This week's LFE Friday was translated with permission from the
Erlang Thursday
series by Steven Proctor.
This week's translator: Robert Virding.
Today's LFE Friday is a short one on c:pid/3.
c:pid/3 takes the three different parts of a pid as its arguments, and returns a constructed Pid type corresponding to those values.
We'll call (self) to get a pid that we know is good, and we can use that to compare the result of calling c:pid/3.
> (self)
<0.87.0>
> (c:pid 0 87 0)
<0.87.0>
> (=:= (self) (c:pid 0 87 0))
true
Why is this useful? Sometimes when inspecting what is going on in a live system there are certain calls in LFE that expect a pid() type, and not just the pid numbers.
> (c:regs)
** Registered procs on node nonode@nohost **
Name Pid Initial Call Reds Msgs
application_controlle <0.7.0> erlang:apply/2 3594 0
code_server <0.12.0> erlang:apply/2 415654 0
erl_prim_loader <0.3.0> erlang:apply/2 900396 0
error_logger <0.6.0> gen_event:init_it/6 9565 0
file_server_2 <0.20.0> file_server:init/1 18072 0
global_group <0.19.0> global_group:init/1 8057 0
global_name_server <0.15.0> global:init/1 6341 0
inet_db <0.18.0> inet_db:init/1 2394 0
init <0.0.0> otp_ring0:start/2 9920 0
kernel_safe_sup <0.31.0> supervisor:kernel/1 4389 0
kernel_sup <0.11.0> supervisor:kernel/1 9798 0
rex <0.14.0> rpc:init/1 3086 0
standard_error <0.22.0> erlang:apply/2 5386 0
standard_error_sup <0.21.0> supervisor_bridge:standar 1402 0
user <0.25.0> group:server/3 3202 0
user_drv <0.24.0> user_drv:server/2 41003 0
** Registered ports on node nonode@nohost **
Name Id Command
ok
> (erlang:is_process_alive (c:pid 0 7 0))
true
So let's see what happens when we try to feed it something to break it, but in a meaningful way.
> (c:pid 0 0 0)
<0.0.0>
> (c:pid 111110 0 1111110)
exception error: badarg
in (: erlang list_to_pid "<111110.0.1111110>")
in c:pid/3 (c.erl, line 426)
So it looks like the pid <0.0.0> is a valid pid, but when we throw it something else, we see it is trying to call erlang:list_to_pid.
So let's take a quick look at erlang:list_to_pid.
> (erlang:list_to_pid "<0.47.0>")
<0.47.0>
> (=:= (c:pid 0 47 0) (erlang:list_to_pid "<0.47.0>"))
true
So c:pid/3 is really just a wrapper function around
erlang:list_to_pid that builds the 3 parts of a pid into a string,
and then gets a pid() type from that call.
As with the earlier c functions this function exists as an LFE repl built-in command which can be called from the repl with just (pid 0 47 0).
-Proctor, Robert
