VIS(3)

NAME
vis - visually encode characters

SYNOPSIS
#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);

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 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() 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 strvisx() function encodes 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 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_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 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 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)
        \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
vis(1), unvis(1), unvis(3)

HISTORY
These functions first appeared in 4.4BSD.