native extension Dart

Question

I'm trying to build a native extension in dart for postgresql. I have compiled the CC file in .o then in .so (shared object i guess). It's now named libpsql.so and I placed it in the the same directory as my .dart file. The first line of dart file is #import(dart-ext:libpsql); but it keeps telling me that the resources is unavailable.

My dart code

#library("psql");

#import("dart-ext:libpsql_dart");

class Database {
  var mDb;
  var mUser;
  var mDbname;
  var mPasswd;
  var mHost;
  var mPort;
  var mTable;

  //String toString() => "<PostgreSQL: $user@$_host:$_port/$_table>";
  Database(host,user,passwd,dbname) : this.mUser = user, this.mHost = host, this.mPasswd = passwd, this.mDbname = dbname  {
    mDb = _connect(host,user,passwd,dbname);
  }

}
_connect(host,user,passwd,dbname) native 'Connect';

And here is my C++ code.

#include <string.h>
#include <stdio.h>
#include <libpq-fe.h>
#include "dart_api.h"

Dart_NativeFunction ResolveName(Dart_Handle name, int argc);

DART_EXPORT Dart_Handle psql_dart_Init(Dart_Handle parent_library) {
  if (Dart_IsError(parent_library)) return parent_library;

  Dart_Handle result_code =
      Dart_SetNativeResolver(parent_library, ResolveName);
  if (Dart_IsError(result_code)) return result_code;

  return Dart_Null();
}

Dart_Handle HandleError(Dart_Handle handle) {
 if (Dart_IsError(handle)) Dart_PropagateError(handle);
 return handle;
}
void Connect(Dart_NativeArguments args) {
    Dart_EnterScope();
    PGconn *conn;
    const char *conninfo = "user=postgres;password=postgres;host=localhost;port=5432;dbname=reviewdb";
    conn = PQconnectdb(conninfo);

    /* Check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "Connection to database failed: %s",
                PQerrorMessage(conn));
        PQfinish(conn);
    }

  Dart_Handle result = HandleError(Dart_NewInteger((int64_t) conn));
  Dart_SetReturnValue(args, result);

  Dart_ExitScope();
}

Dart_NativeFunction ResolveName(Dart_Handle name, int argc) {
  assert(Dart_IsString8(name));
  const char* cname;
  Dart_Handle check_error = Dart_StringToCString(name, &cname);
  if (Dart_IsError(check_error)) Dart_PropagateError(check_error);
  Dart_NativeFunction result = NULL;
  if (strcmp("Connect", cname) == 0) result = Connect;
  Dart_ExitScope();
  return result;
}

Html script include

<script type="application/dart" src="web/lib/psql.dart"></script> 
    <script type="application/dart" src="web/test_dart.dart"></script>

And the last, my compile command line :

g++ -fPIC --verbose -I/home/{linux user}/Documents/dart/dart-sdk/include/ -lpq -I/usr/include/postgresql -c psql_dart.cc

gcc -shared -Wl,-soname,libpsql.so -o libpsql.so psql_dart.o

After testing new code i commented my function Connect like this :

void Connect(Dart_NativeArguments args) {
    Dart_EnterScope();
//    PGconn *conn;
//    const char *conninfo = "user=postgres password=postgres host=localhost port=5432";
//    conn = PQconnectdb(conninfo);
//
//    /* Check to see that the backend connection was successfully made */
//    if (PQstatus(conn) != CONNECTION_OK)
//    {
//        fprintf(stderr, "Connection to database failed: %s",
//                PQerrorMessage(conn));
//        PQfinish(conn);
//        exit(1);
//    }
//  PQfinish(conn);

  Dart_Handle result = HandleError(Dart_NewInteger( 0));
  Dart_SetReturnValue(args, result);

  Dart_ExitScope();
}

The output :

worked?
Segmentation fault (core dumped)

And I still get the SegFault any idea?

My gdb stacktrace :

Starting program: /home/<user>/Documents/dart/dart-sdk/bin/dart test_dart.dart
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
worked?

