xref: /petsc/src/sys/classes/viewer/impls/socket/send.c (revision cc4c1da905d89950b196b027190941013bd3e15a)

#include <petscsys.h> /*I  "petscviewer.h"  I*/

#if defined(PETSC_NEEDS_UTYPE_TYPEDEFS)
/* Some systems have inconsistent include files that use but do not
   ensure that the following definitions are made */
typedef unsigned char  u_char;
typedef unsigned short u_short;
typedef unsigned short ushort;
typedef unsigned int   u_int;
typedef unsigned long  u_long;
#endif

#include <errno.h>
#include <ctype.h>
#if defined(PETSC_HAVE_MACHINE_ENDIAN_H)
  #include <machine/endian.h>
#endif
#if defined(PETSC_HAVE_UNISTD_H)
  #include <unistd.h>
#endif
#if defined(PETSC_HAVE_SYS_SOCKET_H)
  #include <sys/socket.h>
#endif
#if defined(PETSC_HAVE_SYS_WAIT_H)
  #include <sys/wait.h>
#endif
#if defined(PETSC_HAVE_NETINET_IN_H)
  #include <netinet/in.h>
#endif
#if defined(PETSC_HAVE_NETDB_H)
  #include <netdb.h>
#endif
#if defined(PETSC_HAVE_FCNTL_H)
  #include <fcntl.h>
#endif
#if defined(PETSC_HAVE_IO_H)
  #include <io.h>
#endif
#if defined(PETSC_HAVE_WINSOCK2_H)
  #include <Winsock2.h>
#endif
#include <sys/stat.h>
#include <../src/sys/classes/viewer/impls/socket/socket.h>

#if defined(PETSC_NEED_CLOSE_PROTO)
PETSC_EXTERN int close(int);
#endif
#if defined(PETSC_NEED_SOCKET_PROTO)
PETSC_EXTERN int socket(int, int, int);
#endif
#if defined(PETSC_NEED_SLEEP_PROTO)
PETSC_EXTERN int sleep(unsigned);
#endif
#if defined(PETSC_NEED_CONNECT_PROTO)
PETSC_EXTERN int connect(int, struct sockaddr *, int);
#endif

static PetscErrorCode PetscViewerDestroy_Socket(PetscViewer viewer)
{
  PetscViewer_Socket *vmatlab = (PetscViewer_Socket *)viewer->data;

  PetscFunctionBegin;
  if (vmatlab->port) {
    int ierr;

#if defined(PETSC_HAVE_CLOSESOCKET)
    ierr = closesocket(vmatlab->port);
#else
    ierr = close(vmatlab->port);
#endif
    PetscCheck(!ierr, PETSC_COMM_SELF, PETSC_ERR_SYS, "System error closing socket");
  }
  PetscCall(PetscFree(vmatlab));
  PetscCall(PetscObjectComposeFunction((PetscObject)viewer, "PetscViewerBinarySetSkipHeader_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)viewer, "PetscViewerBinaryGetSkipHeader_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)viewer, "PetscViewerBinaryGetFlowControl_C", NULL));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  PetscOpenSocket - handles connected to an open port where someone is waiting.

  Input Parameters:
+ hostname - for example www.mcs.anl.gov
- portnum  - for example 80

  Output Parameter:
. t - the socket number

  Notes:
  Use close() to close the socket connection

  Use read() or `PetscHTTPRequest()` to read from the socket

  Level: advanced

.seealso: `PetscSocketListen()`, `PetscSocketEstablish()`, `PetscHTTPRequest()`, `PetscHTTPSConnect()`
@*/
PetscErrorCode PetscOpenSocket(const char hostname[], int portnum, int *t)
{
  struct sockaddr_in sa;
  struct hostent    *hp;
  int                s      = 0;
  PetscBool          flg    = PETSC_TRUE;
  static int         refcnt = 0;

