VIS(3) BSD Programmer's Manual VIS(3)
vis, strvis, strnvis, strvisx, svis, strsvis, strnsvis, strsvisx -
visually encode characters
#include <stdlib.h>
#include <vis.h>
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);
char *
svis(char *dst, int c, int flag, int nextc, const char *extra);
int
strsvis(char *dst, const char *src, int flag, const char *extra);
int
strnsvis(char *dst, const char *src, size_t size, int flag,
const char *extra);
int
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
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 charac-
ters from src up to the first NUL. The strnvis() function encodes charac-
ters 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(3)). 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 functions svis(), strsvis(), strnsvis() and strsvisx() correspond to
vis(), strvis(), strnvis(), 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(), strnvis(), 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_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 func-
tions. Currently this form allows space, tab, newline, back-
space, bell, and return -- in addition to all graphic charac-
ters -- unencoded.
(The above flags have no effect for svis(), strsvis(), strnsvis(), 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 '\' charac-
ter to introduce a special sequence; two backslashes are used to
represent a real backslash, except VIS_HTTPSTYLE which uses '%'. 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 char-
acters (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 - (040)
\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 representa-
tion 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 hexadecimal digit.
VIS_UTF8 Pass through non-ASCII non-control UTF-8 characters as-is.
Currently only implemented for strnvis() and strnsvis(). Only
characters in the Basic Multilingual Plane are recognised.
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), snprintf(3), strlcpy(3), unvis(3)
T. Berners-Lee, Uniform Resource Locators (URL), RFC1738.
The vis(), strvis() and strvisx() functions first appeared in 4.4BSD. The
strnvis() function first appeared in OpenBSD 2.9. The svis(), strsvis(),
and strsvisx() functions appeared in NetBSD 1.5. The strnsvis() function
as well as the VIS_UTF8 flag appeared in MirBSD #10.
MirBSD #10-current May 15, 2018 2