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  • #1

    Null pointer

    I'm trying to enumerate all the IP addresses on a machine.
    The problem is that when I get the pointer to the list of all the ip's I can't increase it. When I increase the pointer with 4 bytes the pointer is refering to an invalid address.

    Code I use :

    #COMPILE EXE
    #DIM ALL
    #INCLUDE "WIN32API.INC"
    #INCLUDE "WSOCK32.INC"


    DECLARE FUNCTION StartWinsock()AS LONG

    FUNCTION PBMAIN()
    DIM winsck AS LONG
    DIM Hostname AS ASCIIZ * 80
    DIM ptrAsciiz AS ASCIIZ PTR
    DIM ErrNumber AS LONG
    DIM hstEnt AS hostentStru PTR
    DIM varADDR AS in_addr

    winsck = StartWinsock()
    IF winsck <> 0 THEN
    MSGBOX "Error on opening winsock. Error number: " + STR$(winsck)
    CALL wsacleanup()
    EXIT FUNCTION
    END IF

    IF (gethostname(Hostname, SIZEOF(Hostname)) = %SOCKET_ERROR) THEN
    ErrNumber = WSAGetLastError()
    MSGBOX "Error on getting local hostname. Error number: " + STR$(ErrNumber)
    CALL wsacleanup()
    EXIT FUNCTION
    END IF
    MSGBOX "The local hostname is: " + hostname

    hstEnt = gethostbyname(hostname)

    IF hstent = 0 THEN
    MSGBOX "Error on host lookup."
    CALL wsacleanup()
    EXIT FUNCTION
    END IF

    '-------- Here the enum has to start. Currently I can only
    '-------- retrieve 1 address
    '-------- C checks if pointer = 0
    ' FOR (INT i = 0; phe->h_addr_list[i] != 0; ++i) {
    ' struct in_addr ADDR;
    ' memcpy(&addr, phe->h_addr_list[i], SIZEOF(struct n_addr));
    ' cout << "Address " << i << ": " << inet_ntoa(ADDR) << endl;
    ' }

    varADDR.s_addr = @[email protected]@h_list

    ptrAsciiz = inet_ntoa(varADDR.s_addr)

    MSGBOX " Your IP adress is : " + @ptrAsciiz

    CALL wsacleanup()

    END FUNCTION

    FUNCTION StartWinsock()AS LONG
    DIM wsaData AS WSAData
    DIM lngReturn AS LONG

    lngReturn = wsastartup(257,wsaData)
    FUNCTION = lngReturn

    END FUNCTION

    Anybody who know how to check if the pointer reference is 0 (null) ?

    Thanks
    Tags: None
  • #2
    IF ptrASCIIZ = 0 THEN MSGBOX "The pointer is NULL, NULL is the pointer!"

    Do you want to check the target byte? If so, then LEN(@ptrASCIIZ) should return zero, as long as the target address is allocated to your application.

    ------------------
    Lance
    PowerBASIC Support
    mailto:[email protected][email protected]</A>
    Lance
    mailto:[email protected]

    Comment

    • #3
      The problem is if I increase h_list I'm getting a reference to an illigal address. Maybe I'm doing the increase the wrong way.

      When I'm looking at the type declare the member of hostent, h_list is a long pointer adress to the list of all ip addresses. So when I increase this pointer with 4 bytes I should get the next address ?

      But if there's no other address then the pointer refers to an illigal address. So I can't call @hostent[email protected]@h_list because this gives me a GPF. So the problem is that I don't know how to check if the pointer still points to a valid address.

      Erwin



      ------------------

      Comment

      • #4
        Hi Erwin

        try:

        Code:
        #COMPILE EXE
#DIM ALL
#INCLUDE "WIN32API.INC"
#INCLUDE "WSOCK32.INC"


DECLARE FUNCTION StartWinsock()AS LONG

FUNCTION PBMAIN()
DIM winsck AS LONG
DIM Hostname AS ASCIIZ * 80
DIM ptrAsciiz AS ASCIIZ PTR
DIM ErrNumber AS LONG
DIM hstEnt AS hostentStru PTR
DIM varADDR AS in_addr
DIM plAddr AS LONG PTR

    winsck = StartWinsock()
    IF winsck <> 0 THEN
        MSGBOX "Error on opening winsock. Error number: " + STR$(winsck)
        CALL wsacleanup()
        EXIT FUNCTION
    END IF

    IF (gethostname(Hostname, SIZEOF(Hostname)) = %SOCKET_ERROR) THEN
        ErrNumber = WSAGetLastError()
        MSGBOX "Error on getting local hostname. Error number: " + STR$(ErrNumber)
        CALL wsacleanup()
        EXIT FUNCTION
    END IF
    MSGBOX "The local hostname is: " + hostname

    hstEnt = gethostbyname(hostname)

    IF hstent = 0 THEN
        MSGBOX "Error on host lookup."
        CALL wsacleanup()
        EXIT FUNCTION
    END IF

'-------- Here the enum has to start. Currently I can only
'-------- retrieve 1 address
'-------- C checks if pointer = 0
'    FOR (INT i = 0; phe->h_addr_list[i] != 0; ++i) {
'        struct in_addr ADDR;
'        memcpy(&addr, phe->h_addr_list[i], SIZEOF(struct n_addr));
'        cout << "Address " << i << ": " << inet_ntoa(ADDR) << endl;
'    }

    plAddr =  @hstent.h_list
    DO WHILE @plAddr <> 0

        ptrAsciiz = inet_ntoa(BYVAL @@plAddr)

        MSGBOX " Your IP adress is : " + @ptrAsciiz
        INCR plAddr


    LOOP

    CALL wsacleanup()

END FUNCTION

FUNCTION StartWinsock()AS LONG
DIM wsaData AS WSAData
DIM lngReturn AS LONG

    lngReturn = wsastartup(MAKLNG(2,0),wsaData) 'ask for Winsock 2.0
     FUNCTION = lngReturn

END FUNCTION


        ------------------

        Comment

        • #5
          hent.h_list is a pointer, to an array of pointers(which is null terminated)...
          Err..