  PetscFunctionBegin;
  if (!(hp = gethostbyname(hostname))) {
    perror("SEND: error gethostbyname: ");
    SETERRQ(PETSC_COMM_SELF, PETSC_ERR_SYS, "system error open connection to %s", hostname);
  }
  PetscCall(PetscMemzero(&sa, sizeof(sa)));
  PetscCall(PetscMemcpy(&sa.sin_addr, hp->h_addr_list[0], hp->h_length));

  sa.sin_family = hp->h_addrtype;
  sa.sin_port   = htons((u_short)portnum);
  while (flg) {
    if ((s = socket(hp->h_addrtype, SOCK_STREAM, 0)) < 0) {
      perror("SEND: error socket");
      SETERRQ(PETSC_COMM_SELF, PETSC_ERR_SYS, "system error");
    }
    if (connect(s, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
#if defined(PETSC_HAVE_WSAGETLASTERROR)
      ierr = WSAGetLastError();
      if (ierr == WSAEADDRINUSE) (*PetscErrorPrintf)("SEND: address is in use\n");
      else if (ierr == WSAEALREADY) (*PetscErrorPrintf)("SEND: socket is non-blocking \n");
      else if (ierr == WSAEISCONN) {
        (*PetscErrorPrintf)("SEND: socket already connected\n");
        Sleep((unsigned)1);
      } else if (ierr == WSAECONNREFUSED) {
        /* (*PetscErrorPrintf)("SEND: forcefully rejected\n"); */
        Sleep((unsigned)1);
      } else {
        perror(NULL);
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_SYS, "system error");
      }
#else
      if (errno == EADDRINUSE) {
        PetscErrorCode ierr = (*PetscErrorPrintf)("SEND: address is in use\n");
        (void)ierr;
      } else if (errno == EALREADY) {
        PetscErrorCode ierr = (*PetscErrorPrintf)("SEND: socket is non-blocking \n");
        (void)ierr;
      } else if (errno == EISCONN) {
        PetscErrorCode ierr = (*PetscErrorPrintf)("SEND: socket already connected\n");
        (void)ierr;
        sleep((unsigned)1);
      } else if (errno == ECONNREFUSED) {
        refcnt++;
        PetscCheck(refcnt <= 5, PETSC_COMM_SELF, PETSC_ERR_SYS, "Connection refused by remote host %s port %d", hostname, portnum);
        PetscCall(PetscInfo(NULL, "Connection refused in attaching socket, trying again\n"));
        sleep((unsigned)1);
      } else {
        perror(NULL);
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_SYS, "system error");
      }
#endif
      flg = PETSC_TRUE;
#if defined(PETSC_HAVE_CLOSESOCKET)
      closesocket(s);
#else
      close(s);
#endif
    } else flg = PETSC_FALSE;
  }
  *t = s;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
   PetscSocketEstablish - starts a listener on a socket

   Input Parameter:
.    portnumber - the port to wait at

   Output Parameter:
.     ss - the socket to be used with `PetscSocketListen()`

    Level: advanced

.seealso: `PetscSocketListen()`, `PetscOpenSocket()`
*/
static PetscErrorCode PetscSocketEstablish(int portnum, int *ss)
{
  static size_t      MAXHOSTNAME = 100;
  char               myname[MAXHOSTNAME + 1];
  int                s;
  struct sockaddr_in sa;
  struct hostent    *hp;

  PetscFunctionBegin;
  PetscCall(PetscGetHostName(myname, sizeof(myname)));

  PetscCall(PetscMemzero(&sa, sizeof(struct sockaddr_in)));

  hp = gethostbyname(myname);
  PetscCheck(hp, PETSC_COMM_SELF, PETSC_ERR_SYS, "Unable to get hostent information from system");

  sa.sin_family = hp->h_addrtype;
  sa.sin_port   = htons((u_short)portnum);

  PetscCheck((s = socket(AF_INET, SOCK_STREAM, 0)) >= 0, PETSC_COMM_SELF, PETSC_ERR_SYS, "Error running socket() command");
#if defined(PETSC_HAVE_SO_REUSEADDR)
  {
    int optval = 1; /* Turn on the option */
    int ret    = setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (char *)&optval, sizeof(optval));
    PetscCheck(!ret, PETSC_COMM_SELF, PETSC_ERR_LIB, "setsockopt() failed with error code %d", ret);
  }
#endif

  while (bind(s, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
#if defined(PETSC_HAVE_WSAGETLASTERROR)
    ierr = WSAGetLastError();
    if (ierr != WSAEADDRINUSE) {
#else
    if (errno != EADDRINUSE) {
#endif
      close(s);
      SETERRQ(PETSC_COMM_SELF, PETSC_ERR_SYS, "Error from bind()");
    }
  }
  listen(s, 0);
  *ss = s;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
   PetscSocketListen - Listens at a socket created with `PetscSocketEstablish()`

   Input Parameter:
.    listenport - obtained with `PetscSocketEstablish()`

   Output Parameter:
.     t - pass this to read() to read what is passed to this connection

