Add ctx_len field to ring buffer context

Allow extending the structure beyond its original size.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/include/lttng/ringbuffer-config.h b/include/lttng/ringbuffer-config.h
index 5ae0d8a6a4985500e168ab0e529b8642f0cba554..c95d35cb348d820e69adba599ad3f90b9583b130 100644 (file)
--- a/include/lttng/ringbuffer-config.h
+++ b/include/lttng/ringbuffer-config.h
@@ -256,10 +256,21 @@ struct lttng_ust_lib_ring_buffer_ctx {
                                         */
        uint64_t tsc;                   /* time-stamp counter value */
        unsigned int rflags;            /* reservation flags */
-       unsigned int padding1;          /* padding to realign on pointer */
+       /*
+        * The field ctx_len is the length of struct
+        * lttng_ust_lib_ring_buffer_ctx as known by the user of
+        * lib_ring_buffer_ctx_init.
+        */
+       unsigned int ctx_len;
        void *ip;                       /* caller ip address */
        void *priv2;                    /* 2nd priv data */
        char padding2[LTTNG_UST_RING_BUFFER_CTX_PADDING];
+       /*
+        * This is the end of the initial fields expected by the original ABI
+        * between probes and UST. Only the fields above can be used if
+        * ctx_len is 0. Use the value of ctx_len to find out which of the
+        * following fields may be used.
+        */
 };
 
 /**
@@ -291,7 +302,7 @@ void lib_ring_buffer_ctx_init(struct lttng_ust_lib_ring_buffer_ctx *ctx,
        ctx->cpu = cpu;
        ctx->rflags = 0;
        ctx->handle = handle;
-       ctx->padding1 = 0;
+       ctx->ctx_len = sizeof(struct lttng_ust_lib_ring_buffer_ctx);
        ctx->ip = 0;
        ctx->priv2 = priv2;
        memset(ctx->padding2, 0, LTTNG_UST_RING_BUFFER_CTX_PADDING);