Oracle8i Supplied Packages Reference
Release 8.1.5
A68001-01

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DBMS_PIPE

The DBMS_PIPE package lets two or more sessions in the same instance communicate. Oracle pipes are similar in concept to the pipes used in UNIX, but Oracle pipes are not implemented using the operating system pipe mechanisms.

Information sent through Oracle pipes is buffered in the system global area (SGA). All information in pipes is lost when the instance is shut down.

Depending upon your security requirements, you may choose to use either a public or a private pipe.

Caution:

Pipes are independent of transactions. Be careful using pipes when transaction control can be affected.  

Public Pipes

You may create a public pipe either implicitly or explicitly. For implicit public pipes, the pipe is automatically created when it is referenced for the first time, and it disappears when it no longer contains data. Because the pipe descriptor is stored in the SGA, there is some space usage overhead until the empty pipe is aged out of the cache.

You create an explicit public pipe by calling the CREATE_PIPE function with the private flag set to FALSE. You must deallocate explicitly-created pipes by calling the REMOVE_PIPE function.

The domain of a public pipe is the schema in which it was created, either explicitly or implicitly.

Writing and Reading Pipes

Each public pipe works asynchronously. Any number of schema users can write to a public pipe, as long as they have EXECUTE permission on the DBMS_PIPE package, and they know the name of the public pipe. However, once buffered information is read by one user, it is emptied from the buffer, and is not available for other readers of the same pipe.

The sending session builds a message using one or more calls to the PACK_MESSAGE procedure. This procedure adds the message to the session's local message buffer. The information in this buffer is sent by calling the SEND_MESSAGE function, designating the pipe name to be used to send the message. When SEND_MESSAGE is called, all messages that have been stacked in the local buffer are sent.

A process that wants to receive a message calls the RECEIVE_MESSAGE function, designating the pipe name from which to receive the message. The process then calls the UNPACK_MESSAGE procedure to access each of the items in the message.

Private Pipes

You explicitly create a private pipe by calling the CREATE_PIPE function. Once created, the private pipe persists in shared memory until you explicitly deallocate it by calling the REMOVE_PIPE function. A private pipe is also deallocated when the database instance is shut down.

You cannot create a private pipe if an implicit pipe exists in memory and has the same name as the private pipe you are trying to create. In this case, CREATE_PIPE returns an error.

Access to a private pipe is restricted to:

An attempt by any other user to send or receive messages on the pipe, or to remove the pipe, results in an immediate error. Any attempt by another user to create a pipe with the same name also causes an error.

As with public pipes, you must first build your message using calls to PACK_MESSAGE before calling SEND_MESSAGE. Similarly, you must call RECEIVE_MESSAGE to retrieve the message before accessing the items in the message by calling UNPACK_MESSAGE.

Pipe Uses

The pipe functionality has several potential applications:

Security

Security can be achieved by use of 'grant execute' on the DBMS_PIPE package by creating a pipe using the private parameter in the CREATE_PIPE function and by writing cover packages that only expose particular features or pipenames to particular users or roles.

Constants

maxwait   constant integer := 86400000; /* 1000 days */

This is the maximum time to wait attempting to send or receive a message.

Errors

DBMS_PIPE package subprograms can return the following errors:

Table 28-1 DBMS_PIPE Errors
Error  Description  
ORA-23321:
 

Pipename may not be null. This can be returned by the CREATE_PIPE function, or any subprogram that takes a pipe name as a parameter.  

 
ORA-23322:
 

Insufficient privilege to access pipe. This can be returned by any subprogram that references a private pipe in its parameter list.  

Summary of Subprograms

Table 28-2 DBMS_PIPE Package Subprograms
Subprogram  Description  
CREATE_PIPE function
 

Explicitly creates a pipe (necessary for private pipes).  

 
PACK_MESSAGE procedure
 

Builds message in local buffer.  

 
SEND_MESSAGE function
 

Sends message on named pipe: This implicitly creates a public pipe if the named pipe does not exist.  

 
RECEIVE_MESSAGE function
 

Copies message from named pipe into local buffer.  

 
NEXT_ITEM_TYPE function
 

Returns datatype of next item in buffer.  

 
UNPACK_MESSAGE procedure
 

Accesses next item in buffer.  

 
REMOVE_PIPE function
 

Removes the named pipe.  

 
PURGE procedure
 

Purges contents of named pipe.  

 
RESET_BUFFER procedure
 

Purges contents of local buffer.  

 
UNIQUE_SESSION_NAME function
 

Returns unique session name.  

CREATE_PIPE function

This function explicitly creates a public or private pipe. If the private flag is TRUE, then the pipe creator is assigned as the owner of the private pipe.

