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digitalmars.D.learn - Linking D with C structs

reply Anthony <anthoq88 gmail.com> writes:
Hello,


I'm trying to hand write some bindings to mongo-c-driver. (For 
learning purposes and to get the latest bindings).

My binding code to convert <mongoc/mongoc.h> to mongoc/mongoc.d 
is:

===== c interop file ====
module mongoc/mongoc.d;
import core.stdc.stdint;

extern (c) {
struct {
     uint domain;
     uint code;
     char[504] message; //when this exists, the code does not 
compile
}
}
=======================

===== build script ====

export 
LD_LIBRARY_PATH=mongo-c-driver/cmake-build/src/libmongoc:mongo-c-driver/cmake-build/src/libbson:$LD_LIBRARY_PATH

dmd hello_mongo.d \
     -I=mongo-c-driver/src/libmongoc/src/ \
     -I=mongo-c-driver/src/libbson/src/ \
     -I=mongo-c-driver/cmake-build/src/libbson/src/bson/ \
     -I=mongo-c-driver/cmake-build/src/libmongoc/src/mongoc \
     -L=-Lmongo-c-driver/cmake-build/src/libmongoc \
     -L=-Lmongo-c-driver/cmake-build/src/libbson \
     -L=-lmongoc-1.0 \
     -L=-lbson-1.0 \

./hello_mongo
========================


However, when I add the message field in the struct, I cannot 
compile the code:

ld: hello_mongo.o: in function `_Dmain':
hello_mongo.d:(.text._Dmain[_Dmain]+0x1a): undefined reference to 
`_D6mongoc6mongoc13_bson_error_t6__initZ'


What does "__initZ" refer to?
Does this refer to automatic initialization like "this()"?
Jun 28 2020
parent reply kinke <kinke gmx.net> writes:
On Monday, 29 June 2020 at 06:29:38 UTC, Anthony wrote:

 What does "__initZ" refer to?
 Does this refer to automatic initialization like "this()"?


Almost, it's the static initializer for that struct, which is 
omitted because you apparently don't compile/link the module 
containing the struct declaration. Initialize the char array with 
zeros (= 0) to make the struct fully zero-initialized, preventing 
the need for that symbol. chars in D are initialized with 0xFF, 
unlike byte and ubyte.
Jun 29 2020
parent Anthony <anthoq88 gmail.com> writes:
On Monday, 29 June 2020 at 10:44:16 UTC, kinke wrote:

 On Monday, 29 June 2020 at 06:29:38 UTC, Anthony wrote:

 What does "__initZ" refer to?
 Does this refer to automatic initialization like "this()"?


 Almost, it's the static initializer for that struct, which is 
 omitted because you apparently don't compile/link the module 
 containing the struct declaration. Initialize the char array 
 with zeros (= 0) to make the struct fully zero-initialized, 
 preventing the need for that symbol. chars in D are initialized 
 with 0xFF, unlike byte and ubyte.


Thanks for this!

What do you mean by "which is omitted because you apparently 
don't compile/link the module containing the struct declaration"?

Is there a link step that I'm missing that will make things 
easier for me?
Is there a way to have D automatically get the struct from the c 
files directly?
Jun 30 2020