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VIS(3) NetBSD Library Functions Manual VIS(3)
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vis, strvis, strvisx, svis, strsvis, strsvisx -- visually encode charac-
Standard C Library (libc, -lc)
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);
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);
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
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 fol-
lowing 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 char-
acters 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
There are four forms of encoding. All forms use the backslash character
`\' to introduce a special sequence; two backslashes are used to repre-
sent a real backslash, except VIS_HTTPSTYLE that uses `%'. These are the
(default) Use an `M' to represent meta characters (characters with the
8th bit set), and use caret `^' to represent control charac-
ters see (iscntrl(3)). The following formats are used:
\^C Represents the control character `C'. Spans charac-
ters `\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 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 represen-
tation 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 1738. 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.
unvis(1), vis(1), unvis(3)
T. Berners-Lee, Uniform Resource Locators (URL), RFC1738.
The vis, strvis, and strvisx functions first appeared in 4.4BSD. The
svis, strsvis, and strsvisx functions appeared in NetBSD 1.5.
NetBSD 5.0.1 April 9, 2006 NetBSD 5.0.1