VIS(3)                         Library Routines                         VIS(3)

NAME

       vis - visually encode characters

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 NULL); thus, when encoding a set of characters into a buf‐
       fer, the size of the buffer should be four times the number of  charac‐
       ters  encoded,  plus  one for the trailing NULL.  The flag parameter is
       used for altering the default range of characters considered for encod‐
       ing and for altering the visual representation.  The additional charac‐
       ter, 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 NULL.  The strvisx function encodes exactly len characters
       from src (this is useful for encoding a block of data that may  contain
       NULL's).   Both forms NULL terminate dst.  The size of dst must be four
       times the number of characters encoded  from  src  (plus  one  for  the
       NULL).   Both forms return the number of characters in dst (not includ‐
       ing the trailing NULL).

       The encoding is a unique, invertible representation comprised  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  charac‐
       ters  that  are  encoded,  and the type of representation used.  By de‐
       fault, 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 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 charac‐
              ters - unencoded.

       There are three forms of encoding.  All forms use the backslash charac‐
       ter '\' to introduce a special sequence; two backslashes  are  used  to
       represent a real backslash.  These are the visual formats:

       (default)
              Use an 'M' to represent meta characters (characters with the 8th
              bit set), and use carat ^ 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
                      '\M^?').

              \040
                      Represents ASCII space.

              \240
                      Represents Meta-space.


       VIS_CSTYLE
              Use C-style backslash sequences to represent standard non-print‐
              able 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 - NULL (000)


              When  using this format, the nextc parameter is looked at to de‐
              termine if a NULL 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.

       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), strunvis(3), unvis(3)

HISTORY

       These functions first appeared in 4.4BSD.




GNO                             3 January 1999                          VIS(3)