VIS(3) Library Routines VIS(3)
vis - visually encode characters
char *vis (char *dst, int c, int flag, int nextc);
int strvis (char *dst, const char *src, int flag);
int strvisx (char *dst, const char *src, size_t len, int flag);
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 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 NULL); thus, when encoding a set of characters into a
buffer, the size of the buffer should be four times the number of char-
acters encoded, plus one for the trailing NULL. The flag parameter is
used for altering the default range of characters considered for encod-
ing and for altering the visual representation. The additional charac-
ter, nextc, is only used when selecting the VIS_CSTYLE encoding format
The strvis and strvisx functions copy into dst a visual representation
of the string src. The strvis function encodes characters from src up
to the first NULL. The strvisx function encodes exactly len characters
from src (this is useful for encoding a block of data that may contain
NULL's). Both forms NULL terminate dst. The size of dst must be four
times the number of characters encoded from src (plus one for the
NULL). Both forms return the number of characters in dst (not includ-
ing the trailing NULL).
The encoding is a unique, invertible representation comprised 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 charac-
ters 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_SP Also encode space.
Also encode tab.
VIS_NL Also encode newline.
Synonym for VIS_SP | VIS_TAB | VIS_NL .
Only encode "unsafe" characters. Unsafe means control charac-
ters 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 charac-
ters - unencoded.
There are three forms of encoding. All forms use the backslash charac-
ter '\' to introduce a special sequence; two backslashes are used to
represent a real backslash. These are the visual formats:
Use an 'M' to represent meta characters (characters with the 8th
bit set), and use carat ^ to represent control characters see
iscntrl(3). The following formats are used:
Represents the control character C. Spans characters
'\000' through '\037', and '\177' (as '\^?').
Represents character 'C' with the 8th bit set. Spans
characters '\241' through '\376'.
Represents control character C with the 8th bit set.
Spans characters '\200' through '\237', and '\377' (as
Represents ASCII space.
Use C-style backslash sequences to represent standard non-print-
able 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)
\t - HT (011)
\v - VT (013)
\0 - NULL (000)
When using this format, the nextc parameter is looked at to
determine if a NULL character can be encoded as '\0' instead of
'\000'. If nextc is an octal digit, the latter representation
is used to avoid ambiguity.
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.
unvis(1), strunvis(3), unvis(3)
These functions first appeared in 4.4BSD.
GNO 3 January 1999 VIS(3)
Man(1) output converted with