VIS(3) Library Functions Manual VIS(3)
vis, nvis, strvis, strnvis, strvisx, strnvisx, svis, snvis, strsvis,
strsnvis, strsvisx strsnvisx -- visually encode characters
Standard C Library (libc, -lc)
vis(char *dst, int c, int flag, int nextc);
nvis(char *dst, size_t dlen, int c, int flag, int nextc);
strvis(char *dst, const char *src, int flag);
strnvis(char *dst, size_t dlen, const char *src, int flag);
strvisx(char *dst, const char *src, size_t len, int flag);
strnvisx(char *dst, size_t dlen, const char *src, size_t len, int flag);
svis(char *dst, int c, int flag, int nextc, const char *extra);
snvis(char *dst, size_t dlen, int c, int flag, int nextc, const char
strsvis(char *dst, const char *src, int flag, const char *extra);
strsnvis(char *dst, size_t dlen, const char *src, int flag, const char
strsvisx(char *dst, const char *src, size_t len, int flag, const char
strsnvisx(char *dst, size_t dlen, const char *src, size_t len, int
flag, const char *extra);
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
The strvis(), strnvis(), strvisx(), and strnvisx() functions copy into
dst a visual representation of the string src. The strvis() and
strnvis() functions encode characters from src up to the first NUL. The
strvisx() and strnvisx() functions encode 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 ``n'' versions of the functions also take an additional argument dlen
that indicates the length of the dst buffer. If dlen is not large enough
to fix the converted string then the strnvis() and strnvisx() functions
return -1 and set errno to ENOSPC.
The functions svis(), snvis(), strsvis(), strsnvis(), strsvisx(), and
strsnvisx() correspond to vis(), nvis(), strvis(), strnvis(), strvisx(),
and strnvisx() 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), strunvis(3) or strnunvis(3) functions.
There are two parameters that can be controlled: the range of characters
that are encoded (applies only to vis(), nvis(), strvis(), strnvis(),
strvisx(), and strnvisx()), 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(), snvis(), strsvis(),
strsnvis(), strsvisx(), and strsnvisx(). 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
\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.
Use URI encoding as described in RFC 1738. The form is `%xx'
where x represents a lower case hexadecimal digit.
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.
The functions nvis() and snvis() will return NULL and the functions
strnvis(), strnvisx(), strsnvis(), and strsnvisx(), will return -1 when
the dlen destination buffer length size is not enough to perform the
conversion while setting errno to:
[ENOSPC] The destination buffer size is not large enough to
perform the conversion.
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. The
buffer size limited versions of the functions (nvis(), strnvis(),
strnvisx(), snvis(), strsnvis(), and strsnvisx()) appeared in NetBSD 6.0.
NetBSD 6.1.5 March 12, 2011 NetBSD 6.1.5