[erlang-questions] NIF Query

[John Duffy]

Hi Steve
Thanks for you reply. Your help has answered my queries...
1.)  I had a typo in ERL_NIF_INIT(), the module name parameter was pointing to a NIF function name and not the module name.
2.)  I'm compiling with gcc on Debian. With my original compiler options "-Wall -W" the "-W" was generating the uninitialised structure field warning. Without the "-W" flag the warning is suppressed.
Many thanks for your help.
Kind regards
John
----Original message----
>From : vinoski@REDACTED
Date : 02/09/2017 - 14:24 (BST)
To : jb_duffy@REDACTED
Cc : erlang-questions@REDACTED
Subject : Re: [erlang-questions] NIF Query
On Fri, Sep 1, 2017 at 7:13 PM, John Duffy <jb_duffy@REDACTED> wrote:
Hi
I'm trying to follow the Interoperability Tutorial Users Guide (Version 9.0) to create a module containing some simple NIFS. I have managed to do this, however I have needed to digress from the documentation as follows...
1.)  if my module name doesn't match the filename in the call to erlang:load_nif("filename", 0) I get a compile error. This is not as per the example in the documentation, i.e. -module(complex6) & erlang:load_nif("complex6_nif, 0).
Can you supply the details of the error you're getting? I followed the tutorial on my Mac and had no problems.
2.)  The appears to be a "flags" parameter missing in the {"foo", 1, foo_nif} structure in the documentation which generates a compile error...
static ErlNifFunc nif_funcs[] = {
    {"foo", 1, foo_nif},
    {"bar", 1, bar_nif}
};
I have changed this to that below, which compiles, but what is the nature/value of "flags"?
static ErlNifFunc nif_funcs[] = {
    {"foo", 1, foo_nif, 0},
    {"bar", 1, bar_nif, 0}
};
I may be missing something, or the Tutorial may need updating?
The flags field is described in http://erlang.org/doc/man/erl_nif.html -- it's currently used for values specific to dirty NIFs, which are also described on that page. For a regular NIF like the one in the tutorial, flags should be 0. Since the nif_funcs variable in the tutorial is defined as static, the C compiler is guaranteed to initialize flags to 0 because an initial value for it isn't specified.
Whether you get a problem from the flags field or not depends on your compiler and the options you pass to it. For example, copying and pasting the complex6_nif.c from the tutorial into a file and compiling it as follows on my Mac produced no errors or warnings:
clang -o complex6_nif.so complex.c complex6_nif.c -I/usr/local/lib/erlang/usr/include -bundle -flat_namespace -undefined suppress
But adding the -Wmissing-field-initializers option results in warnings:
complex6_nif.c:27:23: warning: missing field 'flags' initializer [-Wmissing-field-initializers]
    {"foo", 1, foo_nif},
                      ^
complex6_nif.c:28:23: warning: missing field 'flags' initializer [-Wmissing-field-initializers]
    {"bar", 1, bar_nif}
                      ^
2 warnings generated.
--steve
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