2.9.6 External Variables

D uses the backquote character (`) as a special scoping operator for accessing variables that are defined in the operating system and not in your D program. For example, the Oracle Linux kernel contains a C declaration of a system variable named max_pfn. This variable is declared in C in the kernel source code as follows:

unsigned long max_pfn

To access the value of this variable in a D program, use the D notation:

`max_pfn

DTrace associates each kernel symbol with the type used for the symbol in the corresponding operating system C code, providing easy source-based access to the native operating system data structures. In order to use external operating system variables, you will need access to the corresponding operating system source code.

When you access external variables from a D program, you are accessing the internal implementation details of another program such as the operating system kernel or its device drivers. These implementation details do not form a stable interface upon which you can rely. Any D programs you write that depend on these details might cease to work when you next upgrade the corresponding piece of software. For this reason, external variables are typically used to debug performance or functionality problems using DTrace. To learn more about the stability of your D programs, refer to Chapter 16, Stability.

Kernel symbol names are kept in a separate namespace from D variable and function identifiers, so you need not worry about these names conflicting with your D variables. When you prefix a variable with a backquote, the D compiler searches the known kernel symbols and uses the list of loaded modules to find a matching variable definition. Because the Oracle Linux kernel supports dynamically loaded modules with separate symbol namespaces, the same variable name might be used more than once in the active operating system kernel. You can resolve these name conflicts by specifying the name of the kernel module whose variable should be accessed prior to the backquote in the symbol name. For example, to refer to the address of the _bar function provided by a kernel module named foo, you would write:

foo`_bar

You can apply any of the D operators to external variables, except those that modify values, subject to the usual rules for operand types. When required, the D compiler loads the variable names that correspond to active kernel modules, so you do not need to declare these variables. You may not apply any operator to an external variable that modifies its value, such as = or +=. For safety reasons, DTrace prevents you from damaging or corrupting the state of the software you are observing.