The arguments to entry probes are the same as the arguments to the corresponding operating system kernel function. These arguments may be accessed in a typed fashion by using the args[] array. These arguments may be accessed as int64_t's by using the arg0 .. arg n variables.

While a given function only has a single point of entry, it may have many different points where it returns to its caller. You are usually interested in either the value that a function returned or the fact that the function returned at all rather than the specific return path taken. FBT therefore collects a function's multiple return sites into a single return probe. If the exact return path is of interest, you can examine the return probe args[0] value, which indicates the offset (in bytes) of the returning instruction in the function text.

If the function has a return value, the return value is stored in args[1]. If a function does not have a return value, args[1] is not defined.