Today’s Erlang Thursday continues to take a look at the c module with c:m/1.

c:m/1 takes an atom of a module name, and returns information about the module. It prints out information about the compliation date, time, and options; the object (BEAM) file that it was loaded from, and a list of functions exported by the module.

We’ll start with taking a look at the string module in Erlang.

c:m(string).
% Module string compiled: Date: November 28 2014, Time: 06.47
% Compiler options:  [{outdir,"/private/tmp/erlang-pY1Kv2/otp-OTP-17.3.4/lib/stdlib/src/../ebin"},
%                     {i,"/private/tmp/erlang-pY1Kv2/otp-OTP-17.3.4/lib/stdlib/src/../include"},
%                     {i,"/private/tmp/erlang-pY1Kv2/otp-OTP-17.3.4/lib/stdlib/src/../../kernel/include"},
%                     warnings_as_errors,debug_info]
% Object file: /usr/local/Cellar/erlang/17.3.4/lib/erlang/lib/stdlib-2.2/ebin/string.beam
% Exports:
% centre/2                      rstr/2
% centre/3                      span/2
% chars/3                       str/2
% chars/2                       strip/1
% chr/2                         strip/2
% concat/2                      strip/3
% copies/2                      sub_string/2
% cspan/2                       sub_string/3
% equal/2                       sub_word/2
% join/2                        sub_word/3
% left/2                        substr/2
% left/3                        substr/3
% len/1                         to_float/1
% module_info/0                 to_integer/1
% module_info/1                 to_lower/1
% rchr/2                        to_upper/1
% right/2                       tokens/2
% right/3                       words/1
%                               words/2
% ok

We can see that this was compiled on my machine on November 28th of 2014, and had the warnings_as_errors and debug_info turned on, as well as the location of the beam file, and all of the different functions the string module exports.

Next, we will look at a module compiled from inside the shell.

c(fizzbuzz).
% {ok,fizzbuzz}
c:m(fizzbuzz).
% Module fizzbuzz compiled: Date: August 5 2015, Time: 22.14
% Compiler options:  []
% Object file: /Users/proctor/tmp/fizzbuzz.beam
% Exports:
%          fizzbuzz/1
%          module_info/0
%          module_info/1
% ok

c:m(fizzbuzz) shows that it was compiled, and was loaded from my tmp directory, and exports fizzbuzz/1 along with the two versions of module_info that every module exports.

Again, this is one of those functions that you might not use everyday, but when it comes to debugging and inspecting your Erlang application becomes a very useful function to know about.

–Proctor