ERROR::SDT(7stap)														 ERROR::SDT(7stap)

NAME

       error::sdt - <sys/sdt.h> marker failures

DESCRIPTION

       Systemtap's <sys/sdt.h> probes are modeled after the dtrace USDT API, but are implemented differently.  They leave a only a NOP instruction
       in the userspace program's text segment, and add an ELF note to the binary with metadata.  This metadata describes the  marker's  name  and
       parameters.   This  encoding  is  designed to be parseable by multiple tools (not just systemtap: GDB, the GNU Debugger, also contains sup-
       port).  These allow the tools to find parameters and their types, wherever they happen to reside, even without DWARF debuginfo.

       The reason finding parameters is tricky is because the STAP_PROBE / DTRACE_PROBE markers store an assembly language expression for each op-
       erand,  as  a result of use of gcc inline-assembly directives.  The compiler is given a broad gcc operand constraint string ("nor") for the
       operands, which usually works well.  Usually, it does not force the compiler to load the parameters into or out of registers,  which  would
       slow  down  an instrumented program.  However, some instrumentation sites with some parameters do not work well with the default "nor" con-
       straint.

       unresolveable at run-time
	      GCC may emit strings that an assembler could resolve (from the context of compiling the original program), but a run-time tool  can-
	      not.   For example, the operand string might refer to a label of a local symbol that is not emitted into the ELF object file at all,
	      which leaves no trace for the run-time.  Reference to such parameters from within systemtap can result in "SDT asm  not  understood"
	      errors.

       too complicated expression
	      GCC  might  synthesize very complicated assembly addressing modes from complex C data types / pointer expressions.  systemtap or gdb
	      may not be able to parse some valid but complicated expressions.	Reference to such parameters from within systemtap can	result	in
	      "SDT asm not understood" errors.

       overly restrictive constraint
	      GCC  might  not be able to even compile the original program with the default "nor" constraint due to shortage of registers or other
	      reasons.	A compile-time gcc error such as "asm operand has impossible constraints" may result.

       There are two general workarounds to this family of problems.

       change the constraints
	      While compiling the original instrumented program, set the STAP_SDT_ARG_CONSTRAINT macro to different constraint strings.   See  the
	      GCC  manual  about  various options.  For example, on many machine architectures, "r" forces operands into registers, and "g" leaves
	      operands essentially unconstrained.

       revert to debuginfo
	      As long as the instrumented program compiles, it may be fine simply to keep using <sys/sdt.h> but eschew extraction of a	few  indi-
	      vidual  parameters.   In	the  worst  case, disable <sys/sdt.h> macros entirely to eschew the compiled-in instrumentation.  If DWARF
	      debuginfo was generated and preserved, a systemtap script could refer to the underlying source  context  variables  instead  of  the
	      positional STAP_PROBE parameters.

SEE ALSO

       stap(1),
       stapprobes(3stap),
       error::dwarf(7stap),
       http://gcc.gnu.org/onlinedocs/gcc/Constraints.html,
       http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation,
       error::reporting(7stap)

																 ERROR::SDT(7stap)