openssl/doc/man3/OSSL_trace_set_channel.pod

=pod

=for openssl foreign manual atexit(3)

=head1 NAME

OSSL_trace_set_channel, OSSL_trace_set_prefix, OSSL_trace_set_suffix,
OSSL_trace_set_callback, OSSL_trace_cb - Enabling trace output

=head1 SYNOPSIS

 #include <openssl/trace.h>

 typedef size_t (*OSSL_trace_cb)(const char *buf, size_t cnt,
                                 int category, int cmd, void *data);

 void OSSL_trace_set_channel(int category, BIO *bio);
 void OSSL_trace_set_prefix(int category, const char *prefix);
 void OSSL_trace_set_suffix(int category, const char *suffix);
 void OSSL_trace_set_callback(int category, OSSL_trace_cb cb, void  *data);

=head1 DESCRIPTION

If available (see L</NOTES> below), the application can request
internal trace output.
This output comes in form of free text for humans to read.

The trace output is divided into categories which can be
enabled individually.
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 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
tracing output.
What constitues an output group is decided by the code that produces
the output.
The lines given here are considered immutable; for more dynamic
tracing prefixes, consider setting a callback with
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. 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.

OSSL_trace_set_channel() and OSSL_trace_set_callback() are mutually
exclusive, calling one of them will clear whatever was set by the
previous call.

Calling OSSL_trace_set_channel() with C<NULL> for C<channel> or
OSSL_trace_set_callback() with C<NULL> for C<cb> disables tracing for
the given C<category>

=head2 Trace callback

The tracer callback must return a C<size_t>, which must be zero on
error and otherwise return the number of bytes that were output.
It receives a text buffer C<buf> with C<cnt> bytes of text, as well as
the C<category>, a control number C<cmd>, and the C<data> that was
passed to OSSL_trace_set_callback().

The possible control numbers are:

=over 4

=item C<OSSL_TRACE_CTRL_BEGIN>

The callback is called from OSSL_trace_begin(), which gives the
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_WRITE>

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>

The callback is called from OSSL_trace_end(), which gives the callback
the possibility to output a dynamic ending line, or reset the line
prefix that was set with OSSL_TRACE_CTRL_BEGIN, or something other.

=back

=head2 Trace categories

The trace categories are simple numbers available through macros.

=over 4

=item C<OSSL_TRACE_CATEGORY_TRACE>

Traces the OpenSSL trace API itself.

More precisely, this will generate trace output any time a new
trace hook is set.

=item C<OSSL_TRACE_CATEGORY_INIT>

Traces OpenSSL library initialization and cleanup.

This needs special care, as OpenSSL will do automatic cleanup after
exit from C<main()>, and any tracing output done during this cleanup
will be lost if the tracing channel or callback were cleaned away
prematurely.
A suggestion is to make such cleanup part of a function that's
registered very early with L<atexit(3)>.

=item C<OSSL_TRACE_CATEGORY_TLS>

Traces the TLS/SSL protocol.

=item C<OSSL_TRACE_CATEGORY_TLS_CIPHER>

Traces the ciphers used by the TLS/SSL protocol.

=item C<OSSL_TRACE_CATEGORY_ENGINE_CONF>

Traces the ENGINE configuration.

=item C<OSSL_TRACE_CATEGORY_ENGINE_TABLE>

Traces the ENGINE algorithm table selection.

More precisely, engine_table_select(), the function that is used by
RSA, DSA (etc) code to select registered ENGINEs, cache defaults and
functional references (etc), will generate trace summaries.

=item C<OSSL_TRACE_CATEGORY_ENGINE_REF_COUNT>

Tracds the ENGINE reference counting.

More precisely, both reference counts in the ENGINE structure will be
monitored with a line of trace output generated for each change.

=item C<OSSL_TRACE_CATEGORY_PKCS5V2>

Traces PKCS#5 v2 key generation.

=item C<OSSL_TRACE_CATEGORY_PKCS12_KEYGEN>

Traces PKCS#12 key generation.