    Level: advanced

.seealso: `PetscSocketEstablish()`
*/
static PetscErrorCode PetscSocketListen(int listenport, int *t)
{
  struct sockaddr_in isa;
#if defined(PETSC_HAVE_ACCEPT_SIZE_T)
  size_t i;
#else
  int i;
#endif

  PetscFunctionBegin;
  /* wait for someone to try to connect */
  i = sizeof(struct sockaddr_in);
  PetscCheck((*t = accept(listenport, (struct sockaddr *)&isa, (socklen_t *)&i)) >= 0, PETSC_COMM_SELF, PETSC_ERR_SYS, "error from accept()");
  PetscFunctionReturn(PETSC_SUCCESS);
}

// "Unknown section 'Environmental Variables'"
// PetscClangLinter pragma disable: -fdoc-section-header-unknown
/*@
  PetscViewerSocketOpen - Opens a connection to a MATLAB or other socket based server.

  Collective

  Input Parameters:
+ comm    - the MPI communicator
. machine - the machine the server is running on, use `NULL` for the local machine, use "server" to passively wait for
             a connection from elsewhere
- port    - the port to connect to, use `PETSC_DEFAULT` for the default

  Output Parameter:
. lab - a context to use when communicating with the server

  Options Database Keys:
   For use with  `PETSC_VIEWER_SOCKET_WORLD`, `PETSC_VIEWER_SOCKET_SELF`,
   `PETSC_VIEWER_SOCKET_()` or if
    `NULL` is passed for machine or PETSC_DEFAULT is passed for port
+ -viewer_socket_machine <machine> - the machine where the socket is available
- -viewer_socket_port <port>       - the socket to connect to

  Environmental variables:
+   `PETSC_VIEWER_SOCKET_MACHINE` - machine name
-   `PETSC_VIEWER_SOCKET_PORT` - portnumber

  Level: intermediate

  Notes:
  Most users should employ the following commands to access the
  MATLAB `PetscViewer`
.vb

    PetscViewerSocketOpen(MPI_Comm comm, char *machine,int port,PetscViewer &viewer)
    MatView(Mat matrix,PetscViewer viewer)
.ve
  or
.vb
    PetscViewerSocketOpen(MPI_Comm comm,char *machine,int port,PetscViewer &viewer)
    VecView(Vec vector,PetscViewer viewer)
.ve

  Currently the only socket client available is MATLAB, PETSc must be configured with --with-matlab for this client. See
  src/dm/tests/ex12.c and ex12.m for an example of usage.

  The socket viewer is in some sense a subclass of the binary viewer, to read and write to the socket
  use `PetscViewerBinaryRead()`, `PetscViewerBinaryWrite()`, `PetscViewerBinarWriteStringArray()`, `PetscViewerBinaryGetDescriptor()`.

  Use this for communicating with an interactive MATLAB session, see `PETSC_VIEWER_MATLAB_()` for writing output to a
  .mat file. Use `PetscMatlabEngineCreate()` or `PETSC_MATLAB_ENGINE_()`, `PETSC_MATLAB_ENGINE_SELF`, or `PETSC_MATLAB_ENGINE_WORLD`
  for communicating with a MATLAB Engine

.seealso: [](sec_viewers), `PETSCVIEWERBINARY`, `PETSCVIEWERSOCKET`, `MatView()`, `VecView()`, `PetscViewerDestroy()`, `PetscViewerCreate()`, `PetscViewerSetType()`,
          `PetscViewerSocketSetConnection()`, `PETSC_VIEWER_SOCKET_`, `PETSC_VIEWER_SOCKET_WORLD`,
          `PETSC_VIEWER_SOCKET_SELF`, `PetscViewerBinaryWrite()`, `PetscViewerBinaryRead()`, `PetscViewerBinaryWriteStringArray()`,
          `PetscBinaryViewerGetDescriptor()`, `PetscMatlabEngineCreate()`
@*/
PetscErrorCode PetscViewerSocketOpen(MPI_Comm comm, const char machine[], int port, PetscViewer *lab)
{
  PetscFunctionBegin;
  PetscCall(PetscViewerCreate(comm, lab));
  PetscCall(PetscViewerSetType(*lab, PETSCVIEWERSOCKET));
  PetscCall(PetscViewerSocketSetConnection(*lab, machine, port));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode PetscViewerSetFromOptions_Socket(PetscViewer v, PetscOptionItems *PetscOptionsObject)
{
  PetscInt  def = -1;
  char      sdef[256];
  PetscBool tflg;

