VIS(3) BSD 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 *extra);
strsvis(char *dst, const char *src, int flag, const char *extra);
strsnvis(char *dst, size_t dlen, const char *src, int flag, const char *extra);
strsvisx(char *dst, const char *src, size_t len, int flag, const char *extra);
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 trail-
ing 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(), strvisx(), and strnvisx() functions copy into dst a visual repre-
sentation 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 charac-
ters 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 addi-
tional 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
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'
\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
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-
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 conver-
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
BSD March 12, 2011 BSD