Logging variables

Logging variables from applications with debug information is simple. In general, it is easy to log a variable using the log() probe directive and the variable's "target name." The variable must be logged from a probe in a scope where the variable is visible.

From a probed function you can log parameters, local variables, function-static variables, file-static variables and globals within the same file using the target expression for the variable. You can log global and static variables from any scope using the target expression modifiers -file or -unit (see examples below).

In most case, the target expression is just the variable name preceded by a dollar sign, for example:

$var
$structvar.field
$structptr->field

For global and file-static variables you may need to use target expression modifiers to locate the variable scope, for example:

$(global-var, "-file file.c")
$(file-static, "-file file.c")
$(ada-global-var, "-unit sec/pkg")

C/C++ Example

For this C program:

int global = 1234;

static int file_local = 5678;

void p1(int param)
{
  static int func_static = 9876;

  int func_local = param * 10;

  {
     int block_local = param * 100;

     printf("b = %d\n", block_local);
     global += block_local;
  }
}

The following probe:

probe thread
{
   on_entry
   {
      // This probe not in any scope so we have to use the "-file" modifier to specify the scope. 
      log ("\"global\" = ", $(global, "-file main.c"));
      log ("\"file_local\" = ", $(file_local, "-file main.c"));
   }

   probe "p1"
   {
      // all target expressions are in the scope containing the variable so no modifiers needed.
      on_entry
      {
         log ("\"global\" = ", $global);
         log ("\"file_local\" = ", $file_local);
         log ("\"func_static\" = ", $func_static);
         log ("\"param\" = ", $param);
      }
      on_line(17)
      {
         log ("\"func_local\" = ", $func_local);
         log ("\"block_local\" = ", $block_local);
      }
   }
}

Produces the following output:

"global" = 1234
"file_local" = 5678
"global" = 1234
"file_local" = 5678
"func_static" = 9876
"param" = 1234
"func_local" = 12340
"block_local" = 123400

Ada Example

For an Ada program:

package Pkg is

  function P1(Param : in Integer) return Integer;

  PkgGlobal : Integer := 53;

end Pkg;

with Text_Io;

package body Pkg is

  PkgLocal : Integer := 27;

  function P1(Param : in Integer) return Integer is
    PLocal : Integer := Param * 5;
  begin
    Text_Io.Put_Line("PkgGlobal = " & Integer'Image(PkgGlobal));
    Text_Io.Put_Line("PkgLocal = " & Integer'Image(PkgLocal));
    Text_Io.Put_Line("Param = " & Integer'Image(Param));
    Text_Io.Put_Line("PLocal = " & Integer'Image(PLocal));
    declare
       PBlock : Integer := Param * 3;
    begin
       Text_Io.Put_Line("PBlock = " & Integer'Image(PBlock));
       PLocal := PLocal + PBlock;
    end;
    return PLocal;
  end P1;

The following probe:

probe thread
{
   probe "pkg.p1"
   {
      on_entry
      {
#ifdef _AIX
         // Must specify the PowerAda unit name to access global
         log ("\"PkgGlobal\" = ", $(PkgGlobal, "-unit lib/pkg"));
#else
         log ("\"PkgGlobal\" = ", $PkgGlobal);
#endif
         log ("\"PkgLocal\" = ", $PkgLocal);
         log ("\"Param\" = ", $Param);
      }
      on_line(18)
      {
         log ("\"PLocal\" = ", $PLocal);
         log ("\"PBlock\" = ", $PBlock);
      }
      on_exit
      {
         log ("\"Return\" = ", $return);
      }
   }
}

Produces the following output:

<nowiki>

"PkgGlobal" = 53 "PkgLocal" = 27 "Param" = 1234 "PLocal" = 6170 "PBlock" = 3702 "Return" = 9872 </nowikik>