Introduce nng_options, nng_setopt, nng_getopt manual pages.

This starts a new section 5 for generic topics, and sets up some links
for things like nng_duration and nng_socket types.  There will some day
be an nng_errors(5) page as well.

Some initial work towards indexing terms for these pages is done now too.
(Indexing will mostly be useful when generating book forms of this
documentation.)

1 files changed, 122 insertions, 0 deletions

diff --git a/docs/man/nng_getopt.adoc b/docs/man/nng_getopt.adoc
new file mode 100644
index 00000000..322927cc
--- /dev/null
+++ b/docs/man/nng_getopt.adoc
@@ -0,0 +1,122 @@
+= nng_getopt(3)
+//
+// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
+// Copyright 2018 Capitar IT Group BV <info@capitar.com>
+//
+// This document is supplied under the terms of the MIT License, a
+// copy of which should be located in the distribution where this
+// file was obtained (LICENSE.txt).  A copy of the license may also be
+// found online at https://opensource.org/licenses/MIT.
+//
+
+== NAME
+
+nng_getopt - get socket option
+
+== SYNOPSIS
+
+[source, c]
+-----------
+#include <nng/nng.h>
+
+int nng_getopt(nng_socket s, const char *opt, void *val, size_t *valszp);
+
+int nng_getopt_int(nng_socket s, const char *opt, int *ivalp);
+
+int nng_getopt_ms(nng_socket s, const char *opt, nng_duration *durp);
+
+int nng_getopt_ptr(nng_socket s, const char *opt, void **ptr);
+
+int nng_getopt_size(nng_socket s, const char *opt, size_t *zp);
+
+int nng_getopt_uint64(nng_socket s, const char *opt, uint64_t *u64p);
+-----------
+
+== DESCRIPTION
+
+The `((nng_getopt))()` functions are used to retrieve option values for
+the socket _s_.
+The actual options that may be retrieved in this way vary.
+A number of them are documented in <<nng_options#,nng_options(5)>>.
+
+Additionally transport-specific options and protocol-specific options are
+documented with the transports and protocols themselves.
+
+In all of these forms, the option _opt_ is retrieved from the socket _s_.
+The forms vary based on the type of the option they take.
+
+TIP: Generally, it will be easier to use one of the typed forms instead.
+
+NOTE: No validation that the option is actually of the associated
+type is performed, so the caller must take care to use the *correct* typed form.
+
+The details of the type, size, and semantics of the option will depend
+on the actual option, and will be documented with the option itself.
+
+
+`nng_getopt()`::
+This function is untyped can be used to retrieve the value of any option.
+The caller must store a pointer to a buffer to receive the value in _val_,
+and the size of the buffer shall be stored at the location referenced by
+_valszp_.
++
+When the function returns, the actual size of the data copied (or that
+would have been copied if sufficient space were present) is stored at
+the location referened by _valszp_.
+If the caller's buffer is not large enough to hold the entire object,
+then the copy is truncated.
+Therefore the caller should check for truncation by verifyng that the
+returned size in _valszp_ does not exceed the original buffer size.
++
+It is acceptable to pass `NULL` for _val_ if the value in _valszp_ is zero.
+This can be used to determine the size of the buffer needed to receive
+the object.
+
+`nng_getopt_int()`::
+
+This function is for options which take an integer (`int`) or boolean (`bool`).
+The value will be stored at _ivalp_.
+For booleans the value will be eiher 0 (`false`) or 1 (`true`).
+
+`nng_getopt_ms()`::
+This function is used to retrieve time durations
+(such as timeouts), stored in _durp_ as a number of milliseconds.
+(The special value ((`NNG_DUR_INFINITE`)) means an infinite amount of time, and
+the special value ((`NNG_DUR_DEFAULT`)) means a context-specific default.)
+
+`nng_getopt_ptr()`::
+This function is used to retrieve a pointer, _ptr_, to structured data.
+The data referenced by _ptr_ is generally managed using other functions.
+Note that this form is somewhat special in that the object is generally
+not copied, but instead the *pointer* to the object is copied.
+
+`nng_getopt_size()`::
+This function is used to retrieve a size into the pointer _zp_,
+typically for buffer sizes, message maximum sizes, and similar options.
+
+`nng_getopt_uint64()`::
+This function is used to retrieve a 64-bit unsigned value into the value
+referenced by _u64p_.
+This is typically used for options related to identifiers, network
+numbers, and similar.
+
+== RETURN VALUES
+
+This function returns 0 on success, and non-zero otherwise.
+
+== ERRORS
+
+`NNG_ECLOSED`:: Parameter _s_ does not refer to an open socket.
+`NNG_ENOTSUP`:: The option _opt_ is not supported.
+`NNG_EWRITEONLY`:: The option _opt_ is write-only.
+
+== SEE ALSO
+
+[.text-left]
+<<nng_dialer_getopt#,nng_dialer_getopt(3)>>,
+<<nng_listener_getopt#,nng_listener_getopt(3)>>,
+<<nng_pipe_getopt#,nng_pipe_getopt(3)>>,
+<<nng_setopt#,nng_setopt(3)>>,
+<<nng_strerror#,nng_strerror(3)>>,
+<<nng#,nng_options(5)>>,
+<<nng#,nng(7)>>