  PetscFunctionBegin;
  /*
       These options are not processed here, they are processed in PetscViewerSocketSetConnection(), they
    are listed here for the GUI to display
  */
  PetscOptionsHeadBegin(PetscOptionsObject, "Socket PetscViewer Options");
  PetscCall(PetscOptionsGetenv(PetscObjectComm((PetscObject)v), "PETSC_VIEWER_SOCKET_PORT", sdef, 16, &tflg));
  if (tflg) {
    PetscCall(PetscOptionsStringToInt(sdef, &def));
  } else def = PETSCSOCKETDEFAULTPORT;
  PetscCall(PetscOptionsInt("-viewer_socket_port", "Port number to use for socket", "PetscViewerSocketSetConnection", def, NULL, NULL));

  PetscCall(PetscOptionsString("-viewer_socket_machine", "Machine to use for socket", "PetscViewerSocketSetConnection", sdef, NULL, sizeof(sdef), NULL));
  PetscCall(PetscOptionsGetenv(PetscObjectComm((PetscObject)v), "PETSC_VIEWER_SOCKET_MACHINE", sdef, sizeof(sdef), &tflg));
  if (!tflg) PetscCall(PetscGetHostName(sdef, sizeof(sdef)));
  PetscOptionsHeadEnd();
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode PetscViewerBinaryGetSkipHeader_Socket(PetscViewer viewer, PetscBool *skip)
{
  PetscViewer_Socket *vsocket = (PetscViewer_Socket *)viewer->data;

  PetscFunctionBegin;
  *skip = vsocket->skipheader;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode PetscViewerBinarySetSkipHeader_Socket(PetscViewer viewer, PetscBool skip)
{
  PetscViewer_Socket *vsocket = (PetscViewer_Socket *)viewer->data;

  PetscFunctionBegin;
  vsocket->skipheader = skip;
  PetscFunctionReturn(PETSC_SUCCESS);
}

PETSC_INTERN PetscErrorCode PetscViewerBinaryGetFlowControl_Binary(PetscViewer, PetscInt *);

/*MC
   PETSCVIEWERSOCKET - A viewer that writes to a Unix socket

  Level: beginner

.seealso: [](sec_viewers), `PETSC_VIEWERBINARY`, `PetscViewerSocketOpen()`, `PetscViewerDrawOpen()`, `PETSC_VIEWER_DRAW_()`, `PETSC_VIEWER_DRAW_SELF`, `PETSC_VIEWER_DRAW_WORLD`,
          `PetscViewerCreate()`, `PetscViewerASCIIOpen()`, `PetscViewerBinaryOpen()`, `PETSCVIEWERBINARY`, `PETSCVIEWERDRAW`,
          `PetscViewerMatlabOpen()`, `VecView()`, `DMView()`, `PetscViewerMatlabPutArray()`, `PETSCVIEWERASCII`, `PETSCVIEWERMATLAB`,
          `PetscViewerFileSetName()`, `PetscViewerFileSetMode()`, `PetscViewerFormat`, `PetscViewerType`, `PetscViewerSetType()`
M*/

PETSC_EXTERN PetscErrorCode PetscViewerCreate_Socket(PetscViewer v)
{
  PetscViewer_Socket *vmatlab;

  PetscFunctionBegin;
  PetscCall(PetscNew(&vmatlab));
  vmatlab->port          = 0;
  vmatlab->flowcontrol   = 256; /* same default as in PetscViewerCreate_Binary() */
  v->data                = (void *)vmatlab;
  v->ops->destroy        = PetscViewerDestroy_Socket;
  v->ops->flush          = NULL;
  v->ops->setfromoptions = PetscViewerSetFromOptions_Socket;

  /* lie and say this is a binary viewer; then all the XXXView_Binary() methods will work correctly on it */
  PetscCall(PetscObjectChangeTypeName((PetscObject)v, PETSCVIEWERBINARY));
  PetscCall(PetscObjectComposeFunction((PetscObject)v, "PetscViewerBinarySetSkipHeader_C", PetscViewerBinarySetSkipHeader_Socket));
  PetscCall(PetscObjectComposeFunction((PetscObject)v, "PetscViewerBinaryGetSkipHeader_C", PetscViewerBinaryGetSkipHeader_Socket));
  PetscCall(PetscObjectComposeFunction((PetscObject)v, "PetscViewerBinaryGetFlowControl_C", PetscViewerBinaryGetFlowControl_Binary));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  PetscViewerSocketSetConnection - Sets the machine and port that a PETSc socket
  viewer is to use

