---
name: "svis"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/nvis"
---

# svis(3)

visually encode characters

Part of [nvis(3)](/nvis/3.md).

## NAME

**vis**, **nvis**, **strvis**, **stravis**, **strnvis**, **strvisx**, **strnvisx**, **strenvisx**, **svis**, **snvis**, **strsvis**, **strsnvis**, **strsvisx**, **strsnvisx**, **strsenvisx** – visually encode characters

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<vis.h\>**

*char* *\**\

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

*char* *\**\

**nvis**(*char \*dst*, *size\_t dlen*, *int c*, *int flag*, *int nextc*);

*int*\

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

*int*\

**stravis**(*char \*\*dst*, *const char \*src*, *int flag*);

*int*\

**strnvis**(*char \*dst*, *size\_t dlen*, *const char \*src*, *int flag*);

*int*\

**strvisx**(*char \*dst*, *const char \*src*, *size\_t len*, *int flag*);

*int*\

**strnvisx**(*char \*dst*, *size\_t dlen*, *const char \*src*, *size\_t len*, *int flag*);

*int*\

**strenvisx**(*char \*dst*, *size\_t dlen*, *const char \*src*, *size\_t len*, *int flag*, *int \*cerr\_ptr*);

*char* *\**\

**svis**(*char \*dst*, *int c*, *int flag*, *int nextc*, *const char \*extra*);

*char* *\**\

**snvis**(*char \*dst*, *size\_t dlen*, *int c*, *int flag*, *int nextc*, *const char \*extra*);

*int*\

**strsvis**(*char \*dst*, *const char \*src*, *int flag*, *const char \*extra*);

*int*\

**strsnvis**(*char \*dst*, *size\_t dlen*, *const char \*src*, *int flag*, *const char \*extra*);

*int*\

**strsvisx**(*char \*dst*, *const char \*src*, *size\_t len*, *int flag*, *const char \*extra*);

*int*\

**strsnvisx**(*char \*dst*, *size\_t dlen*, *const char \*src*, *size\_t len*, *int flag*, *const char \*extra*);

*int*\

**strsenvisx**(*char \*dst*, *size\_t dlen*, *const char \*src*, *size\_t len*, *int flag*, *const char \*extra*, *int \*cerr\_ptr*);

## 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 bytes (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 bytes 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**(), **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)](/unvis/3.md), [strunvis(3)](/strunvis/3.md) or [strnunvis(3)](/strnunvis/3.md) 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)](/isgraph/3.md)). The following flags alter this:

- VIS\_DQ

  Also encode double quotes

- VIS\_GLOB

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

- 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

  Leave “safe” characters unencoded. Characters are only considered unsafe if they cause common terminals to perform unexpected functions. Setting the VIS\_SAFE flag exempts the backspace, bell, and return characters from being encoded.

(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)](/iscntrl/3.md)). 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.

## MULTIBYTE CHARACTER SUPPORT

These functions support multibyte character input. The encoding conversion is influenced by the setting of the LC\_CTYPE environment variable which defines the set of characters that can be copied without encoding.

If VIS\_NOLOCALE is set, processing is done assuming the C locale and overriding any other environment settings.

When 8-bit data is present in the input, LC\_CTYPE must be set to the correct locale or to the C locale. If the locales of the data and the conversion are mismatched, multibyte character recognition may fail and encoding will be performed byte-by-byte instead.

As noted above, *dst* must be four times the number of bytes processed from *src*. But note that each multibyte character can be up to MB\_LEN\_MAX bytes so in terms of multibyte characters, *dst* must be four times MB\_LEN\_MAX times the number of characters processed from *src*.

## ENVIRONMENT

- LC\_CTYPE

  Specify the locale of the input data. Set to C if the input data locale is unknown.

## ERRORS

The functions **nvis**() and **snvis**() will return NULL and the functions **strnvis**(), **strnvisx**(), **strsnvis**(), and **strsnvisx**(), will return -1 when the *dlen* destination buffer size is not enough to perform the conversion while setting *errno* to:

- \[ENOSPC\]

  The destination buffer size is not large enough to perform the conversion.

## SEE ALSO

[unvis(1)](/unvis/1.md), [vis(1)](/vis/1.md), [glob(3)](/glob/3.md), [unvis(3)](/unvis/3.md) T. Berners-Lee, *Uniform Resource Locators (URL)*, RFC 1738. *Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies*, RFC 2045.

## HISTORY

The **vis**(), **strvis**(), and **strvisx**() functions first appeared in 4.4BSD. The **svis**(), **strsvis**(), and **strsvisx**() functions appeared in NetBSD 1.5 and FreeBSD 9.2. The buffer size limited versions of the functions (**nvis**(), **strnvis**(), **strnvisx**(), **snvis**(), **strsnvis**(), and **strsnvisx**(*Pc*) appeared in NetBSD 6.0 and FreeBSD 9.2. Multibyte character support was added in NetBSD 7.0 and FreeBSD 9.2.