NAME

     vis, strvis, strnvis, strvisx — visually encode characters

LIBRARY

     library “libbsd”

SYNOPSIS

     #include <stdlib.h>
     #include <vis.h>
     (See libbsd(7) for include usage.)

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

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

     int
     strnvis(char *dst, const char *src, size_t size, 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 character c.  If c needs no
     encoding, it is copied in unaltered.  The string is NUL 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(), strnvis() 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
     strnvis() function encodes characters from src up to the first NUL or the end of dst, as
     indicated by size.  The strvisx() function encodes exactly len characters from src (this is
     useful for encoding a block of data that may contain NULs).  All three forms NUL terminate
     dst, except for strnvis() when size is zero, in which case dst is not touched.  For strvis()
     and strvisx(), the size of dst must be four times the number of characters encoded from src
     (plus one for the NUL).  strvis() and strvisx() return the number of characters in dst (not
     including the trailing NUL).  strnvis() returns the length that dst would become if it were
     of unlimited size (similar to snprintf(3) or strlcpy(3bsd)).  This can be used to detect
     truncation but it also means that the return value of strnvis() must not be used without
     checking it against size.

     The encoding is a unique, invertible representation composed entirely of graphic characters;
     it can be decoded back into the original form using the unvis(3bsd) or strunvis(3bsd)
     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 recognized by glob(3) (‘*’, ‘?’, ‘[’) and ‘#’.

     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 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.

     There are three forms of encoding.  All forms use the backslash ‘\’ character 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 a 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 ‘\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 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.

     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), snprintf(3), strlcpy(3bsd), unvis(3bsd)

HISTORY

     The vis(), strvis() and strvisx() functions first appeared in 4.4BSD.  The strnvis()
     function first appeared in OpenBSD 2.9.