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DESCRIPTION

The strvis(), stravis(), 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 bytes encoded from src (plus one for the NUL). Both forms return the number of characters in dst (not including the trailing NUL). The stravis() function allocates space dynamically to hold the string. 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 fit the converted string then the strnvis() and strnvisx() functions return -1 and set errno to ENOSPC. The strenvisx() function takes an additional argument, cerr_ptr, that is used to pass in and out a multibyte conversion error flag. This is useful when processing single characters at a time when it is possible that the locale may be set to something other than the locale of the characters in the input data.

The functions svis(), snvis(), strsvis(), strsnvis(), strsvisx(), strsnvisx(), and strsenvisx() correspond to vis(), nvis(), strvis(), strnvis(), strvisx(), strnvisx(), and strenvisx() 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_DQ

Also encode double quotes

VIS_GLOB

Also encode the magic characters (‘*’, ‘?’, ‘[’, and ‘#’) recognized by glob(3).

VIS_SHELL

Also encode the meta characters used by shells (in addition to the glob characters): (‘'’, ‘`’, ‘"’, ‘;’, ‘&’, ‘<’, ‘>’, ‘(’, ‘)’, ‘|’, ‘]’, ‘\’, ‘$’, ‘!’, ‘^’, and ‘~’).

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_META

Synonym for VIS_WHITE | VIS_GLOB | VIS_SHELL.

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(), 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 six 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 ‘\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.

Non-printable characters without C-style backslash sequences use the default representation.

VIS_OCTAL

Use a three digit octal sequence. The form is ‘\ddd’ where d represents an octal digit.

VIS_CSTYLE | VIS_OCTAL

Same as VIS_CSTYLE except that non-printable characters without C-style backslash sequences use a three digit octal sequence.

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.