Explicitly-created pipes can only be removed by calling REMOVE_PIPE, or by shutting down the instance.

Syntax

DBMS_PIPE.CREATE_PIPE (
   pipename    IN VARCHAR2,
   maxpipesize IN INTEGER DEFAULT 8192,
   private     IN BOOLEAN DEFAULT TRUE)
  RETURN INTEGER;

Pragmas 

pragma restrict_references(create_pipe,WNDS,RNDS);

Parameters

Table 28-3 CREATE_PIPE Function Parameters
Parameter  Description  
pipename
 

Name of the pipe you are creating.

You must use this name when you call SEND_MESSAGE and RECEIVE_MESSAGE. This name must be unique across the instance.

Caution: Do not use pipe names beginning with ORA$. These are reserved for use by procedures provided by Oracle Corporation. Pipename should not be longer than 128 bytes, and is case_insensitive. At this time, the name cannot contain NLS characters.  

 
maxpipesize
 

The maximum size allowed for the pipe, in bytes.

The total size of all of the messages on the pipe cannot exceed this amount. The message is blocked if it exceeds this maximum. The default maxpipesize is 8192 bytes.

The maxpipesize for a pipe becomes a part of the characteristics of the pipe and persists for the life of the pipe. Callers of SEND_MESSAGE with larger values cause the maxpipesize to be increased. Callers with a smaller value use the existing, larger value.  

 
private
 

Uses the default, TRUE, to create a private pipe.

Public pipes can be implicitly created when you call SEND_MESSAGE.  

Returns

Table 28-4 CREATE_PIPE Function Returns
Return  Description  
0
 

Successful.

If the pipe already exists and the user attempting to create it is authorized to use it, then Oracle returns 0, indicating success, and any data already in the pipe remains.

If a user connected as SYSDBA/SYSOPER re-creates a pipe, then Oracle returns status 0, but the ownership of the pipe remains unchanged.  

 
ORA-23322
 

Failure due to naming conflict.

If a pipe with the same name exists and was created by a different user, then Oracle signals error ORA-23322, indicating the naming conflict.  

Exceptions

Table 28-5 CREATE_PIPE Function Exception
Exception  Description 

Null pipe name  

Permission error: Pipe with the same name already exists, and you are not allowed to use it.  

PACK_MESSAGE procedure

This procedure builds your message in the local message buffer.

To send a message, first make one or more calls to PACK_MESSAGE. Then, call SEND_MESSAGE to send the message in the local buffer on the named pipe.

The PACK_MESSAGE procedure is overloaded to accept items of type VARCHAR2, NUMBER, or DATE. In addition to the data bytes, each item in the buffer requires one byte to indicate its type, and two bytes to store its length. One additional byte is needed to terminate the message.The overhead for all types other than VARCHAR is 4 bytes.

In Oracle8, the char-set-id (2 bytes) and the char-set-form (1 byte) are stored with each data item. Therefore, the overhead when using Oracle8 is 7 bytes.

When you call SEND_MESSAGE to send this message, you must indicate the name of the pipe on which you want to send the message. If this pipe already exists, then you must have sufficient privileges to access this pipe. If the pipe does not already exist, then it is created automatically.

Syntax

DBMS_PIPE.PACK_MESSAGE       (item  IN  VARCHAR2);
DBMS_PIPE.PACK_MESSAGE       (item  IN  NCHAR);
DBMS_PIPE.PACK_MESSAGE       (item  IN  NUMBER);
DBMS_PIPE.PACK_MESSAGE       (item  IN  DATE);
DBMS_PIPE.PACK_MESSAGE_RAW   (item  IN  RAW);
DBMS_PIPE.PACK_MESSAGE_ROWID (item  IN  ROWID);

Note:

The PACK_MESSAGE procedure is overloaded to accept items of type VARCHAR2, NCHAR, NUMBER, or DATE. There are two additional procedures to pack RAW and ROWID items.  

Pragmas

pragma restrict_references(pack_message,WNDS,RNDS);
pragma restrict_references(pack_message_raw,WNDS,RNDS);  
pragma restrict_references(pack_message_rowid,WNDS,RNDS);

Parameters

Table 28-6 PACK_MESSAGE Procedure Parameters
Parameter  Description  
item
 

Item to pack into the local message buffer.  

Exceptions

ORA-06558 is raised if the message buffer overflows (currently 4096 bytes). Each item in the buffer takes one byte for the type, two bytes for the length, plus the actual data. There is also one byte needed to terminate the message.

SEND_MESSAGE function

This function sends a message on the named pipe.

