header internals description.

+/* GNU mailutils - a suite of utilities for electronic mail
+   Copyright (C) 1999, 2000 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Library Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+#ifndef _HEADER0_H
+#define _HEADER0_H
+
+#include <header.h>
+#include <io0.h>
+#include <sys/types.h>
+
+#ifdef _cplusplus
+extern "C" {
+#endif
+
+#ifndef __P
+# ifdef __STDC__
+#  define __P(args) args
+# else
+#  define __P(args) ()
+# endif
+#endif /*__P */
+
+/* The structure members are offset that point to the begin/end of header
+   fields.  */
+struct _hdr
+{
+  char *fn;
+  char *fn_end;
+  char *fv;
+  char *fv_end;
+};
+
+/* The blurb member represents the headers, hdr_count the number of distinct
+   header field and the layout is done by struct_hdr *hdr.  */
+struct _header
+{
+  /* Owner.  */
+  void *owner;
+  /* Data.  */
+  char *temp_blurb;
+  size_t temp_blurb_len;
+  char *blurb;
+  size_t blurb_len;
+  size_t hdr_count;
+  struct _hdr *hdr;
+
+  /* Streams.  */
+  stream_t stream;
+  int (*_get_value) __P ((header_t, const char *, char *, size_t , size_t *));
+  int (*_set_value) __P ((header_t, const char *, const char *, int));
+};
+
+#ifdef _cplusplus
+}
+#endif
+
+#endif /* _HEADER0_H */