Fix crash and latest update.

doc/man3/OSSL_trace_set_channel.pod

@@ -25,14 +25,21 @@ This output comes in form of free text for humans to read.
 
 The trace output is divided into categories which can be
 enabled individually.
-They are enabled by giving them a channel in form of a BIO, or a
-tracer callback, which is responsible for performing the actual
-output.
+Every category can be enabled individually by attaching a so called
+I<trace channel> to it, which in the simplest case is just a BIO object
+to which the application can write the tracing output for this category.
+Alternatively, the application can provide a tracer callback in order to
+get more finegrained trace information. This callback will be wrapped
+internally by a dedicated BIO object.
+
+For the tracing code, both trace channel types are indistinguishable.
+These are called a I<simple trace channel> and a I<callback trace channel>,
+respectively.
 
 =head2 Functions
 
 OSSL_trace_set_channel() is used to enable the given trace C<category>
-by giving it the B<BIO> C<bio>.
+by attaching the B<BIO> C<bio> object as (simple) trace channel.
 
 OSSL_trace_set_prefix() and OSSL_trace_set_suffix() can be used to add
 an extra line for each channel, to be output before and after group of
@@ -46,7 +53,8 @@ OSSL_trace_set_callback() instead.
 OSSL_trace_set_callback() is used to enable the given trace
 C<category> by giving it the tracer callback C<cb> with the associated
 data C<data>, which will simply be passed through to C<cb> whenever
-it's called.
+it's called. The callback function is internally wrapped by a
+dedicated BIO object, the so called I<callback trace channel>.
 This should be used when it's desirable to do form the trace output to
 something suitable for application needs where a prefix and suffix
 line aren't enough.
@@ -78,9 +86,13 @@ callback the possibility to output a dynamic starting line, or set a
 prefix that should be output at the beginning of each line, or
 something other.
 
-=item C<OSSL_TRACE_CTRL_DURING>
+=item C<OSSL_TRACE_CTRL_WRITE>
 
-The callback is called from any regular BIO output routine.
+This callback is called whenever data is written to the BIO by some
+regular BIO output routine.
+An arbitrary number of C<OSSL_TRACE_CTRL_WRITE> callbacks can occur
+inside a group marked by a pair of C<OSSL_TRACE_CTRL_BEGIN> and
+C<OSSL_TRACE_CTRL_END> calls, but never outside such a group.
 
 =item C<OSSL_TRACE_CTRL_END>
 
@@ -177,8 +189,8 @@ success, or 0 on failure.
 
 =head1 EXAMPLES
 
-In all examples below, we assume that the trace producing code is
-this:
+In all examples below, the trace producing code is assumed to be
+the following:
 
  int foo = 42;
  const char bar[] = { 0,  1,  2,  3,  4,  5,  6,  7,
@@ -261,10 +273,11 @@ The output is almost the same as for the simple example above.
 
 =head1 NOTES
 
-=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, the macro
 C<OPENSSL_NO_TRACE> is defined in C<openssl/opensslconf.h> and all