The message is contained in the local message buffer, which was filled with calls to PACK_MESSAGE. A pipe could be explicitly using CREATE_PIPE; otherwise, it is created implicitly.

Syntax

DBMS_PIPE.SEND_MESSAGE (
    pipename     IN VARCHAR2,
    timeout      IN INTEGER DEFAULT MAXWAIT,
    maxpipesize  IN INTEGER DEFAULT 8192)
  RETURN INTEGER;

Pragmas

pragma restrict_references(send_message,WNDS,RNDS);

Parameters

Table 28-7 SEND_MESSAGE Function Parameters
Parameter  Description  
pipename
 

Name of the pipe on which you want to place the message.

If you are using an explicit pipe, then this is the name that you specified when you called CREATE_PIPE.

Caution: Do not use pipe names beginning with 'ORA$'. These names are reserved for use by procedures provided by Oracle Corporation. Pipename should not be longer than 128 bytes, and is case-insensitive. At this time, the name cannot contain NLS characters.  

 
timeout
 

Time to wait while attempting to place a message on a pipe, in seconds.

The default value is the constant MAXWAIT, which is defined as 86400000 (1000 days).  

 
maxpipesize
 

Maximum size allowed for the pipe, in bytes.

The total size of all the messages on the pipe cannot exceed this amount. The message is blocked if it exceeds this maximum. The default is 8192 bytes.

The maxpipesize for a pipe becomes a part of the characteristics of the pipe and persists for the life of the pipe. Callers of SEND_MESSAGE with larger values cause the maxpipesize to be increased. Callers with a smaller value simply use the existing, larger value.

Specifying maxpipesize as part of the SEND_MESSAGE procedure eliminates the need for a separate call to open the pipe. If you created the pipe explicitly, then you can use the optional maxpipesize parameter to override the creation pipe size specifications.  

Returns

Table 28-8 SEND_MESSAGE Function Returns
Return  Description  
0
 

Success.

If the pipe already exists and the user attempting to create it is authorized to use it, then Oracle returns 0, indicating success, and any data already in the pipe remains.

If a user connected as SYSDBS/SYSOPER re-creates a pipe, then Oracle returns status 0, but the ownership of the pipe remains unchanged.  

 
1
 

Timed out.

This procedure can timeout either because it cannot get a lock on the pipe, or because the pipe remains too full to be used. If the pipe was implicitly-created and is empty, then it is removed.  

 
3
 

An interrupt occurred.

If the pipe was implicitly created and is empty, then it is removed.  

 
ORA-23322
 

Insufficient privileges.

If a pipe with the same name exists and was created by a different user, then Oracle signals error ORA-23322, indicating the naming conflict.  

Exceptions

Table 28-9 SEND_MESSAGE Function Exception
Exception  Description 

Null pipe name  

Permission error. Insufficient privilege to write to the pipe. The pipe is private and owned by someone else.  

RECEIVE_MESSAGE function

This function copies the message into the local message buffer.

To receive a message from a pipe, first call RECEIVE_MESSAGE. When you receive a message, it is removed from the pipe; hence, a message can only be received once. For implicitly-created pipes, the pipe is removed after the last record is removed from the pipe.

If the pipe that you specify when you call RECEIVE_MESSAGE does not already exist, then Oracle implicitly creates the pipe and waits to receive the message. If the message does not arrive within a designated timeout interval, then the call returns and the pipe is removed.

After receiving the message, you must make one or more calls to UNPACK_MESSAGE to access the individual items in the message. The UNPACK_MESSAGE procedure is overloaded to unpack items of type DATE, NUMBER, VARCHAR2, and there are two additional procedures to unpack RAW and ROWID items. If you do not know the type of data that you are attempting to unpack, then call NEXT_ITEM_TYPE to determine the type of the next item in the buffer.

Syntax

DBMS_PIPE.RECEIVE_MESSAGE (
   pipename     IN VARCHAR2,
   timeout      IN INTEGER      DEFAULT maxwait)
  RETURN INTEGER;

Pragmas

pragma restrict_references(receive_message,WNDS,RNDS);

Parameters

Table 28-10 RECEIVE_MESSAGE Function Parameters
Parameter  Description  
pipename
 

Name of the pipe on which you want to receive a message.

Names beginning with ORA$ are reserved for use by Oracle  

 
timeout
 

Time to wait for a message, in seconds.

The default value is the constant MAXWAIT, which is defined as 86400000 (1000 days). A timeout of 0 allows you to read without blocking.  

Returns

Table 28-11 RECEIVE_MESSAGE Function Returns
Return  Description  
0
 

Success  

 
1
 

Timed out. If the pipe was implicitly-created and is empty, then it is removed.  

