In most cases SystemTap can determine a variable's type from the debug information. However, code may use void pointers for variables (for example memory allocation routines) and typing information is not available. Also the typing information available within a probe handler is not available within a function; SystemTap functions arguments use a long in place of a typed pointer. SystemTap's @cast operator (first available in SystemTap 0.9) can be used to indicate the correct type of the object.
The Example 3.9, “Casting Example” is from the task.stp tapset. The function returns the value of the state field from a task_struct pointed to by the long task. The first argument of the @cast operator, task, is the pointer to the object. The second argument is the type to cast the object to, task_struct. The third argument lists what file that the type definition information comes from and is optional. With the @cast operator the various fields of this particular task_struct task can be accessed; in this example the state field is obtained.

Example 3.9. Casting Example

function task_state:long (task:long)
    return @cast(task, "task_struct", "kernel<linux/sched.h>")->state