VIS(3)                    OpenBSD Programmer's Manual                   VIS(3)

NAME
     vis, strvis, strvisx - visually encode characters



SYNOPSIS
     #include <vis.h>

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

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

     int
     strvisx(char *dst, char *src, int 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 parameter 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 charac-
     ters from src (this is useful for encoding a block of data that may con-
     tain NULs).  Both forms null 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_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.  These are control charac-
                 ters 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 three forms of encoding.  All forms use the backslash `\' char-
     acter 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 a caret `^' to represent control char-
                 acters (see iscntrl(3)).  The following formats are used:

                 \^C    Represents the control character `C'. Spans characters
                        `\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)
                       \t - HT (011)
                       \v - VT (013)
                       \0 - NUL (000)

                 When using this format, the nextc parameter 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 representa-
                 tion is used to avoid ambiguity.

     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),  vis(1),  unvis(3)

HISTORY
     These functions first appeared in 4.4BSD.


OpenBSD 2.6                      June 9, 1993                                2