 
2
 

Record in the pipe is too large for the buffer. (This should not happen.)  

 
3
 

An interrupt occurred.  

 
ORA-23322
 

User has insufficient privileges to read from the pipe.  

Exceptions

Table 28-12 RECEIVE_MESSAGE Function Exceptions
Exception  Description 

Null pipe name  

Permission error. Insufficient privilege to remove the record from the pipe. The pipe is owned by someone else.  

NEXT_ITEM_TYPE function

This function determines the datatype of the next item in the local message buffer.

After you have called RECEIVE_MESSAGE to place pipe information in a local buffer, call NEXT_ITEM_TYPE.

Syntax

DBMS_PIPE.NEXT_ITEM_TYPE 
  RETURN INTEGER;

Pragmas

pragma restrict_references(next_item_type,WNDS,RNDS);

Returns

Table 28-13 NEXT_ITEM_TYPE Function Returns
Return  Description  
0
 

No more items  

 
6
 		 
NUMBER
 		 
9
 		 
VARCHAR2
 		 
11
 		 
ROWID
 		 
12
 		 
DATE
 		 
23
 		 
RAW
 

UNPACK_MESSAGE procedure

This procedure retrieves items from the buffer.

After you have called RECEIVE_MESSAGE to place pipe information in a local buffer, call UNPACK_MESSAGE.

Syntax

DBMS_PIPE.UNPACK_MESSAGE       (item  OUT VARCHAR2);
DBMS_PIPE.UNPACK_MESSAGE       (item  OUT NCHAR);
DBMS_PIPE.UNPACK_MESSAGE       (item  OUT NUMBER);
DBMS_PIPE.UNPACK_MESSAGE       (item  OUT DATE);
DBMS_PIPE.UNPACK_MESSAGE_RAW   (item  OUT RAW);
DBMS_PIPE.UNPACK_MESSAGE_ROWID (item  OUT ROWID);

Note:

The UNPACK_MESSAGE procedure is overloaded to return items of type VARCHAR2, NCHAR, NUMBER, or DATE. There are two additional procedures to unpack RAW and ROWID items.  

Pragmas

pragma restrict_references(unpack_message,WNDS,RNDS);  
pragma restrict_references(unpack_message_raw,WNDS,RNDS);  
pragma restrict_references(unpack_message_rowid,WNDS,RNDS);

Parameters

Table 28-14 UNPACK_MESSAGE Procedure Parameters
Parameter  Description  
item
 

Argument to receive the next unpacked item from the local message buffer.  

Exceptions

ORA-06556 or 06559 are generated if the buffer contains no more items, or if the item is not of the same type as that requested.

REMOVE_PIPE function

This function removes explicitly-created pipes.

Pipes created implicitly by SEND_MESSAGE are automatically removed when empty. However, pipes created explicitly by CREATE_PIPE are removed only by calling REMOVE_PIPE, or by shutting down the instance. All unconsumed records in the pipe are removed before the pipe is deleted.

This is similar to calling PURGE on an implicitly-created pipe.

Syntax

DBMS_PIPE.REMOVE_PIPE (
   pipename  IN  VARCHAR2)
  RETURN INTEGER;

Pragmas

pragma restrict_references(remove_pipe,WNDS,RNDS);

Parameters

Table 28-15 REMOVE_PIPE Function Parameters
Parameter  Description  
pipename
 

Name of pipe that you want to remove.  

Returns

Table 28-16 REMOVE_PIPE Function Returns
Return  Description  
0
 

Success

If the pipe does not exist, or if the pipe already exists and the user attempting to remove it is authorized to do so, then Oracle returns 0, indicating success, and any data remaining in the pipe is removed.  

 
ORA-23322
 

Insufficient privileges.

If the pipe exists, but the user is not authorized to access the pipe, then Oracle signals error ORA-23322, indicating insufficient privileges.  

Exceptions

Table 28-17 REMOVE_PIPE Function Exception
Exception  Description 

Null pipe name  

Permission error: Insufficient privilege to remove pipe. The pipe was created and is owned by someone else.  

PURGE procedure

This procedure empties the contents of the named pipe.

An empty implicitly-created pipe is aged out of the shared global area according to the least-recently-used algorithm. Thus, calling PURGE lets you free the memory associated with an implicitly-created pipe.

Because PURGE calls RECEIVE_MESSAGE, the local buffer might be overwritten with messages as they are purged from the pipe. Also, you can receive an ORA-23322 (insufficient privileges) error if you attempt to purge a pipe with which you have insufficient access rights.