=item C<OSSL_TRACE_CATEGORY_PKCS12_DECRYPT>

Traces PKCS#12 decryption.

=item C<OSSL_TRACE_CATEGORY_X509V3_POLICY>

Traces X509v3 policy processing.

More precisely, this generates the complete policy tree at various
point during evaluation.

=item C<OSSL_TRACE_CATEGORY_BN_CTX>

Traces BIGNUM context operations.

=item C<OSSL_TRACE_CATEGORY_CONF>

Traces details about the provider and engine configuration.

=back

There is also C<OSSL_TRACE_CATEGORY_ALL>, which works as a fallback
and can be used to get I<all> trace output.

Note, however, that in this case all trace output will effectively be
associated with the 'ALL' category, which is undesirable if the
application intends to include the category name in the trace output.
In this case it is better to register separate channels for each
trace category instead.

=head1 RETURN VALUES

OSSL_trace_set_channel(), OSSL_trace_set_prefix(),
OSSL_trace_set_suffix(), and OSSL_trace_set_callback() return 1 on
success, or 0 on failure.

=head1 EXAMPLES

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,
                      8,  9, 10, 11, 12, 13, 14, 15 };

 OSSL_TRACE_BEGIN(TLS) {
     BIO_puts(trc_out, "foo: ");
     BIO_printf(trc_out, "%d\n", foo);
     BIO_dump(trc_out, bar, sizeof(bar));
 } OSSL_TRACE_END(TLS);

=head2 Simple example

An example with just a channel and constant prefix / suffix.

 int main(int argc, char *argv[])
 {
     BIO *err = BIO_new_fp(stderr, BIO_NOCLOSE | BIO_FP_TEXT);
     OSSL_trace_set_channel(OSSL_TRACE_CATEGORY_SSL, err);
     OSSL_trace_set_prefix(OSSL_TRACE_CATEGORY_SSL, "BEGIN TRACE[TLS]");
     OSSL_trace_set_suffix(OSSL_TRACE_CATEGORY_SSL, "END TRACE[TLS]");

     /* ... work ... */
 }

When the trace producing code above is performed, this will be output
on standard error:

 BEGIN TRACE[TLS]
 foo: 42
 0000 - 00 01 02 03 04 05 06 07-08 09 0a 0b 0c 0d 0e 0f   ................
 END TRACE[TLS]

=head2 Advanced example

This example uses the callback, and depends on pthreads functionality.

 static size_t cb(const char *buf, size_t cnt,
                 int category, int cmd, void *vdata)
 {
     BIO *bio = vdata;
     const char *label = NULL;

     switch (cmd) {
     case OSSL_TRACE_CTRL_BEGIN:
         label = "BEGIN";
         break;
     case OSSL_TRACE_CTRL_END:
         label = "END";
         break;
     }

     if (label != NULL) {
         union {
             pthread_t tid;
             unsigned long ltid;
         } tid;

         tid.tid = pthread_self();
         BIO_printf(bio, "%s TRACE[%s]:%lx\n",
                    label, OSSL_trace_get_category_name(category), tid.ltid);
     }
     return (size_t)BIO_puts(bio, buf);
 }

 int main(int argc, char *argv[])
 {
     BIO *err = BIO_new_fp(stderr, BIO_NOCLOSE | BIO_FP_TEXT);
     OSSL_trace_set_callback(OSSL_TRACE_CATEGORY_SSL, cb, err);

     /* ... work ... */
 }

The output is almost the same as for the simple example above.

 BEGIN TRACE[TLS]:7f9eb0193b80
 foo: 42
 0000 - 00 01 02 03 04 05 06 07-08 09 0a 0b 0c 0d 0e 0f   ................
 END TRACE[TLS]:7f9eb0193b80

=head1 NOTES

=head2 Configure Tracing

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
functions described here are inoperational, i.e. will do nothing.

=head1 HISTORY

OSSL_trace_set_channel(), OSSL_trace_set_prefix(),
OSSL_trace_set_suffix(), and OSSL_trace_set_callback() were all added
in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2019-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

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