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NAME    [Toc]    [Back]

     vis, strvis, strvisx, svis, strsvis, strsvisx - visually encode characters

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <vis.h>

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

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

     strvisx(char *dst, const char *src, size_t len, int flag);

     char *
     svis(char *dst, int c, int flag, int nextc, const char *extra);

     strsvis(char *dst, const char *src, int flag, const char *extra);

     strsvisx(char *dst, const char *src, size_t len, int flag,
             const char *extra);

DESCRIPTION    [Toc]    [Back]

     The vis() function copies into dst a string which represents the character
 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 representation
 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 functions svis(), strsvis(), and strsvisx() correspond to vis(),
     strvis(), and strvisx() but have an additional argument extra, pointing
     to a NUL terminated list of characters. These characters will be copied
     encoded or backslash-escaped into dst.  These functions are useful e. g.
     to remove the special meaning of certain characters to shells.

     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 (applies only to vis(), strvis(), and strvisx()), 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.  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.

     (The above flags have no effect for svis(), strsvis(), and strsvisx().
     When using these functions, place all graphic characters to be encoded in
     an array pointed to by extra.  In general, the backslash character should
     be included in this array, see the warning on the use of the VIS_NOSLASH
     flag below).

     There are four forms of encoding.  All forms use the backslash character
     `\' to introduce a special sequence; two backslashes are used to represent
 a real backslash, except VIS_HTTPSTYLE that uses `$'.  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 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

                 \040   Represents ASCII space.

                 \240   Represents Meta-space.

     VIS_CSTYLE  Use C-style backslash sequences to represent standard nonprintable
 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 representation
 is used to avoid ambiguity.

     VIS_OCTAL   Use a three digit octal sequence.  The form is `\ddd' where d
                 represents an octal digit.

                 Use URI encoding as described in RFC 1808.  The form is `%xx'
                 where x represents a hexadecimal 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    [Toc]    [Back]

     unvis(1), vis(1), unvis(3)

     R. Fielding, Relative Uniform Resource Locators, RFC1808.

HISTORY    [Toc]    [Back]

     The vis, strvis, and strvisx functions first appeared in 4.4BSD.  The
     svis, strsvis, and strsvisx functions appeared in NetBSD 1.5.

BSD                              June 9, 1993                              BSD
[ Back ]
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