Syntax

DBMS_PIPE.PURGE (
   pipename  IN  VARCHAR2);

Pragmas

pragma restrict_references(purge,WNDS,RNDS);

Parameters

Table 28-18 Purge Procedure Parameters
Parameter  Description  
pipename
 

Name of pipe from which to remove all messages.

The local buffer may be overwritten with messages as they are discarded. Pipename should not be longer than 128 bytes, and is case-insensitive.  

Exceptions

Permission error if pipe belongs to another user.

RESET_BUFFER procedure

This procedure resets the PACK_MESSAGE and UNPACK_MESSAGE positioning indicators to 0.

Because all pipes share a single buffer, you may find it useful to reset the buffer before using a new pipe. This ensures that the first time you attempt to send a message to your pipe, you do not inadvertently send an expired message remaining in the buffer.

Syntax 

DBMS_PIPE.RESET_BUFFER;

Parameters

None.

Pragmas

pragma restrict_references(reset_buffer,WNDS,RNDS);

UNIQUE_SESSION_NAME function

This function receives a name that is unique among all of the sessions that are currently connected to a database.

Multiple calls to this function from the same session always return the same value. You might find it useful to use this function to supply the PIPENAME parameter for your SEND_MESSAGE and RECEIVE_MESSAGE calls.

Syntax

DBMS_PIPE.UNIQUE_SESSION_NAME 
  RETURN VARCHAR2;

Parameters

None.

Pragmas

pragma restrict_references(unique_session_name,WNDS,RNDS,WNPS);

Returns

This function returns a unique name. The returned name can be up to 30 bytes.

Examples

Example 1: Debugging

This example shows a procedure a PL/SQL program can call to place debugging information in a pipe. 

CREATE OR REPLACE PROCEDURE debug (msg VARCHAR2) AS
    status  NUMBER;
BEGIN
  DBMS_PIPE.PACK_MESSAGE(LENGTH(msg));
  DBMS_PIPE.PACK_MESSAGE(msg);
  status := DBMS_PIPE.SEND_MESSAGE('plsql_debug');
  IF status != 0 THEN
    raise_application_error(-20099, 'Debug error');
  END IF;
END debug;

This example shows the Pro*C code that receives messages from the PLSQL_DEBUG pipe in the PL/SQL example above and displays the messages. If the Pro*C session is run in a separate window, then it can be used to display any messages that are sent to the debug procedure from a PL/SQL program executing in a separate session. 

#include <stdio.h> 
#include <string.h> 
 
EXEC SQL BEGIN DECLARE SECTION; 
   VARCHAR username[20]; 
   int     status; 
   int     msg_length;
   char    retval[2000];
EXEC SQL END DECLARE SECTION; 
 
EXEC SQL INCLUDE SQLCA; 
 
void sql_error(); 
 
main() 
{ 

-- Prepare username:
   strcpy(username.arr, "SCOTT/TIGER"); 
   username.len = strlen(username.arr); 
 
   EXEC SQL WHENEVER SQLERROR DO sql_error(); 
   EXEC SQL CONNECT :username; 
 
   printf("connected\n"); 
 
-- Start an endless loop to look for and print messages on the pipe:
   FOR (;;) 
   { 
      EXEC SQL EXECUTE 
         DECLARE 
            len INTEGER;
            typ INTEGER;
            sta INTEGER; 
            chr VARCHAR2(2000); 
         BEGIN 
            chr := ''; 
            sta := dbms_pipe.receive_message('plsql_debug'); 
            IF sta = 0 THEN 
               DBMS_PIPE.UNPACK_MESSAGE(len);
               DBMS_PIPE.UNPACK_MESSAGE(chr); 
            END IF; 
            :status := sta; 
            :retval := chr; 
            IF len IS NOT NULL THEN
               :msg_length := len;
            ELSE
               :msg_length := 2000;
            END IF;
         END; 
      END-EXEC; 
      IF (status == 0) 
         printf("\n%.*s\n", msg_length, retval);
      ELSE 
         printf("abnormal status, value is %d\n", status); 
   }
}

void sql_error() 
{ 
   char msg[1024]; 
   int rlen, len; 
   len = sizeof(msg); 
   sqlglm(msg, &len, &rlen); 
   printf("ORACLE ERROR\n"); 
   printf("%.*s\n", rlen, msg); 
   exit(1); 
}

Example 2: Execute System Commands

This example shows PL/SQL and Pro*C code let a PL/SQL stored procedure (or anonymous block) call PL/SQL procedures to send commands over a pipe to a Pro*C program that is listening for them.

