vis man page
vis, strvis, strnvis, strvisx — visually encode characters
vis(char *dst, int c, int flag, int nextc);
strvis(char *dst, const char *src, int flag);
strnvis(char *dst, const char *src, size_t size, int flag);
strvisx(char *dst, const char *src, size_t len, int flag);
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:
- Also encode magic characters recognized by glob(3) (‘
[’) and ‘
- Also encode space.
- Also encode tab.
- Also encode newline.
- Synonym for VIS_SP | VIS_TAB | VIS_NL.
- Only encode “unsafe” characters. These are 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:
- 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:
- 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 ‘
- Represents control character ‘
C’ with the 8th bit set. Spans characters ‘
\200’ through ‘
\237’, and ‘
\377’ (as ‘
- 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:
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.
- Use a three digit octal sequence. The form is ‘
drepresents 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), vis(1), snprintf(3), strlcpy(3), unvis(3)
The vis(), strvis() and strvisx() functions first appeared in 4.4BSD. The strnvis() function first appeared in OpenBSD 2.9.
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