Today’s Erlang Thursday 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().
% <0.42.0>
c:pid(0, 42, 0).
% <0.42.0>
self() =:= c:pid(0, 42, 0).
% true

Why is this useful? Sometimes when inspecting what is going on in a live system there are certain calls in Erlang 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                     463    0
% code_server           <0.19.0>     erlang:apply/2                  128774    0
% erl_prim_loader       <0.3.0>      erlang:apply/2                  163760    0
% error_logger          <0.6.0>      gen_event:init_it/6                220    0
% file_server_2         <0.18.0>     file_server:init/1                 448    0
% global_group          <0.17.0>     global_group:init/1                 59    0
% global_name_server    <0.13.0>     global:init/1                       51    0
% inet_db               <0.16.0>     inet_db:init/1                     206    0
% init                  <0.0.0>      otp_ring0:start/2                 3398    0
% kernel_safe_sup       <0.27.0>     supervisor:kernel/1                 58    0
% kernel_sup            <0.11.0>     supervisor:kernel/1              49109    0
% rex                   <0.12.0>     rpc:init/1                          35    0
% standard_error        <0.21.0>     erlang:apply/2                       9    0
% standard_error_sup    <0.20.0>     supervisor_bridge:standar           41    0
% user                  <0.24.0>     group:server/3                      36    0
% user_drv              <0.23.0>     user_drv:server/2                17940    0
%
% ** Registered ports on node nonode@nohost **
% Name                  Id              Command
% ok
erlang:is_process_alive(c:pid(0, 6, 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: bad argument
%      in function  list_to_pid/1
%         called as list_to_pid("<111110.0.1111110>")
%      in call from c:pid/3 (c.erl, line 424)

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 list_to_pid.

So let’s take a quick look at list_to_pid.

erlang:list_to_pid("<0.42.0>").
% <0.42.0>
c:pid(0, 42, 0) =:= erlang:list_to_pid("<0.42.0>").
% true

So it looks like c:pid/3 is a wrapper function around list_to_pid that builds the 3 parts of a pid into a string, and then gets a pid() type from that call.

–Proctor