error::pass4(7stap) [centos man page]

ERROR::PASS4(7stap)													       ERROR::PASS4(7stap)

NAME

       error::pass4 - systemtap pass-4 errors

DESCRIPTION

       Errors that occur during pass 4 (compilation) have generally only a few causes:

       kernel or OS version changes
	      The systemtap runtime and embedded-C fragments in the tapset library are designed to be portable across a wide range of OS versions.
	      However, incompatibilities can occur when some OS changes occur, such as kernel  modifications  that  change  functions,	types,	or
	      macros referenced by systemtap.  Upstream builds of systemtap are often quickly updated to include relevant fixes, so try getting or
	      making an updated build.	If the issue persists, report the problem to the systemtap developers.

       buggy embedded-C code
	      Embedded-C code in your own guru-mode script cannot be checked by systemtap, and is passed through verbatim to the compiler.  Errors
	      in such snippets of code may be found during the pass-4 compiler invocation, though may be hard to identify by the compiler errors.

       incompatible embedded-C code
	      The  interface standards between systemtap-generated code and embedded-C code occasionally change.  For example, before version 1.8,
	      arguments were passed using macros THIS->foo and THIS->__retvalue but from version 1.8 onward, using STAP_ARG_foo and STAP_RETVALUE.
	      Adjust your embedded-C code to current standards, or use the stap --compatible=VERSION option to make systemtap use a different one.

GATHERING MORE INFORMATION

       It  may	be  necessary  to run systemtap with -k or -p3 to examine the generated C code.  Increasing the verbosity of pass-4 with an option
       such as --vp 0001 can also help pinpoint the problem.

SEE ALSO

       stap(1),
       error::reporting(7stap)

															       ERROR::PASS4(7stap)

ERROR::DWARF(7stap)													       ERROR::DWARF(7stap)

NAME

       error::dwarf - dwarf debuginfo quality problems

DESCRIPTION

       Systemtap  sometimes  relies  on  ELF/DWARF  debuginfo  for  programs being instrumented to locate places to probe, or context variables to
       read/write, just like a symbolic debugger does.	Even though examination of the program's source code may show  variables  or  lines  where
       probes  may  be desired, the compiler must preserve information about them for systemtap (or a debugger such as gdb) to get pinpoint access
       to the desired information.  If a script requires such data, but the compiler did not preserve enough of it, pass-2 errors may result.

       Common conditions that trigger these problems include;

       compiler version
	      Prior to GCC version 4.5, debuginfo quality was fairly limited.  Often developers were advised to build their programs with  -O0	-g
	      flags  to disable optimization.  GCC version 4.5 introduced a facility called "variable-tracking assignments" that allows it to gen-
	      erate high-quality debuginfo under full -O2 -g optimization.  It is not perfect, but much better than before.   Note  that,  due	to
	      another gcc bug (PR51358) -O0 -g can actually sometimes make debuginfo quality worse than for -O2 -g.

       function inlining
	      Even  modern gcc sometimes has problems with parameters for inlined functions.  It may be necessary to change the script to probe at
	      a slightly different place (try a .statement() probe, instead of a .function() probe, somewhere a few source lines into the body	of
	      the inlined function.  Or try putting a probe at the call site of the inlined function.  Or use the if @defined($var) { ... } script
	      language construct to test for the resolvability of the context variable before using it.

       instruction reordering
	      Heavily optimized code often smears the instructions from multiple source statements together.  This can	leave  systemtap  with	no
	      place  to choose to place a probe, especially a statement probe specified by line number.  Systemtap may advise to try a nearby line
	      number, but these may not work well either.  Consider placing a probe by a statement wildcard or line number range.

ALTERNATIVES

       In order to reduce reliance on ELF/DWARF debuginfo, consider the use of statically compiled-in instrumentation, such as kernel tracepoints,
       or <sys/sdt.h> userspace markers.  Such instrumentation hook sites are relatively low cost (just one NOP instruction for sdt.h), and nearly
       guarantee the availability of parameter data and a reliable probe site, all without reliance on debuginfo.

SEE ALSO

       stap(1),
       http://dwarfstd.org/,
       http://sourceware.org/systemtap/wiki/TipContextVariables,
       http://gcc.gnu.org/wiki/Var_Tracking_Assignments,
       warning::debuginfo(7stap),
       error::reporting(7stap)

															       ERROR::DWARF(7stap)