Fix crash and latest update.

doc/man3/OSSL_trace_enabled.pod

@@ -2,7 +2,8 @@
 
 =head1 NAME
 
-OSSL_trace_enabled, OSSL_trace_begin, OSSL_trace_end
+OSSL_trace_enabled, OSSL_trace_begin, OSSL_trace_end,
+OSSL_TRACE_BEGIN, OSSL_TRACE_END, OSSL_TRACE1, OSSL_TRACE2, OSSL_TRACE9
 - OpenSSL Tracing API
 
 =head1 SYNOPSIS
@@ -14,13 +15,25 @@ OSSL_trace_enabled, OSSL_trace_begin, OSSL_trace_end
  BIO *OSSL_trace_begin(int category);
  void OSSL_trace_end(int category, BIO *channel);
 
+ /* trace group macros */
+ OSSL_TRACE_BEGIN(category) {
+    ...
+ } OSSL_TRACE_END(category);
+
+ /* one-shot trace macros */
+ OSSL_TRACE1(category, format, arg1)
+ OSSL_TRACE2(category, format, arg1, arg2)
+ ...
+ OSSL_TRACE9(category, format, arg1, ..., arg9)
+
+
 =head1 DESCRIPTION
 
 The functions described here are mainly interesting for those who provide
 OpenSSL functionality, either in OpenSSL itself or in engine modules
 or similar.
 
-If operational (see L</NOTES> below), these functions are used to
+If tracing is enabled (see L</NOTES> below), these functions are used to
 generate free text tracing output.
 
 The tracing output is divided into types which are enabled
@@ -30,6 +43,30 @@ L<OSSL_trace_set_callback(3)/Trace types>.
 The fallback type C<OSSL_TRACE_CATEGORY_ANY> should I<not> be used
 with the functions described here.
 
+Tracing for a specific category is enabled if a so called
+I<trace channel> is attached to it. A trace channel is simply a
+BIO object to which the application can write its trace output.
+
+The application has two different ways of registering a trace channel,
+either by directly providing a BIO object using OSSL_trace_set_channel(),
+or by providing a callback routine using OSSL_trace_set_callback().
+The latter is wrapped internally by a dedicated BIO object, so for the
+tracing code both channel types are effectively indistinguishable.
+We call them a I<simple trace channel> and a I<callback trace channel>,
+respectively.
+
+To produce trace output, it is necessary to obtain a pointer to the
+trace channel (i.e., the BIO object) using OSSL_trace_begin(), write
+to it using arbitrary BIO output routines, and finally releases the
+channel using OSSL_trace_end(). The OSSL_trace_begin()/OSSL_trace_end()
+calls surrounding the trace output create a group, which acts as a
+critical section (guarded by a mutex) to ensure that the trace output
+of different threads does not get mixed up.
+
+The tracing code normally does not call OSSL_trace_{begin,end}() directly,
+but rather uses a set of convenience macros, see the L</Macros> section below.
+
+
 =head2 Functions
 
 OSSL_trace_enabled() can be used to check if tracing for the given
@@ -46,7 +83,7 @@ is I<mandatory>.
 The result of trying to produce tracing output outside of such
 sections is undefined.
 
-=head2 Convenience Macros
+=head2 Macros
 
 There are a number of convenience macros defined, to make tracing
 easy and consistent.
@@ -60,7 +97,7 @@ the B<BIO> C<trc_out> and are used as follows to wrap a trace section:
 
  } OSSL_TRACE_END(TLS);
 
-This will normally expands to:
+This will normally expand to:
 
  do {
      BIO *trc_out = OSSL_trace_begin(OSSL_TRACE_CATEGORY_TLS);
@@ -98,6 +135,16 @@ This will normally expand to:
      OSSL_trace_end(OSSL_TRACE_CATEGORY_TLS, trc_out);
  } while (0);
 
+
+C<OSSL_TRACE1()>, ... C<OSSL_TRACE9()> are one-shot macros which essentially wrap
+a single BIO_printf() into a tracing group.
+
+The call OSSL_TRACEn(category, format, arg1, ..., argN) expands to:
+
+  OSSL_TRACE_BEGIN(category) {
+    BIO_printf(trc_out, format, arg1, ..., argN)
+  } OSSL_TRACE_END(category)
+
 =head1 NOTES
 
 It is advisable to always check that a trace type is enabled with
@@ -110,10 +157,11 @@ OSSL_trace_enabled() before generating any output, for example:
         OSSL_trace_end(OSSL_TRACE_CATEGORY_TLS, trace);
     }
 
-=head2 Tracing disabled
+=head2 Configure Tracing
 
-The OpenSSL library may be built with tracing disabled, which makes
-everything documented here inoperational.
+By default, the OpenSSL library is built with tracing disabled. To
+use the tracing functionality documented here, it is therefore
+necessary to configure and build OpenSSL with the 'enable-trace' option.
 
 When the library is built with tracing disabled:
 
@@ -132,7 +180,7 @@ nothing.
 =item *
 
 the convenience macros are defined to produce dead code.
-For example, take this example from L</Convenience Macros> above:
+For example, take this example from L</Macros> section above:
 
  OSSL_TRACE_BEGIN(TLS) {