  Logically Collective

  Input Parameters:
+ v       - viewer to connect
. machine - host to connect to, use `NULL` for the local machine,use "server" to passively wait for
             a connection from elsewhere
- port    - the port on the machine one is connecting to, use `PETSC_DEFAULT` for default

  Level: advanced

.seealso: [](sec_viewers), `PETSCVIEWERMATLAB`, `PETSCVIEWERSOCKET`, `PetscViewerSocketOpen()`
@*/
PetscErrorCode PetscViewerSocketSetConnection(PetscViewer v, const char machine[], int port)
{
  PetscMPIInt         rank;
  char                mach[256];
  PetscBool           tflg;
  PetscViewer_Socket *vmatlab;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(v, PETSC_VIEWER_CLASSID, 1);
  if (machine) PetscAssertPointer(machine, 2);
  vmatlab = (PetscViewer_Socket *)v->data;
  /* PetscValidLogicalCollectiveInt(v,port,3); not a PetscInt */
  if (port <= 0) {
    char portn[16];
    PetscCall(PetscOptionsGetenv(PetscObjectComm((PetscObject)v), "PETSC_VIEWER_SOCKET_PORT", portn, 16, &tflg));
    if (tflg) {
      PetscInt pport;
      PetscCall(PetscOptionsStringToInt(portn, &pport));
      port = (int)pport;
    } else port = PETSCSOCKETDEFAULTPORT;
  }
  if (!machine) {
    PetscCall(PetscOptionsGetenv(PetscObjectComm((PetscObject)v), "PETSC_VIEWER_SOCKET_MACHINE", mach, sizeof(mach), &tflg));
    if (!tflg) PetscCall(PetscGetHostName(mach, sizeof(mach)));
  } else {
    PetscCall(PetscStrncpy(mach, machine, sizeof(mach)));
  }

  PetscCallMPI(MPI_Comm_rank(PetscObjectComm((PetscObject)v), &rank));
  if (rank == 0) {
    PetscCall(PetscStrcmp(mach, "server", &tflg));
    if (tflg) {
      int listenport = 0;

      PetscCall(PetscInfo(v, "Waiting for connection from socket process on port %d\n", port));
      PetscCall(PetscSocketEstablish(port, &listenport));
      PetscCall(PetscSocketListen(listenport, &vmatlab->port));
      close(listenport);
    } else {
      PetscCall(PetscInfo(v, "Connecting to socket process on port %d machine %s\n", port, mach));
      PetscCall(PetscOpenSocket(mach, port, &vmatlab->port));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
    The variable Petsc_Viewer_Socket_keyval is used to indicate an MPI attribute that
  is attached to a communicator, in this case the attribute is a PetscViewer.
*/
PetscMPIInt Petsc_Viewer_Socket_keyval = MPI_KEYVAL_INVALID;

/*@C
   PETSC_VIEWER_SOCKET_ - Creates a socket viewer shared by all processors in a communicator.

   Collective

   Input Parameter:
.  comm - the MPI communicator to share the  `PETSCVIEWERSOCKET` `PetscViewer`

   Level: intermediate

   Options Database Keys:
   For use with the default `PETSC_VIEWER_SOCKET_WORLD` or if
   `NULL` is passed for machine or `PETSC_DEFAULT` is passed for port
+  -viewer_socket_machine <machine> - machine to connect to
-  -viewer_socket_port <port> - port to connect to

   Environmental variables:
+  `PETSC_VIEWER_SOCKET_PORT` - portnumber
-  `PETSC_VIEWER_SOCKET_MACHINE` - machine name

   Notes:
   This object is destroyed in `PetscFinalize()`, `PetscViewerDestroy()` should never be called on it

   Unlike almost all other PETSc routines, `PETSC_VIEWER_SOCKET_()` does not return
   an error code, it returns `NULL` if it fails. The  `PETSCVIEWERSOCKET`  `PetscViewer` is usually used in the form `XXXView(XXX object, PETSC_VIEWER_SOCKET_(comm))`

   Currently the only socket client available is MATLAB. See
   src/dm/tests/ex12.c and ex12.m for an example of usage.

   Connects to a waiting socket and stays connected until `PetscViewerDestroy()` is called.

