docs/reference_guide: add auto-generation of perf event data structur…

…e in Python (iovisor#2211)

Follow-up to iovisor#2198.

docs/reference_guide.md

@@ -1263,7 +1263,7 @@ while 1:
  b.perf_buffer_poll()
 ```
 
-Note that the data structure transferred will need to be declared in C in the BPF program, and in Python. For example:
+Note that the data structure transferred will need to be declared in C in the BPF program. For example:
 
 ```C
 // define output data structure in C
@@ -1272,18 +1272,32 @@ struct data_t {
  u64 ts;
  char comm[TASK_COMM_LEN];
 };
+BPF_PERF_OUTPUT(events);
+[...]
+```
+
+In Python, you can either let bcc generate the data structure from C declaration automatically (recommanded):
+
+```Python
+def print_event(cpu, data, size):
+ event = b["events"].event(data)
+[...]
 ```
 
+or define it manually:
+
 ```Python
 # define output data structure in Python
 TASK_COMM_LEN = 16 # linux/sched.h
 class Data(ct.Structure):
  _fields_ = [("pid", ct.c_ulonglong),
  ("ts", ct.c_ulonglong),
  ("comm", ct.c_char * TASK_COMM_LEN)]
-```
 
-Perhaps in a future bcc version, the Python data structure will be automatically generated from the C declaration.
+def print_event(cpu, data, size):
+ event = ct.cast(data, ct.POINTER(Data)).contents
+[...]
+```
 
 Examples in situ:
 [code](https://github.com/iovisor/bcc/blob/08fbceb7e828f0e3e77688497727c5b2405905fd/examples/tracing/hello_perf_output.py#L59),