.\"	Id: mandoc_html.3,v 1.19 2019/01/11 12:56:43 schwarze Exp 
.\"
.\" Copyright (c) 2014, 2017, 2018 Ingo Schwarze <schwarze@openbsd.org>
.\"
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.Dd January 11, 2019
.Dt MANDOC_HTML 3
.Os
.Sh NAME
.Nm mandoc_html
.Nd internals of the mandoc HTML formatter
.Sh SYNOPSIS
.In "html.h"
.Ft void
.Fn print_gen_decls "struct html *h"
.Ft void
.Fn print_gen_comment "struct html *h" "struct roff_node *n"
.Ft void
.Fn print_gen_head "struct html *h"
.Ft struct tag *
.Fo print_otag
.Fa "struct html *h"
.Fa "enum htmltag tag"
.Fa "const char *fmt"
.Fa ...
.Fc
.Ft void
.Fo print_tagq
.Fa "struct html *h"
.Fa "const struct tag *until"
.Fc
.Ft void
.Fo print_stagq
.Fa "struct html *h"
.Fa "const struct tag *suntil"
.Fc
.Ft void
.Fo print_text
.Fa "struct html *h"
.Fa "const char *word"
.Fc
.Ft char *
.Fo html_make_id
.Fa "const struct roff_node *n"
.Fc
.Ft int
.Fo html_strlen
.Fa "const char *cp"
.Fc
.Sh DESCRIPTION
The mandoc HTML formatter is not a formal library.
However, as it is compiled into more than one program, in particular
.Xr mandoc 1
and
.Xr man.cgi 8 ,
and because it may be security-critical in some contexts,
some documentation is useful to help to use it correctly and
to prevent XSS vulnerabilities.
.Pp
The formatter produces HTML output on the standard output.
Since proper escaping is usually required and best taken care of
at one central place, the language-specific formatters
.Po
.Pa *_html.c ,
see
.Sx FILES
.Pc
are not supposed to print directly to
.Dv stdout
using functions like
.Xr printf 3 ,
.Xr putc 3 ,
.Xr puts 3 ,
or
.Xr write 2 .
Instead, they are expected to use the output functions declared in
.Pa html.h
and implemented as part of the main HTML formatting engine in
.Pa html.c .
.Ss Data structures
These structures are declared in
.Pa html.h .
.Bl -tag -width Ds
.It Vt struct html
Internal state of the HTML formatter.
.It Vt struct tag
One entry for the LIFO stack of HTML elements.
Members are
.Fa "enum htmltag tag"
and
.Fa "struct tag *next" .
.El
.Ss Private interface functions
The function
.Fn print_gen_decls
prints the opening
.Ao Pf \&? Ic xml ? Ac
and
.Aq Pf \&! Ic DOCTYPE
declarations required for the current document type.
.Pp
The function
.Fn print_gen_comment
prints the leading comments, usually containing a Copyright notice
and license, as an HTML comment.
It is intended to be called right after opening the
.Aq Ic HTML
element.
Pass the first
.Dv ROFFT_COMMENT
node in
.Fa n .
.Pp
The function
.Fn print_gen_head
prints the opening
.Aq Ic META
and
.Aq Ic LINK
elements for the document
.Aq Ic HEAD ,
using the
.Fa style
member of
.Fa h
unless that is
.Dv NULL .
It uses
.Fn print_otag
which takes care of properly encoding attributes,
which is relevant for the
.Fa style
link in particular.
.Pp
The function
.Fn print_otag
prints the start tag of an HTML element with the name
.Fa tag ,
optionally including the attributes specified by
.Fa fmt .
If
.Fa fmt
is the empty string, no attributes are written.
Each letter of
.Fa fmt
specifies one attribute to write.
Most attributes require one
.Va char *
argument which becomes the value of the attribute.
The arguments have to be given in the same order as the attribute letters.
If an argument is
.Dv NULL ,
the respective attribute is not written.
