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_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 ‘%', or
VIS_MIMESTYLE 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 ‘\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.
VIS_HTTPSTYLE
Use URI encoding as described in RFC 1738. The form is ‘%xx' where x represents a lower case hexadecimal digit.
VIS_MIMESTYLE
Use MIME Quoted-Printable encoding as described in RFC 2045, only don't break lines and don't handle CRLF. The form is: ‘%XX' where X represents an upper case 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.