The Pro*C program sleeps and waits for a message to arrive on the named pipe. When a message arrives, the C program processes it, carrying out the required action, such as executing a UNIX command through the system() call or executing a SQL command using embedded SQL.

DAEMON.SQL is the source code for the PL/SQL package. This package contains procedures that use the DBMS_PIPE package to send and receive message to and from the Pro*C daemon. Note that full handshaking is used. The daemon always sends a message back to the package (except in the case of the STOP command). This is valuable, because it allows the PL/SQL procedures to be sure that the Pro*C daemon is running.

You can call the DAEMON packaged procedures from an anonymous PL/SQL block using SQL*Plus or Enterprise Manager. For example: 

SVRMGR> variable rv number
SVRMGR> execute :rv := DAEMON.EXECUTE_SYSTEM('ls -la');

This would, on a UNIX system, cause the Pro*C daemon to execute the command system("ls -la").

Remember that the daemon needs to be running first. You might want to run it in the background, or in another window beside the SQL*Plus or Enterprise Manager session from which you call it.

The DAEMON.SQL also uses the DBMS_OUTPUT package to display the results. For this example to work, you must have execute privileges on this package.

DAEMON.SQL example

This is the code for the PL/SQL DAEMON package: 

CREATE OR REPLACE PACKAGE daemon AS
  FUNCTION execute_sql(command VARCHAR2, 
                       timeout NUMBER DEFAULT 10)
    RETURN NUMBER;

  FUNCTION execute_system(command VARCHAR2,
                          timeout NUMBER DEFAULT 10)
    RETURN NUMBER;

  PROCEDURE stop(timeout NUMBER DEFAULT 10);
END daemon;
/
CREATE OR REPLACE PACKAGE BODY daemon AS

  FUNCTION execute_system(command VARCHAR2,
                          timeout NUMBER DEFAULT 10)
  RETURN NUMBER IS

    status       NUMBER;
    result       VARCHAR2(20);
    command_code NUMBER;
    pipe_name    VARCHAR2(30);
  BEGIN
    pipe_name := DBMS_PIPE.UNIQUE_SESSION_NAME;

    DBMS_PIPE.PACK_MESSAGE('SYSTEM');
    DBMS_PIPE.PACK_MESSAGE(pipe_name);
    DBMS_PIPE.PACK_MESSAGE(command);
    status := DBMS_PIPE.SEND_MESSAGE('daemon', timeout);
    IF status <> 0 THEN
      RAISE_APPLICATION_ERROR(-20010,
        'Execute_system: Error while sending.  Status = ' ||
         status);
    END IF;

    status := DBMS_PIPE.RECEIVE_MESSAGE(pipe_name, timeout);
    IF status <> 0 THEN
      RAISE_APPLICATION_ERROR(-20011,
        'Execute_system: Error while receiving. 
         Status = ' || status);
    END IF;

    DBMS_PIPE.UNPACK_MESSAGE(result);
    IF result <> 'done' THEN
      RAISE_APPLICATION_ERROR(-20012,
        'Execute_system: Done not received.');
    END IF;

    DBMS_PIPE.UNPACK_MESSAGE(command_code);
    DBMS_OUTPUT.PUT_LINE('System command executed.  result = ' ||
                         command_code);
    RETURN command_code;
  END execute_system;

  FUNCTION execute_sql(command VARCHAR2,
                       timeout NUMBER DEFAULT 10)
  RETURN NUMBER IS

    status       NUMBER;
    result       VARCHAR2(20);
    command_code NUMBER;
    pipe_name    VARCHAR2(30);

  BEGIN
    pipe_name := DBMS_PIPE.UNIQUE_SESSION_NAME;

    DBMS_PIPE.PACK_MESSAGE('SQL');
    DBMS_PIPE.PACK_MESSAGE(pipe_name);
    DBMS_PIPE.PACK_MESSAGE(command);
    status := DBMS_PIPE.SEND_MESSAGE('daemon', timeout);
    IF status <> 0 THEN
      RAISE_APPLICATION_ERROR(-20020,
        'Execute_sql: Error while sending.  Status = ' || status);
    END IF;

    status := DBMS_PIPE.RECEIVE_MESSAGE(pipe_name, timeout);

    IF status <> 0 THEN
      RAISE_APPLICATION_ERROR(-20021,
        'execute_sql: Error while receiving.  
         Status = ' || status);
    END IF;

    DBMS_PIPE.UNPACK_MESSAGE(result);
    IF result <> 'done' THEN
      RAISE_APPLICATION_ERROR(-20022,
        'execute_sql: done not received.');
    END IF;