   Use this for communicating with an interactive MATLAB session, see `PETSC_VIEWER_MATLAB_()` for writing output to a
   .mat file. Use `PetscMatlabEngineCreate()` or `PETSC_MATLAB_ENGINE_()`, `PETSC_MATLAB_ENGINE_SELF`, or `PETSC_MATLAB_ENGINE_WORLD`
   for communicating with a MATLAB Engine

.seealso: [](sec_viewers), `PETSCVIEWERMATLAB`, `PETSCVIEWERSOCKET`, `PETSC_VIEWER_SOCKET_WORLD`, `PETSC_VIEWER_SOCKET_SELF`, `PetscViewerSocketOpen()`, `PetscViewerCreate()`,
          `PetscViewerSocketSetConnection()`, `PetscViewerDestroy()`, `PETSC_VIEWER_SOCKET_()`, `PetscViewerBinaryWrite()`, `PetscViewerBinaryRead()`,
          `PetscViewerBinaryWriteStringArray()`, `PetscViewerBinaryGetDescriptor()`, `PETSC_VIEWER_MATLAB_()`
@*/
PetscViewer PETSC_VIEWER_SOCKET_(MPI_Comm comm)
{
  PetscErrorCode ierr;
  PetscMPIInt    mpi_ierr;
  PetscBool      flg;
  PetscViewer    viewer;
  MPI_Comm       ncomm;

  PetscFunctionBegin;
  ierr = PetscCommDuplicate(comm, &ncomm, NULL);
  if (ierr) {
    ierr = PetscError(PETSC_COMM_SELF, __LINE__, "PETSC_VIEWER_SOCKET_", __FILE__, PETSC_ERR_PLIB, PETSC_ERROR_INITIAL, " ");
    PetscFunctionReturn(NULL);
  }
  if (Petsc_Viewer_Socket_keyval == MPI_KEYVAL_INVALID) {
    mpi_ierr = MPI_Comm_create_keyval(MPI_COMM_NULL_COPY_FN, MPI_COMM_NULL_DELETE_FN, &Petsc_Viewer_Socket_keyval, NULL);
    if (mpi_ierr) {
      ierr = PetscError(PETSC_COMM_SELF, __LINE__, "PETSC_VIEWER_SOCKET_", __FILE__, PETSC_ERR_PLIB, PETSC_ERROR_INITIAL, " ");
      PetscFunctionReturn(NULL);
    }
  }
  mpi_ierr = MPI_Comm_get_attr(ncomm, Petsc_Viewer_Socket_keyval, (void **)&viewer, (int *)&flg);
  if (mpi_ierr) {
    ierr = PetscError(PETSC_COMM_SELF, __LINE__, "PETSC_VIEWER_SOCKET_", __FILE__, PETSC_ERR_PLIB, PETSC_ERROR_INITIAL, " ");
    PetscFunctionReturn(NULL);
  }
  if (!flg) { /* PetscViewer not yet created */
    ierr                              = PetscViewerSocketOpen(ncomm, NULL, 0, &viewer);
    ((PetscObject)viewer)->persistent = PETSC_TRUE;
    if (ierr) {
      ierr = PetscError(PETSC_COMM_SELF, __LINE__, "PETSC_VIEWER_SOCKET_", __FILE__, PETSC_ERR_PLIB, PETSC_ERROR_REPEAT, " ");
      PetscFunctionReturn(NULL);
    }
    ierr = PetscObjectRegisterDestroy((PetscObject)viewer);
    if (ierr) {
      ierr = PetscError(PETSC_COMM_SELF, __LINE__, "PETSC_VIEWER_SOCKET_", __FILE__, PETSC_ERR_PLIB, PETSC_ERROR_REPEAT, " ");
      PetscFunctionReturn(NULL);
    }
    mpi_ierr = MPI_Comm_set_attr(ncomm, Petsc_Viewer_Socket_keyval, (void *)viewer);
    if (mpi_ierr) {
      ierr = PetscError(PETSC_COMM_SELF, __LINE__, "PETSC_VIEWER_SOCKET_", __FILE__, PETSC_ERR_PLIB, PETSC_ERROR_INITIAL, " ");
      PetscFunctionReturn(NULL);
    }
  }
  ierr = PetscCommDestroy(&ncomm);
  if (ierr) {
    ierr = PetscError(PETSC_COMM_SELF, __LINE__, "PETSC_VIEWER_SOCKET_", __FILE__, PETSC_ERR_PLIB, PETSC_ERROR_REPEAT, " ");
    PetscFunctionReturn(NULL);
  }
  PetscFunctionReturn(viewer);
}