From: Philippe Proulx <eeppeliteloop@gmail.com>
Date: Tue, 22 Mar 2016 17:44:36 +0000 (-0400)
Subject: instrumenting-linux-kernel-itself: document LTTNG_TRACEPOINT_EVENT_CODE()
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=d154e24c2db033c61759a14dbda074cad6d71d6c;p=lttng-docs.git

instrumenting-linux-kernel-itself: document LTTNG_TRACEPOINT_EVENT_CODE()

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
---

diff --git a/contents/using-lttng/instrumenting/instrumenting-linux-kernel/instrumenting-linux-kernel-itself/lttng-adaptation-layer.md b/contents/using-lttng/instrumenting/instrumenting-linux-kernel/instrumenting-linux-kernel-itself/lttng-adaptation-layer.md
index 1595cf2..5adc088 100644
--- a/contents/using-lttng/instrumenting/instrumenting-linux-kernel/instrumenting-linux-kernel-itself/lttng-adaptation-layer.md
+++ b/contents/using-lttng/instrumenting/instrumenting-linux-kernel/instrumenting-linux-kernel-itself/lttng-adaptation-layer.md
@@ -71,6 +71,11 @@ Some possible entries for `TP_STRUCT__entry()` and `TP_fast_assign()`,
 in the case of LTTng-modules, are shown in the
 [LTTng-modules reference](#doc-lttng-modules-ref) section.
 
+You may also be interested in using the
+[`LTTNG_TRACEPOINT_EVENT_CODE()` macro](#doc-lttng-tracepoint-event-code),
+instead of using `LTTNG_TRACEPOINT_EVENT()`, which allows custom local
+variables and C code to be executed before the event fields are recorded.
+
 The best way to learn how to use the above macros is to inspect
 existing LTTng tracepoint definitions in `instrumentation/events/lttng-module`
 header files. Compare them with the Linux kernel mainline versions
diff --git a/contents/using-lttng/instrumenting/instrumenting-linux-kernel/instrumenting-linux-kernel-itself/lttng-tracepoint-event-code.md b/contents/using-lttng/instrumenting/instrumenting-linux-kernel/instrumenting-linux-kernel-itself/lttng-tracepoint-event-code.md
new file mode 100644
index 0000000..d4aeae8
--- /dev/null
+++ b/contents/using-lttng/instrumenting/instrumenting-linux-kernel/instrumenting-linux-kernel-itself/lttng-tracepoint-event-code.md
@@ -0,0 +1,73 @@
+---
+id: lttng-tracepoint-event-code
+since: 2.7
+---
+
+Although it is recommended to always use the
+[`LTTNG_TRACEPOINT_EVENT()`](#doc-lttng-adaptation-layer)
+macro to describe the arguments and fields of an LTTng tracepoint when
+possible, sometimes a more complex process is needed to access the data
+to be recorded as tracepoint fields. In other words, local variables
+and multiple C statements are required instead of simple argument-based
+expressions passed to the
+[`ctf_*()` macros of `TP_FIELDS()`](#doc-lttng-modules-tp-fields).
+
+The `LTTNG_TRACEPOINT_EVENT_CODE()` macro can be used instead of
+`LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and
+define a block of C code to be executed before the fields are
+recorded. The structure of this macro is:
+
+~~~ c
+LTTNG_TRACEPOINT_EVENT_CODE(
+    /* format identical to LTTNG_TRACEPOINT_EVENT() version for those */
+    hello_world,
+    TP_PROTO(int foo, const char *bar),
+    TP_ARGS(foo, bar),
+
+    /* declarations of custom local variables */
+    TP_locvar(
+        int a = 0;
+        unsigned long b = 0;
+        const char *name = "(undefined)";
+        struct my_struct *my_struct;
+    ),
+
+    /*
+     * Custom code using which use both tracepoint arguments
+     * (in TP_ARGS()) and local variables (in TP_locvar()).
+     *
+     * Local variables are actually members of a structure pointed
+     * to by the special variable tp_locvar.
+     */
+    TP_code(
+        if (foo) {
+            tp_locvar->a = foo + 17;
+            tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
+            tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
+            tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
+            put_my_struct(tp_locvar->my_struct);
+
+            if (tp_locvar->b) {
+                tp_locvar->a = 1;
+            }
+        }
+    ),
+
+    /*
+     * Format identical to LTTNG_TRACEPOINT_EVENT() version for this,
+     * except that tp_locvar members can be used in the argument
+     * expression parameters of the ctf_*() macros.
+     */
+    TP_FIELDS(
+        ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
+        ctf_integer(int, my_struct_a, tp_locvar->a)
+        ctf_string(bar_field, bar)
+        ctf_string(my_struct_name, tp_locvar->name)
+    )
+)
+~~~
+
+Make sure that the C code defined in `TP_code()` has no side effects
+when executed. In particular, the code should not allocate memory or get
+resources without deallocating this memory or putting those resources
+afterwards.
diff --git a/toc/docs.yml b/toc/docs.yml
index 6d817f0..84ff824 100644
--- a/toc/docs.yml
+++ b/toc/docs.yml
@@ -149,6 +149,8 @@ cats:
                     title: Defining/using tracepoints with mainline <code>TRACE_EVENT()</code> infrastructure
                   - id: lttng-adaptation-layer
                     title: Adding the <span class="reset-text-transform">LTTng</span> adaptation layer
+                  - id: lttng-tracepoint-event-code
+                    title: Using custom C code to access the data for tracepoint fields
                   - id: instrumenting-linux-kernel-tracing
                     title: Tracing
               - id: instrumenting-out-of-tree-linux-kernel