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VIS(3) Library Functions Manual VIS(3)

vis, strvis, strnvis, strvisx, stravis — visually encode characters

#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 dstsize, int flag);

int
strvisx(char *dst, const char *src, size_t srclen, int flag);

int
stravis(char **outp, const char *src, int flag);

The () function copies into dst a string which represents the character c. If c needs no encoding, it is copied in unaltered. dst will be NUL-terminated and must be at least 5 bytes long (maximum encoding requires 4 bytes plus the NUL). The additional character, nextc, is only used when selecting the VIS_CSTYLE encoding format (explained below).

The (), strnvis() and strvisx() functions copy into dst a visual representation of the string src.

The () function encodes characters from src up to the first NUL, into a buffer dst (which must be at least 4 * strlen(src) + 1 long).

The () function encodes characters from src up to the first NUL or the end of the buffer dst, as indicated by dstsize.

The () function encodes exactly srclen characters from src into a buffer dst (which must be at least 4 * srclen + 1 long). This is useful for encoding a block of data that may contain NULs.

The () function writes a visual representation of the string src into a newly allocated string outp; it does not attempt to realloc(3) outp. outp should be passed to free(3) to release the allocated storage when it is no longer needed. stravis() checks for integer overflow when allocating memory.

All forms NUL-terminate dst, except for () when dstsize is zero, in which case dst is not touched.

The flag parameter is used for altering the default range of characters considered for encoding and for altering the visual representation.

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:

Encode all characters, whether visible or not.
Also encode double quote characters (‘"’).
Also encode magic characters recognized by glob(3) (‘*’, ‘?’, ‘[’) and ‘#’.
Also encode space.
Also encode tab.
Also encode newline.
Synonym for VIS_SP | | .
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:

(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:
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 ‘\376’.
Represents control character ‘C’ with the 8th bit set. Spans characters ‘\200’ through ‘\237’, and ‘\377’ (as ‘\M^?’).
Represents ASCII space.
Represents Meta-space.
Represents character ‘C’. Only used with VIS_ALL.
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.

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.

vis() returns a pointer to the terminating NUL character of the string dst.

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 dstsize.

Upon successful completion, stravis() returns the number of characters in *outp (not including the trailing NUL). Otherwise, stravis() returns -1 and sets errno to ENOMEM.

strvis() has unusual storage requirements that can lead to stack or heap corruption if the destination is not carefully constructed. A common mistake is to use the same size for the source and destination when the destination actually needs up to 4 * strlen(source) + 1 bytes.

If the length of a string to be encoded is not known at compile time, use stravis():

char *src, *dst;

...
if (stravis(&dst, src, VIS_OCTAL) == -1)
	err(1, "stravis");

...
free(dst);

To encode a fixed size buffer, strnvis() can be used with a fixed size target buffer:

char src[MAXPATHLEN];
char dst[4 * MAXPATHLEN + 1];

...
if (strnvis(dst, src, sizeof(dst), VIS_OCTAL) >= sizeof(dst))
	err(1, "strnvis");

unvis(1), vis(1), free(3), snprintf(3), strlcpy(3), unvis(3)

The vis(), strvis() and strvisx() functions first appeared in 4.4BSD, strnvis() in OpenBSD 2.9 and stravis() in OpenBSD 5.7.

The VIS_ALL flag first appeared in OpenBSD 4.9.

March 16, 2018 OpenBSD-6.9