VIS(3)

NAME

vis, strvis, strnvis, strvisx -- visually encode characters

LIBRARY

Utility functions from BSD systems (libbsd, -lbsd)

SYNOPSIS

#include <stdlib.h>
#include <vis.h>

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

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

int
strnvis(char *dst, const char *src, size_t size, int flag);

int
strvisx(char *dst, const char *src, size_t 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 NUL 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(), strnvis() 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 strnvis() function encodes characters from src up to the first NUL or the end of dst, as indicated by size. The strvisx() function encodes exactly len characters from src (this is useful for encoding a block of data that may contain NULs). All three forms NUL terminate dst, except for strnvis() when size is zero, in which case dst is not touched. For strvis() and strvisx(), the size of dst must be four times the number of characters encoded from src (plus one for the NUL). strvis() and strvisx() return the number of characters in dst (not including the trailing NUL). strnvis() returns the length that dst would become if it were of unlimited size (similar to
snprintf(3) or strlcpy(3)). This can be used to detect truncation but it also means that the return value of strnvis() must not be used without checking it against size.

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_GLOB Also encode magic characters recognized by glob(3) ('*', '?',

'[') 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_SAFE Only encode ``unsafe'' characters. These are 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
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 a caret '^' to represent control characters (see iscntrl(3)). The following formats are used:

\^C Represents the control character 'C'. Spans charac

ters '\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.

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), snprintf(3), strlcpy(3), unvis(3)

HISTORY

The vis(), strvis() and strvisx() functions first appeared in 4.4BSD. The strnvis() function first appeared in OpenBSD 2.9.

BSD November 24, 2010 BSD