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VIS(3) Library Functions Manual VIS(3)

vis — visually encode characters

#include <vis.h>

char *
vis(char *dst, char c, int flag, char nextc);

int
strvis(char *dst, char *src, int flag);

int
strvisx(char *dst, char *src, int len, int flag);

The () 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 NULL); 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 NULL. 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 () and () 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 including 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 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:

Also encode space.
Also encode tab.
Also encode newline.
Synonym for VIS_SP | VIS_TAB | VIS_NL.
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.

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 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 ‘\M^?’).
Represents ASCII space.
Represents Meta-space.
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)
\t - HT (011)
\v - VT (013)
\0 - NUL (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 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), unvis(3) strunvis(3)

These functions first appeared in 4.4BSD.

4.4BSD-Lite2 June 9, 1993 VIS(3)