Refactoring: Introduce extensibility scheme for tracepoint structures

[lttng-ust.git] / include / lttng / tracepoint-types.h

diff --git a/include/lttng/tracepoint-types.h b/include/lttng/tracepoint-types.h
index ceb476a6f9891d760f0e43e60bda928aed2acae0..5354c3698abc478dd41c6167f5cff07098f7de63 100644 (file)
--- a/include/lttng/tracepoint-types.h
+++ b/include/lttng/tracepoint-types.h
@@ -7,19 +7,44 @@
 #ifndef _LTTNG_TRACEPOINT_TYPES_H
 #define _LTTNG_TRACEPOINT_TYPES_H
 
+#include <stdint.h>
+
+/*
+ * Tracepoint probe definition
+ *
+ * IMPORTANT: this structure is part of the ABI between instrumented
+ * applications and UST. This structure is fixed-size because it is part
+ * of a public array of structures. Rather than extending this
+ * structure, struct lttng_ust_tracepoint should be extended instead.
+ */
+
 struct lttng_ust_tracepoint_probe {
        void (*func)(void);
        void *data;
 };
 
-#define LTTNG_UST_TRACEPOINT_PADDING   16
+/*
+ * Tracepoint definition
+ *
+ * IMPORTANT: this structure is part of the ABI between instrumented
+ * applications and UST. Fields need to be only added at the end, never
+ * reordered, never removed.
+ *
+ * The field @struct_size should be used to determine the size of the
+ * structure. It should be queried before using additional fields added
+ * at the end of the structure.
+ */
+
 struct lttng_ust_tracepoint {
+       uint32_t struct_size;
+
        const char *name;
        int state;
        struct lttng_ust_tracepoint_probe *probes;
        int *tracepoint_provider_ref;
        const char *signature;
-       char padding[LTTNG_UST_TRACEPOINT_PADDING];
+
+       /* End of base ABI. Fields below should be used after checking struct_size. */
 };
 
 #endif /* _LTTNG_TRACEPOINT_TYPES_H */