          Anyways
          This code works for me:

          Code:
          #COMPILE EXE
#DIM ALL
#INCLUDE "WIN32API.INC"
#INCLUDE "WSOCK32.INC"

FUNCTION PBMAIN()
  DIM wd       AS WSAData
  DIM ret      AS LONG
  DIM hostname AS ASCIIZ * 256
  DIM hent     AS hostentStru PTR
  DIM i        AS LONG
  DIM ptrIP    AS ASCIIZ PTR

  ret = WSAStartup(&H101, wd)
  IF ret <> 0 THEN
    MSGBOX "Blah."
    EXIT FUNCTION
  END IF

  ' Set hostname to your machine name...
  hostname = "www.altavista.com"
  hent = gethostbyname(hostname)

  IF hent = 0 THEN
    MSGBOX "gethostbyname()... beep."
    WSACleanup
    EXIT FUNCTION
  END IF

  DO
    INCR i
    ptrIP = inet_ntoa(@[email protected]@h_list)
    MSGBOX "IP Address #" + FORMAT$(i) + ": " + @ptrIP
    @hent.h_list = @hent.h_list + @hent.h_len
    IF @[email protected]_list = 0 THEN EXIT DO
  LOOP

  WSACleanup
END FUNCTION

          ------------------
          http://www.sublevel6.f2s.com/

          Comment

          • #6
            Thanks, problem solved ...

            Erwin van de wiel


            ------------------

            Comment

            • #7
              If this helps:


              Code:
              '------------------------------------------------------------------------------
Global hostent          As HostEntStru Ptr
Global lpAddr           As In_Addr
Global ptr1             As Asciiz Ptr 


Function Host_to_IP(hName As Asciiz) As String
hostent = GetHostByName(hName)

If hostent=0 Then
  WSAErMsg = WSAGetLastError
  Function = WSAErMsgToStr(WSAErMsg)
Else
  lpaddr.s_addr = @[email protected]@h_list
  'Get the ip address
  ptr1 = inet_ntoa(lpaddr.s_addr)
  Function = @ptr1
End If
End Function

'------------------------------------------------------------------------------

Function IP_to_Host(haddr As Asciiz) As String
' IP to Host Name
' ascii IP to dword addr
lpaddr.s_addr = Inet_Addr(haddr)
' get host by address
hostent = GetHostByAddr(lpaddr.s_addr, 4, %PF_INET)
' null =  error
If hostent = 0 Then
  WSAErMsg = WSAGetLastError
  Function = WSAErMsgToStr(Result)
Else
  Function = @[email protected]_name
End If
End Function

'------------------------------------------------------------------------------
              ------------------
              Scott
              mailto:[email protected][email protected]</A>

              [This message has been edited by Scott Turchin (edited May 04, 2000).]
              Scott Turchin
              MCSE, MCP+I
              http://www.tngbbs.com
              ----------------------
              True Karate-do is this: that in daily life, one's mind and body be trained and developed in a spirit of humility; and that in critical times, one be devoted utterly to the cause of justice. -Gichin Funakoshi

              Comment

              • #8
                Originally posted by Andreas_Allerdahl:
                hent.h_list is a pointer, to an array of pointers(which is null terminated)...
                Err..

                Anyways
                This code works for me:

                <font face="Courier New, Courier" size="3"><pre>
                #COMPILE EXE
                #DIM ALL
                #INCLUDE "WIN32API.INC"
                #INCLUDE "WSOCK32.INC"

                FUNCTION PBMAIN()
                DIM wd AS WSAData
                DIM ret AS LONG
                DIM hostname AS ASCIIZ * 256
                DIM hent AS hostentStru PTR
                DIM i AS LONG
                DIM ptrIP AS ASCIIZ PTR

                ret = WSAStartup(&H101, wd)
                IF ret <> 0 THEN
                MSGBOX "Blah."
                EXIT FUNCTION
                END IF

                ' Set hostname to your machine name...
                hostname = "www.altavista.com"
                hent = gethostbyname(hostname)

                IF hent = 0 THEN
                MSGBOX "gethostbyname()... beep."
                WSACleanup
                EXIT FUNCTION
                END IF

                DO
                INCR i
                ptrIP = inet_ntoa(@[email protected]@h_list)
                MSGBOX "IP Address #" + FORMAT$(i) + ": " + @ptrIP
                @hent.h_list = @hent.h_list + @hent.h_len
                IF @[email protected]_list = 0 THEN EXIT DO
                LOOP

                WSACleanup
                END FUNCTION
                </pre></font>
                I've found the line "IF @[email protected]_list = 0 THEN EXIT DO"
                will repeatably often cause a crash under circumstances where,
                for example, there are network cards active and a modem
                is also logged into another network. However
                "IF @hent.h_list = 0 THEN EXIT DO"
                seems to not cause a crash.

                ------------------
                Michael Burns
                Michael Burns

                Comment

                • #9
                  Exactly... test for a non-null pointer before looking at the pointers target - which was where this thread started all those months (years?) ago!

                  ------------------
                  Lance
                  PowerBASIC Support
                  mailto:[email protected][email protected]</A>
                  Lance
                  mailto:[email protected]

                  Comment

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