Limitar Banco de Dados Firebird para 3 usuarios

Ola eu dei uma pesquisada e encontrei esse comando para saber as conexões, no entando só funciona no firebird 2.1 ou superior

select count(MON$STATE) as conexoes from MON$ATTACHMENTS

É interessante testar o comando repassado pelo bruno nas versões anteriores, já que tem essa base instalada.

Peguei na net algumas funções:

Olha Ghost, acho que esse código pode lhe ajudar.......
Sufolk...
Código:

{

Unit para retornar a quantidade de usuários conectados num banco Firebird

Sintaxe:

function UsuariosLogados(DBPath, Username, Password) : integer

DBPath - path/nome do arquivo FDB. Ex: c:\programa\dados.fdb ou 192.168.0.1:c:\dados\banco.fdb
Username - Usuário que efetuará a conexão, geralmente SYSDBA
Password - senha para validar o usuário

RETORNO
-Retorna a quantidade(integer) de usuários conectados no banco, mais 1 referente
a própria conexão que a Unit cria, então se existir 2 usuários conectados, o
resultado será 3.

-Retorna -1 caso acontecer algum erro e não ser capaz de conectar ao banco

Baseado no exemplo criado por: Willian de Carvalho
http://www.delphi.eti.br/downloads.php?id=4347

-:-
12/12/2006
Enio Marconcini
email/msn/gtalk: eniorm at gmail dot com
skype: eniorm
www.delfosistemas.com

}

unit U_UsuariosLogados;

interface

uses
Windows, SysUtils, Dialogs;

const
isc_dpb_version1 = 1;
isc_dpb_user_name = 28;
isc_dpb_password = 29;

isc_info_end = 1;
isc_info_truncated = 2;
isc_info_error = 3;
isc_info_user_names = 53;

IBASE_DLL = FBCLIENT.DLL;
KILOBYTE = 1024;

type
ISC_LONG = Longint;
ISC_STATUS = ISC_LONG;
ISC_STATUS_VECTOR = array[0..19] of ISC_STATUS;
PSTATUS_VECTOR = ^ISC_STATUS_VECTOR;
PPSTATUS_VECTOR = ^PSTATUS_VECTOR;

Tisc_db_handle = Pointer;
pisc_db_handle = ^Tisc_db_handle;

TParamBlock = array [0..KILOBYTE-1] of Char;
TLargePB = array [0..(4*KILOBYTE)-1] of Char;
TSmallPB = array [0..(KILOBYTE div 4)-1] of Char;

var
E : Exception;
UserFound: boolean;
Mode: (mdDBName, mdUserName, mdPassword);
s, DBName, UserName, Password: string;

ErrorCode: ISC_STATUS;
StatusVector: ISC_STATUS_VECTOR;
DBHandle: Tisc_db_handle;
DPB: TParamBlock; //parameter block for database connection
DPBLen: Integer; //length of Paramblock
ItemList: TSmallPB;
UserNames: TLargePB;
UserCount: Integer;

Item, //InfoItem we are testing for
Pos, //marker for position in array
Len, //Length of section
namelength: SmallInt;
UserStr: array[0..255] of char;

function isc_interprete(buffer: PChar; status_vector_ptr: PPSTATUS_VECTOR): ISC_STATUS;
stdcall; external IBASE_DLL name isc_interprete;

function isc_attach_database(status_vector: PSTATUS_VECTOR; db_name_length: Short;
db_name: PChar; db_handle: pisc_db_handle; parm_buffer_length: Short;
parm_buffer: PChar): ISC_STATUS; stdcall; external IBASE_DLL name isc_attach_database;

function isc_database_info(status_vector: PSTATUS_VECTOR; db_handle: pisc_db_handle;
item_list_buffer_length: Smallint; item_list_buffer: Pointer;
result_buffer_length: Smallint; result_buffer: Pointer): ISC_STATUS;
stdcall; external IBASE_DLL name isc_database_info;

function isc_vax_integer(result_buffer : PChar; result_length : SmallInt): ISC_LONG;
stdcall; external IBASE_DLL name isc_vax_integer;

function isc_detach_database(status_vector: PSTATUS_VECTOR; db_handle:
pisc_db_handle): ISC_STATUS; stdcall; external IBASE_DLL name isc_detach_database;

procedure Error;
procedure BuildPBString( var PB: array of char; var PBLen: Integer; item: byte; contents: string);
function UsuariosLogados(DBFile,Username,Password : String) : Integer;


implementation

procedure Error;
var
buffer: array[0..511] of char;
ErrorMessages, lastMsg: string;
pStatus: PSTATUS_VECTOR;
begin
fillchar(buffer,512,#0);
pStatus:=@StatusVector;
ErrorMessages:=;
repeat
ErrorCode := isc_interprete( @buffer, @pstatus);
if lastMsg strPas( buffer) then
begin
lastMsg := strPas( buffer);
if length(ErrorMessages) 0 then ErrorMessages := ErrorMessages+#13#10;
ErrorMessages := ErrorMessages+lastMsg;
end;
until ErrorCode = 0;
raise Exception.Create(ErrorMessages);
end;

procedure BuildPBString( var PB: array of char; var PBLen: Integer; item: byte; contents: string);
//Add a string value to a parameter block
var len: Integer;
begin
PB[PBLen] := char(item);
inc(PBLen);
len:=Length(Contents);
PB[PBLen] := char(len);
inc(PBLen);
StrPCopy(@PB[PBLen],Contents);
inc(PBLen,len);
end;

// =============================================================================
// =============================================================================

function UsuariosLogados(DBFile,Username,Password : String) : Integer;
Var
i: Integer;
begin
try
UserFound := false;
Mode := mdDBName;
//DBName := ;
DBName := trim(DBFile);

if ( UpperCase(ExtractFileExt(DBName)) .FDB) then begin
raise EAccessViolation.Create(Formato de arquivo inválido!);
Abort;
end;

//UserName := ;
UserName := Trim(Username);
//Password := ;
Password := trim(Password);
if (DBName = ) or (UserName = ) then
halt(2);
for i:= low(StatusVector) to high(StatusVector) do StatusVector[i] := 0;
DBHandle := nil;
fillchar(DPB,sizeof(DPB),#0);
DPB[0] := char(isc_dpb_version1);
DPBLen := 1;
BuildPBString(DPB,DPBLen,isc_dpb_user_name,Username);
BuildPBString(DPB,DPBLen,isc_dpb_password,Password);
ErrorCode := isc_attach_database(@StatusVector, Length(DBName), PChar(DBName),
@DBHandle, DPBLen, @DPB);
if ErrorCode 0 1);
end
else
Error;
if assigned(DBHandle) then
begin
ErrorCode := isc_detach_database(@StatusVector, @DBHandle);
if ErrorCode 0 then
Error;
end;
except
on E:Exception do
begin
s := E.Message + #13#10;
ShowMessage(Houve um erro: + s[1]);
s := E.Message + #13#10;
WriteFile(GetStdHandle(STD_ERROR_HANDLE), s[1], Length(s), DWORD(i), nil);
Result := -1;
ExitCode := 2;
end;
end;
end;


end.

Créditos: http://www.activedelphi.com.br/forum/viewtopic.php?t=32010&sid=3d4c4f5a59220f2bb720e2294518c0b2

verifique também o seguinte arquivo:

http://www.firebirdsql.org/pdfmanual/Firebird-Commandline-Brazilian-Portuguese.pdf

Nele constam várias orientações referente a sua versão do firebird.