Program received signal SIGSEGV, Segmentation fault.
dart::Api::NewHandle (isolate=0x948a40, raw=0x7ffff7f80021) at runtime/vm/dart_api_impl.cc:114
114 runtime/vm/dart_api_impl.cc: No such file or directory.
(gdb) bt
#0  dart::Api::NewHandle (isolate=0x948a40, raw=0x7ffff7f80021) at runtime/vm/dart_api_impl.cc:114
#1  0x000000000042e121 in dart::Dart_Invoke (target=<optimized out>, name=0x959b90, number_of_arguments=<optimized out>, arguments=<optimized out>) at runtime/vm/dart_api_impl.cc:3543
#2  0x00000000004097ee in main (argc=<optimized out>, argv=<optimized out>) at runtime/bin/main.cc:724

Answer 1

After playing around some with your code and getting the postgresql-dev-9.1 package installed this is where I am. Currently it still does not run, however it is due to a linking error not due to the import itself.

Note a change to your C++ file: I renamed your initialization function from: psql_dart_Init to just psql_Init

// libpsql.cc
#include <string.h>
#include <stdio.h>
#include <libpq-fe.h>
#include "dart_api.h"

Dart_NativeFunction ResolveName(Dart_Handle name, int argc);

DART_EXPORT Dart_Handle psql_Init(Dart_Handle parent_library) {
  if (Dart_IsError(parent_library)) return parent_library;

  Dart_Handle result_code =
      Dart_SetNativeResolver(parent_library, ResolveName);
  if (Dart_IsError(result_code)) return result_code;

  return Dart_Null();
}

Dart_Handle HandleError(Dart_Handle handle) {
 if (Dart_IsError(handle)) Dart_PropagateError(handle);
 return handle;
}

void Connect(Dart_NativeArguments args) {
    Dart_EnterScope();
    PGconn *conn;
    const char *conninfo = "user=postgres;password=postgres;host=localhost;port=5432;dbname=reviewdb";
    conn = PQconnectdb(conninfo);

    /* Check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "Connection to database failed: %s",
                PQerrorMessage(conn));
        PQfinish(conn);
    }

  Dart_Handle result = HandleError(Dart_NewInteger((int64_t) conn));
  Dart_SetReturnValue(args, result);

  Dart_ExitScope();
}

Dart_NativeFunction ResolveName(Dart_Handle name, int argc) {
  assert(Dart_IsString8(name));
  const char* cname;
  Dart_Handle check_error = Dart_StringToCString(name, &cname);
  if (Dart_IsError(check_error)) Dart_PropagateError(check_error);
  Dart_NativeFunction result = NULL;
  if (strcmp("Connect", cname) == 0) result = Connect;
  Dart_ExitScope();
  return result;
}

And the following is my 1st dart file:

// psql.dart
#library("psql");

#import("dart-ext:psql");

class Database {
  var mDb;
  var mUser;
  var mDbname;
  var mPasswd;
  var mHost;
  var mPort;
  var mTable;

  //String toString() => "<PostgreSQL: $user@$_host:$_port/$_table>";
  Database(host,user,passwd,dbname) : this.mUser = user, this.mHost = host, this.mPasswd = passwd, this.mDbname = dbname  {
    mDb = _connect(host,user,passwd,dbname);
  }

}
_connect(host,user,passwd,dbname) native 'Connect';

And then the actual VERY minimal application (command line instead of dartium-based) to test it.

// test.dart
#import('psql.dart');

main() {
  var database = new Database('localhost', 'mbutler', 'test', 'test');

  if(database != null) {
    print('worked?');
  }
}

I used the following command to compile and link in one go and I it does work correctly. I segfault because I don't have a valid database to connect to but the following does load the native library properly:

g++ -O2 -DDART_SHARED_LIB -I/home/<user>/dart/dart-sdk/include -rdynamic -fPIC -shared libpsql.cc -lpq -I/usr/include/postgresql -o libpsql.so

(Thanks to dart-sqlite build script was able to piece together the linking I required)

Answer 2

If you want to compile the small projects in the language C/C++.

In particular native dart extension. Then you can try this one small set of tools. It allows you to organize the following tasks.

1. Organize a project into a single file as yaml or json file.
2. Organize a project for support multi-platform building.
3. Building these projects directly from Dart language scripts.
4. Eliminates the need to write batch files for different platforms.

You can install these tools as Dart package through pub package manager and use it.

Also, it contains a simple example of creating a native extension and example of how to quickly build it (complile, link and clean project) from Dart language script.

This set of tools is called ccompile.

You can find this set of tools on github at https://github.com/mezoni/ccompile

I hope this will be helpful to you!