    DBMS_PIPE.UNPACK_MESSAGE(command_code);
    DBMS_OUTPUT.PUT_LINE
        ('SQL command executed.  sqlcode = ' || command_code);
    RETURN command_code;
  END execute_sql;

  PROCEDURE stop(timeout NUMBER DEFAULT 10) IS
    status NUMBER;
  BEGIN
    DBMS_PIPE.PACK_MESSAGE('STOP');
    status := DBMS_PIPE.SEND_MESSAGE('daemon', timeout);
    IF status <> 0 THEN
      RAISE_APPLICATION_ERROR(-20030,
        'stop: error while sending.  status = ' || status);
    END IF;
  END stop;
END daemon;
daemon.pc example

This is the code for the Pro*C daemon. You must precompile this using the Pro*C Precompiler, Version 1.5.x or later. You must also specify the USERID and SQLCHECK options, as the example contains embedded PL/SQL code.

For example: 

proc iname=daemon userid=scott/tiger sqlcheck=semantics

Then C-compile and link in the normal way. 

#include <stdio.h>
#include <string.h>

EXEC SQL INCLUDE SQLCA;

EXEC SQL BEGIN DECLARE SECTION;
  char *uid = "scott/tiger";
  int status;
  VARCHAR command[20];
  VARCHAR value[2000];
  VARCHAR return_name[30];
EXEC SQL END DECLARE SECTION;

void
connect_error()
{
  char msg_buffer[512];
  int msg_length;
  int buffer_size = 512;

  EXEC SQL WHENEVER SQLERROR CONTINUE;
  sqlglm(msg_buffer, &buffer_size, &msg_length);
  printf("Daemon error while connecting:\n");
  printf("%.*s\n", msg_length, msg_buffer);
  printf("Daemon quitting.\n");
  exit(1);
}

void
sql_error()
{
  char msg_buffer[512];
  int msg_length;
  int buffer_size = 512;

  EXEC SQL WHENEVER SQLERROR CONTINUE;
  sqlglm(msg_buffer, &buffer_size, &msg_length);
  printf("Daemon error while executing:\n");
  printf("%.*s\n", msg_length, msg_buffer);
  printf("Daemon continuing.\n");
}
main()
{
  EXEC SQL WHENEVER SQLERROR DO connect_error();
  EXEC SQL CONNECT :uid;
  printf("Daemon connected.\n");

  EXEC SQL WHENEVER SQLERROR DO sql_error();
  printf("Daemon waiting...\n");
  while (1) {
    EXEC SQL EXECUTE
      BEGIN
        :status := DBMS_PIPE.RECEIVE_MESSAGE('daemon');
        IF :status = 0 THEN
          DBMS_PIPE.UNPACK_MESSAGE(:command);
        END IF;
      END;
    END-EXEC;
    IF (status == 0)
    {
      command.arr[command.len] = '\0';
      IF (!strcmp((char *) command.arr, "STOP"))
      {
        printf("Daemon exiting.\n");
        break;
      }

      ELSE IF (!strcmp((char *) command.arr, "SYSTEM"))
      {
        EXEC SQL EXECUTE
          BEGIN
            DBMS_PIPE.UNPACK_MESSAGE(:return_name);
            DBMS_PIPE.UNPACK_MESSAGE(:value);
          END;
        END-EXEC;
        value.arr[value.len] = '\0';
        printf("Will execute system command '%s'\n", value.arr);

        status = system(value.arr);
        EXEC SQL EXECUTE
          BEGIN
            DBMS_PIPE.PACK_MESSAGE('done');
            DBMS_PIPE.PACK_MESSAGE(:status);
            :status := DBMS_PIPE.SEND_MESSAGE(:return_name);
          END;
        END-EXEC;

        IF (status)
        {
          printf
           ("Daemon error while responding to system command.");
          printf("  status: %d\n", status);
        }
      }
      ELSE IF (!strcmp((char *) command.arr, "SQL")) {
        EXEC SQL EXECUTE
          BEGIN
            DBMS_PIPE.UNPACK_MESSAGE(:return_name);
            DBMS_PIPE.UNPACK_MESSAGE(:value);
          END;
        END-EXEC;
        value.arr[value.len] = '\0';
        printf("Will execute sql command '%s'\n", value.arr);

        EXEC SQL WHENEVER SQLERROR CONTINUE;
        EXEC SQL EXECUTE IMMEDIATE :value;
        status = sqlca.sqlcode;

        EXEC SQL WHENEVER SQLERROR DO sql_error();
        EXEC SQL EXECUTE
          BEGIN
            DBMS_PIPE.PACK_MESSAGE('done');
            DBMS_PIPE.PACK_MESSAGE(:status);
            :status := DBMS_PIPE.SEND_MESSAGE(:return_name);
          END;
        END-EXEC;

