntrl(3)).  The following formats are
		    used:

		    \^C	   Represents the control character ‘C’.  Spans char‐
			   acters ‘\000’ through ‘\037’, and ‘\177’ (as
			   ‘\^?’).

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

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

		    \040   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:

			  \a  BEL (007)
			  \b  BS (010)
			  \f  NP (014)
			  \n  NL (012)
			  \r  CR (015)
			  \s  SP (040)
			  \t  HT (011)
			  \v  VT (013)
			  \0  NUL (000)

		    When using this format, the nextc argument is looked at to
		    determine if a NUL character can be encoded as ‘\0’
		    instead of ‘\000’.	If nextc is an octal digit, the latter
		    representation is used to avoid ambiguity.

     VIS_HTTPSTYLE  Use URI encoding as described in RFC 1808.	The form is
		    ‘%dd’ where d represents a hexadecimal digit.

     VIS_OCTAL	    Use a three digit octal sequence.  The form is ‘\ddd’
		    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.

SEE ALSO
     unvis(1), unvis(3)

     R. Fielding, Relative Uniform Resource Locators, RFC1808.

HISTORY
     These functions first appeared in 4.4BSD.

BUGS
     The vis family of functions do not recognize multibyte characters, and
     thus may consider them to be non-printable when they are in fact print‐
     able (and vice versa.)

BSD				 April 9, 2006				   BSD
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