.Bl -tag -width 1n -offset indent
.It Cm c
Print a
.Cm class
attribute.
.It Cm h
Print a
.Cm href
attribute.
This attribute letter can optionally be followed by a modifier letter.
If followed by
.Cm R ,
it formats the link as a local one by prefixing a
.Sq #
character.
If followed by
.Cm I ,
it interpretes the argument as a header file name
and generates a link using the
.Xr mandoc 1
.Fl O Cm includes
option.
If followed by
.Cm M ,
it takes two arguments instead of one, a manual page name and
section, and formats them as a link to a manual page using the
.Xr mandoc 1
.Fl O Cm man
option.
.It Cm i
Print an
.Cm id
attribute.
.It Cm \&?
Print an arbitrary attribute.
This format letter requires two
.Vt char *
arguments, the attribute name and the value.
The name must not be
.Dv NULL .
.It Cm s
Print a
.Cm style
attribute.
If present, it must be the last format letter.
It requires two
.Va char *
arguments.
The first is the name of the style property, the second its value.
The name must not be
.Dv NULL .
The
.Cm s
.Ar fmt
letter can be repeated, each repetition requiring an additional pair of
.Va char *
arguments.
.El
.Pp
.Fn print_otag
uses the private function
.Fn print_encode
to take care of HTML encoding.
If required by the element type, it remembers in
.Fa h
that the element is open.
The function
.Fn print_tagq
is used to close out all open elements up to and including
.Fa until ;
.Fn print_stagq
is a variant to close out all open elements up to but excluding
.Fa suntil .
.Pp
The function
.Fn print_text
prints HTML element content.
It uses the private function
.Fn print_encode
to take care of HTML encoding.
If the document has requested a non-standard font, for example using a
.Xr roff 7
.Ic \ef
font escape sequence,
.Fn print_text
wraps
.Fa word
in an HTML font selection element using the
.Fn print_otag
and
.Fn print_tagq
functions.
.Pp
The function
.Fn html_make_id
takes a node containing one or more text children
and returns a newly allocated string containing the concatenation
of the child strings, with blanks replaced by underscores.
If the node
.Fa n
contains any non-text child node,
.Fn html_make_id
returns
.Dv NULL
instead.
The caller is responsible for freeing the returned string.
.Pp
The function
.Fn html_strlen
counts the number of characters in
.Fa cp .
It is used as a crude estimate of the width needed to display a string.
.Pp
The functions
.Fn print_eqn ,
.Fn print_tbl ,
and
.Fn print_tblclose
are not yet documented.
.Sh FILES
.Bl -tag -width mandoc_aux.c -compact
.It Pa main.h
declarations of public functions for use by the main program,
not yet documented
.It Pa html.h
declarations of data types and private functions
for use by language-specific HTML formatters
.It Pa html.c
main HTML formatting engine and utility functions
.It Pa mdoc_html.c
.Xr mdoc 7
HTML formatter
.It Pa man_html.c
.Xr man 7
HTML formatter
.It Pa tbl_html.c
.Xr tbl 7
HTML formatter
.It Pa eqn_html.c
.Xr eqn 7
HTML formatter
.It Pa out.h
declarations of data types and private functions
for shared use by all mandoc formatters,
not yet documented
.It Pa out.c
private functions for shared use by all mandoc formatters
.It Pa mandoc_aux.h
declarations of common mandoc utility functions, see
.Xr mandoc 3
.It Pa mandoc_aux.c
implementation of common mandoc utility functions
.El
.Sh SEE ALSO
.Xr mandoc 1 ,
.Xr mandoc 3 ,
.Xr man.cgi 8
.Sh AUTHORS
.An -nosplit
The mandoc HTML formatter was written by
.An Kristaps Dzonsons Aq Mt kristaps@bsd.lv .
It is maintained by
.An Ingo Schwarze Aq Mt schwarze@openbsd.org ,
who also wrote this manual.