VIS(3) BSD Library Functions Manual VIS(3)
NAME
vis, nvis, strvis, strnvis, strvisx, strnvisx, svis, snvis, strsvis, strsnvis, strsvisx strsnvisx -- visually encode characters
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <vis.h>
char *
vis(char *dst, int c, int flag, int nextc);
char *
nvis(char *dst, size_t dlen, int c, int flag, int nextc);
int
strvis(char *dst, const char *src, int flag);
int
strnvis(char *dst, size_t dlen, const char *src, int flag);
int
strvisx(char *dst, const char *src, size_t len, int flag);
int
strnvisx(char *dst, size_t dlen, const char *src, size_t len, int flag);
char *
svis(char *dst, int c, int flag, int nextc, const char *extra);
char *
snvis(char *dst, size_t dlen, int c, int flag, int nextc, const char *extra);
int
strsvis(char *dst, const char *src, int flag, const char *extra);
int
strsnvis(char *dst, size_t dlen, const char *src, int flag, const char *extra);
int
strsvisx(char *dst, const char *src, size_t len, int flag, const char *extra);
int
strsnvisx(char *dst, size_t dlen, const char *src, size_t len, int flag, const char *extra);
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 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 charac-
ters 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(), 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 func-
tions. 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 ' 00' through ' 37', 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^?').
40 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)
- BS (010)
f - NP (014)
- NL (012)
- CR (015)
s - SP (040)
- HT (011)
v - VT (013)
- NUL (000)
When using this format, the nextc parameter is looked at to determine if a NUL character can be encoded as ' ' instead of
' 00'. 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.
VIS_HTTPSTYLE
Use URI encoding as described in RFC 1738. The form is '%xx' where x represents a lower case hexadecimal digit.
VIS_MIMESTYLE
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.
ERRORS
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.
SEE ALSO
unvis(1), vis(1), unvis(3)
T. Berners-Lee, Uniform Resource Locators (URL), RFC1738.
HISTORY
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.
BSD
March 12, 2011 BSD