From mboxrd@z Thu Jan 1 00:00:00 1970 From: simon.marchi@polymtl.ca (Simon Marchi) Date: Wed, 10 Jun 2015 18:20:48 -0400 Subject: [lttng-dev] Printing bit fields with LTTng In-Reply-To: References: Message-ID: On 10 June 2015 at 17:28, Simon Marchi wrote: > On 10 June 2015 at 17:16, Chidhu R wrote: >> Hello, >> >> I am getting compilation issues while trying to print bit field values with >> LTTng. >> >> Error: >> >> hello.c: In function ?main?: >> hello.c:36:1: error: ?typeof? applied to a bit-field >> hello.c:36:1: error: ?typeof? applied to a bit-field >> hello.c:36:1: error: ?typeof? applied to a bit-field >> hello.c:36:1: error: ?sizeof? applied to a bit-field >> >> Snippet: >> >> struct abc{ >> int a:16; >> int b:8; >> }; >> >> Line 36: tracepoint(hello_world, my_third_tracepoint, aa.a, "welcome"); >> Line 37: printf("val = %x\n",aa.a); >> >> tracepoint line results in compilation error. >> printf succeeds. >> >> Tp definition looks like this. >> >> TRACEPOINT_EVENT( >> hello_world, >> my_third_tracepoint, >> TP_ARGS( >> uint32_t, arg, >> const char * , my_string_arg >> ), >> TP_FIELDS( >> ctf_integer_hex(uint32_t, my_arg, arg) >> ctf_string(field, my_string_arg) >> ) >> ) >> >> How to print bit field values? >> >> Thanks >> Chid > > I have no idea what the right fix would be, but an easy workaround > could be to add 0 to your value: > > tracepoint(hello_world, my_third_tracepoint, aa.a + 0, "welcome"); > > I just tried it and it works. > > Simon Alright, after a bit more investigation (thanks to Philippe Proulx), we found that this is caused by the SystemTap integration. If the SystemTap integration is enabled, each LTTng tracepoint also defines a SystemTap tracepoint. SystemTap uses sizeof() on the tracepoint's input arguments to find their sizes and typeof() to find their signedness. It encodes this info into a special section that then allows tools like gdb to re-use the tracepoints and get the argument values. For more information, look at the definitions of _SDT_ARGSIZE and _SDT_ARGSIGNED in /usr/include/sys/sdt.h. Pragmatically, I think that the "+ 0" trick is good enough, since it pleases SystemTap and is otherwise harmless. You could get the same result by casting your value to an integer type. However, you would need to carefully choose the size and signedness of the type you cast to, or it might alter your value. With + 0, it should work with any type. Thanks for raising this, I learned something new today.