UNVIS(3) FreeBSD Library Functions Manual UNVIS(3)


unvis, strunvisdecode a visual representation of characters


Standard C Library (libc, -lc)


#include < vis.h>

unvis( char *cp, int c, int *astate, int flag);

strunvis( char *dst, const char *src);

strnunvis( char *dst, size_t dlen, const char *src);

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

strnunvisx( char *dst, size_t dlen, const char *src, int flag);


The unvis(), strunvis() and strunvisx() functions are used to decode a visual representation of characters, as produced by the vis(3) function, back into the original form.

The unvis() function is called with successive characters in c until a valid sequence is recognized, at which time the decoded character is available at the character pointed to by cp.

The strunvis() function decodes the characters pointed to by src into the buffer pointed to by dst. The strunvis() function simply copies src to dst, decoding any escape sequences along the way, and returns the number of characters placed into dst, or -1 if an invalid escape sequence was detected. The size of dst should be equal to the size of src (that is, no expansion takes place during decoding).

The strunvisx() function does the same as the strunvis() function, but it allows you to add a flag that specifies the style the string src is encoded with. Currently, the supported flags are: VIS_HTTPSTYLE and VIS_MIMESTYLE.

The unvis() function implements a state machine that can be used to decode an arbitrary stream of bytes. All state associated with the bytes being decoded is stored outside the unvis() function (that is, a pointer to the state is passed in), so calls decoding different streams can be freely intermixed. To start decoding a stream of bytes, first initialize an integer to zero. Call unvis() with each successive byte, along with a pointer to this integer, and a pointer to a destination character. The unvis() function has several return codes that must be handled properly. They are:

0 (zero)
Another character is necessary; nothing has been recognized yet.
A valid character has been recognized and is available at the location pointed to by cp.
A valid character has been recognized and is available at the location pointed to by cp; however, the character currently passed in should be passed in again.
A valid sequence was detected, but no character was produced. This return code is necessary to indicate a logical break between characters.
An invalid escape sequence was detected, or the decoder is in an unknown state. The decoder is placed into the starting state.

When all bytes in the stream have been processed, call unvis() one more time with flag set to UNVIS_END to extract any remaining character (the character passed in is ignored).

The flag argument is also used to specify the encoding style of the source. If set to VIS_HTTPSTYLE or VIS_HTTP1808, unvis() will decode URI strings as specified in RFC 1808. If set to VIS_HTTP1866, unvis() will decode entity references and numeric character references as specified in RFC 1866. If set to VIS_MIMESTYLE, unvis() will decode MIME Quoted-Printable strings as specified in RFC 2045. If set to VIS_NOESCAPE, unvis() will not decode ‘ \’ quoted characters.

The following code fragment illustrates a proper use of unvis().

int state = 0; 
char out; 
while ((ch = getchar()) != EOF) { 
 switch(unvis(&out, ch, &state, 0)) { 
 case 0: 
  goto again; 
  errx(EXIT_FAILURE, "Bad character sequence!"); 
if (unvis(&out, '\0', &state, UNVIS_END) == UNVIS_VALID) 


The functions strunvis(), strnunvis(), strunvisx(), and strnunvisx() will return -1 on error and set errno to:
An invalid escape sequence was detected, or the decoder is in an unknown state.

In addition the functions strnunvis() and strnunvisx() will can also set errno on error to:

Not enough space to perform the conversion.


unvis(1), vis(1), vis(3)

R. Fielding, Relative Uniform Resource Locators, RFC1808.


The unvis() function first appeared in 4.4BSD. The strnunvis() and strnunvisx() functions appeared in NetBSD 6.0 and FreeBSD 9.2.


The names VIS_HTTP1808 and VIS_HTTP1866 are wrong. Percent-encoding was defined in RFC 1738, the original RFC for URL. RFC 1866 defines HTML 2.0, an application of SGML, from which it inherits concepts of numeric character references and entity references.
March 12, 2011 FreeBSD