OSSL_trace_enabled, OSSL_trace_begin, OSSL_trace_end, OSSL_TRACE_BEGIN, OSSL_TRACE_END, OSSL_TRACE_CANCEL, OSSL_TRACE, OSSL_TRACE1, OSSL_TRACE2, OSSL_TRACE3, OSSL_TRACE4, OSSL_TRACE5, OSSL_TRACE6, OSSL_TRACE7, OSSL_TRACE8, OSSL_TRACE9, OSSL_TRACEV, OSSL_TRACE_ENABLED - OpenSSL Tracing API
#include <openssl/trace.h>
int OSSL_trace_enabled(int category);
BIO *OSSL_trace_begin(int category);
void OSSL_trace_end(int category, BIO *channel);
/* trace group macros */
OSSL_TRACE_BEGIN(category) {
...
if (some_error) {
/* Leave trace group prematurely in case of an error */
OSSL_TRACE_CANCEL(category);
goto err;
}
...
} 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)
/* check whether a trace category is enabled */
if (OSSL_TRACE_ENABLED(category)) {
...
}The functions described here are mainly interesting for those who provide OpenSSL functionality, either in OpenSSL itself or in engine modules or similar.
If tracing is enabled (see "NOTES" below), these functions are used to generate free text tracing output.
The tracing output is divided into types which are enabled individually by the application. The tracing types are described in detail in "Macros" section below.
OSSL_trace_enabled() can be used to check if tracing for the given category is enabled.
OSSL_trace_begin() is used to starts a tracing section, and get the channel for the given category in form of a BIO. This BIO can only be used for output.
OSSL_trace_end() is used to end a tracing section.
Using OSSL_trace_begin() and OSSL_trace_end() to wrap tracing sections is mandatory. The result of trying to produce tracing output outside of such sections is undefined.
There are a number of convenience macros defined, to make tracing easy and consistent.
OSSL_TRACE_BEGIN(category) and OSSL_TRACE_END(category) reserve the BIO trc_out and are used as follows to wrap a trace section:
OSSL_TRACE_BEGIN(TLS) {
BIO_fprintf(trc_out, ... );
} OSSL_TRACE_END(TLS);This will normally expand to:
do {
BIO *trc_out = OSSL_trace_begin(OSSL_TRACE_CATEGORY_TLS);
if (trc_out != NULL) {
...
BIO_fprintf(trc_out, ...);
}
OSSL_trace_end(OSSL_TRACE_CATEGORY_TLS, trc_out);
} while (0);OSSL_TRACE_CANCEL(category) must be used before returning from or jumping out of a trace section:
OSSL_TRACE_BEGIN(TLS) {
if (some_error) {
OSSL_TRACE_CANCEL(TLS);
goto err;
}
BIO_fprintf(trc_out, ... );
} OSSL_TRACE_END(TLS);This will normally expand to:
do {
BIO *trc_out = OSSL_trace_begin(OSSL_TRACE_CATEGORY_TLS);
if (trc_out != NULL) {
if (some_error) {
OSSL_trace_end(OSSL_TRACE_CATEGORY_TLS, trc_out);
goto err;
}
BIO_fprintf(trc_out, ... );
}
OSSL_trace_end(OSSL_TRACE_CATEGORY_TLS, trc_out);
} while (0);OSSL_TRACE() and OSSL_TRACE1(), OSSL_TRACE2(), ... OSSL_TRACE9() are so-called one-shot macros:
The macro call OSSL_TRACE(category, text), produces literal text trace output.
The macro call OSSL_TRACEn(category, format, arg1, ..., argn) produces printf-style trace output with n format field arguments (n=1,...,9). It expands to:
OSSL_TRACE_BEGIN(category) {
BIO_printf(trc_out, format, arg1, ..., argN)
} OSSL_TRACE_END(category)Internally, all one-shot macros are implemented using a generic OSSL_TRACEV() macro, since C90 does not support variadic macros. This helper macro has a rather weird synopsis and should not be used directly.
The OSSL_TRACE_ENABLED(category) macro can be used to conditionally execute some code only if a specific trace category is enabled. In some situations this is simpler than entering a trace section using OSSL_TRACE_BEGIN(category) and OSSL_TRACE_END(category). For example, the code
if (OSSL_TRACE_ENABLED(TLS)) {
...
}expands to
if (OSSL_trace_enabled(OSSL_TRACE_CATEGORY_TLS) {
...
}If producing the trace output requires carrying out auxiliary calculations, this auxiliary code should be placed inside a conditional block which is executed only if the trace category is enabled.
The most natural way to do this is to place the code inside the trace section itself because it already introduces such a conditional block.
OSSL_TRACE_BEGIN(TLS) {
int var = do_some_auxiliary_calculation();
BIO_printf(trc_out, "var = %d\n", var);
} OSSL_TRACE_END(TLS);In some cases it is more advantageous to use a simple conditional group instead of a trace section. This is the case if calculations and tracing happen in different locations of the code, or if the calculations are so time consuming that placing them inside a (critical) trace section would create too much contention.
if (OSSL_TRACE_ENABLED(TLS)) {
int var = do_some_auxiliary_calculation();
OSSL_TRACE1("var = %d\n", var);
}Note however that premature optimization of tracing code is in general futile and it's better to keep the tracing code as simple as possible. Because most often the limiting factor for the application's speed is the time it takes to print the trace output, not to calculate it.
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 OPENSSL_NO_TRACE is defined in openssl/opensslconf.h.
all functions are still present, bu OSSL_trace_enabled() will always report the categories as disabled, and all other functions will do nothing.
the convenience macros are defined to produce dead code. For example, take this example from "Macros" section above:
OSSL_TRACE_BEGIN(TLS) {
if (condition) {
OSSL_TRACE_CANCEL(TLS);
goto err;
}
BIO_fprintf(trc_out, ... );
} OSSL_TRACE_END(TLS);When the tracing API isn't operational, that will expand to:
do {
BIO *trc_out = NULL;
if (0) {
if (condition) {
((void)0);
goto err;
}
BIO_fprintf(trc_out, ... );
}
} while (0);OSSL_trace_enabled() returns 1 if tracing for the given type is operational and enabled, otherwise 0.
OSSL_trace_begin() returns a BIO * if the given type is enabled, otherwise NULL.
The OpenSSL Tracing API was added ino OpenSSL 3.0.
Copyright 2019 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 https://www.openssl.org/source/license.html.