VIS(3)

NAME
vis, strvis, strvisx, svis, strsvis, strsvisx - visually encode characters

LIBRARY
libc

SYNOPSIS
#include <vis.h>
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);
char *
svis(char *dst, int c, int flag, int nextc, const char *extra);
int
strsvis(char *dst, const char *src, int flag, const char *extra);
int
strsvisx(char *dst, 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 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 functions svis(), strsvis(), and strsvisx() correspond to vis(), strvis(), and strvisx() 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) or strunvis(3) functions.

There are two parameters that can be controlled: the range of characters that are encoded (applies only to vis(), strvis(), and strvisx()), 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.

(The above flags have no effect for svis(), strsvis(), and strsvisx(). 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 ‘%’. 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)
        \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.

VIS_HTTPSTYLE
    Use URI encoding as described in RFC 1738. The form is ‘%xx’ where x represents a 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.

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.