[PATCH] tracetool: work around ust <sys/sdt.h> include conflict

[PATCH] tracetool: work around ust <sys/sdt.h> include conflict

[Stefan Hajnoczi]

Both the dtrace and ust backends may include <sys/sdt.h> but LTTng
Userspace Tracer 2.11 and later requires SDT_USE_VARIADIC to be defined
before including the header file.

This is a classic problem with C header files included from different
parts of a program. If the same header is included twice within the same
compilation unit then the first inclusion determines the macro
environment.

Work around this by defining SDT_USE_VARIADIC in the dtrace backend too.
It doesn't hurt and fixes a missing STAP_PROBEV() compiler error when
the ust backend is enabled together with the dtrace backend.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
I am also sending a fix to the LTTng Userspace Tracer project to add
-DSDT_USE_VARIADIC to the pkg-config CFLAGS so that applications define
the macro consistently. But this patch is still needed as a workaround
for lttng-ust 2.11.
---
 scripts/tracetool/backend/dtrace.py | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/scripts/tracetool/backend/dtrace.py b/scripts/tracetool/backend/dtrace.py
index 638990db79..79dbc2fa27 100644
--- a/scripts/tracetool/backend/dtrace.py
+++ b/scripts/tracetool/backend/dtrace.py
@@ -40,6 +40,12 @@ def generate_h_begin(events, group):
     else:
         header = "trace-dtrace.h"
 
+    # Workaround for ust backend, which also includes <sys/sdt.h> and may
+    # require SDT_USE_VARIADIC to be defined. If dtrace includes <sys/sdt.h>
+    # first without defining SDT_USE_VARIADIC then ust breaks because the
+    # STAP_PROBEV() macro is not defined.
+    out('#define SDT_USE_VARIADIC 1')
+
     out('#include "%s"' % header,
         '')
 
-- 
2.26.2

Re: [PATCH] tracetool: work around ust <sys/sdt.h> include conflict

[Stefan Hajnoczi]

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On Thu, Jun 25, 2020 at 03:07:57PM +0100, Stefan Hajnoczi wrote:
> Both the dtrace and ust backends may include <sys/sdt.h> but LTTng
> Userspace Tracer 2.11 and later requires SDT_USE_VARIADIC to be defined
> before including the header file.
> 
> This is a classic problem with C header files included from different
> parts of a program. If the same header is included twice within the same
> compilation unit then the first inclusion determines the macro
> environment.
> 
> Work around this by defining SDT_USE_VARIADIC in the dtrace backend too.
> It doesn't hurt and fixes a missing STAP_PROBEV() compiler error when
> the ust backend is enabled together with the dtrace backend.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> I am also sending a fix to the LTTng Userspace Tracer project to add
> -DSDT_USE_VARIADIC to the pkg-config CFLAGS so that applications define
> the macro consistently. But this patch is still needed as a workaround
> for lttng-ust 2.11.
> ---
>  scripts/tracetool/backend/dtrace.py | 6 ++++++
>  1 file changed, 6 insertions(+)

Thanks, applied to my tracing tree:
https://github.com/stefanha/qemu/commits/tracing

Stefan

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