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. Unvis 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. Strunvis 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 only supported flag is VIS_HTTPSTYLE.
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: