NAME
vis, strvis, strnvis, strvisx - visually encode characters
LIBRARY
Utility functions from BSD systems (libbsd, -lbsd)
SYNOPSIS
#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);
DESCRIPTION
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 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
characters from src up to the first NUL. The strnvis() function encodes
characters 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 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, 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 functions. Currently this form allows space, tab,
newline, backspace, bell, and return -- in addition to all
graphic characters -- unencoded.
There are three forms of encoding. All forms use the backslash ‘\’
character to introduce a special sequence; two backslashes are used to
represent a real backslash. 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
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.
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.
SEE ALSO
unvis(1), vis(1), snprintf(3), strlcpy(3), unvis(3)
HISTORY
The vis(), strvis() and strvisx() functions first appeared in 4.4BSD.
The strnvis() function first appeared in OpenBSD 2.9.