        IF (status)
        {
          printf("Daemon error while responding to sql command.");
          printf("  status: %d\n", status);
        }
      }
      ELSE
      {
        printf
          ("Daemon error: invalid command '%s' received.\n",
            command.arr);
      }
    }
    ELSE
    {
      printf("Daemon error while waiting for signal.");
      printf("  status = %d\n", status);
    }
  }
  EXEC SQL COMMIT WORK RELEASE;
  exit(0);

Example 3: External Service Interface

Put the user-written 3GL code into an OCI or Precompiler program. The program connects to the database and executes PL/SQL code to read its request from the pipe, computes the result, and then executes PL/SQL code to send the result on a pipe back to the requestor.

Below is an example of a stock service request. The recommended sequence for the arguments to pass on the pipe for all service requests is: 

      protocol_version      VARCHAR2        - '1', 10 bytes or less
      returnpipe            VARCHAR2        - 30 bytes or less
      service               VARCHAR2        - 30 bytes or less
      arg1                  VARCHAR2/NUMBER/DATE
         ...
      argn                  VARCHAR2/NUMBER/DATE

The recommended format for returning the result is: 

      success               VARCHAR2        - 'SUCCESS' if OK,
                                              otherwise error message
      arg1                  VARCHAR2/NUMBER/DATE
         ...
      argn                  VARCHAR2/NUMBER/DATE

The "stock price request server" would do, using OCI or PRO* (in pseudo-code): 

    <loop forever>
      BEGIN dbms_stock_server.get_request(:stocksymbol); END;
      <figure out price based on stocksymbol (probably from some radio
            signal), set error if can't find such a stock>
      BEGIN dbms_stock_server.return_price(:error, :price); END;

A client would do: 

    BEGIN :price := stock_request('YOURCOMPANY'); end;

The stored procedure, dbms_stock_server, which is called by the "stock price request server" above is: 

    CREATE OR REPLACE PACKAGE dbms_stock_server IS
      PROCEDURE get_request(symbol OUT VARCHAR2);
      PROCEDURE return_price(errormsg IN VARCHAR2, price IN VARCHAR2);
    END;
  
    CREATE OR REPLACE PACKAGE BODY dbms_stock_server IS
      returnpipe    VARCHAR2(30);
  
      PROCEDURE returnerror(reason VARCHAR2) IS
        s INTEGER;
      BEGIN
        dbms_pipe.pack_message(reason);
        s := dbms_pipe.send_message(returnpipe);
        IF s <> 0 THEN
          raise_application_error(-20000, 'Error:' || to_char(s) ||
            ' sending on pipe');
        END IF;
      END;
  
      PROCEDURE get_request(symbol OUT VARCHAR2) IS
        protocol_version VARCHAR2(10);
        s                INTEGER;
        service          VARCHAR2(30);
      BEGIN
        s := dbms_pipe.receive_message('stock_service');
        IF s <> 0 THEN
          raise_application_error(-20000, 'Error:' || to_char(s) ||
            'reading pipe');
        END IF;
        dbms_pipe.unpack_message(protocol_version);
        IF protocol_version <> '1' THEN
          raise_application_error(-20000, 'Bad protocol: ' || 
            protocol_version);
        END IF;
        dbms_pipe.unpack_message(returnpipe);
        dbms_pipe.unpack_message(service);
        IF service != 'getprice' THEN
          returnerror('Service ' || service || ' not supported');
        END IF;
        dbms_pipe.unpack_message(symbol);
      END;
  
      PROCEDURE return_price(errormsg in VARCHAR2, price in VARCHAR2) IS
        s INTEGER;
      BEGIN
        IF errormsg is NULL THEN
          dbms_pipe.pack_message('SUCCESS');
          dbms_pipe.pack_message(price);
        ELSE
          dbms_pipe.pack_message(errormsg);
        END IF;
        s := dbms_pipe.send_message(returnpipe);
        IF s <> 0 THEN
          raise_application_error(-20000, 'Error:'||to_char(s)||
            ' sending on pipe');
        END IF;
      END;
    END;

The procedure called by the client is: 

    CREATE OR REPLACE FUNCTION stock_request (symbol VARCHAR2) 
        RETURN VARCHAR2 IS
      s        INTEGER;
      price    VARCHAR2(20);
      errormsg VARCHAR2(512);
    BEGIN
      dbms_pipe.pack_message('1');  -- protocol version
      dbms_pipe.pack_message(dbms_pipe.unique_session_name); -- return pipe
      dbms_pipe.pack_message('getprice');
      dbms_pipe.pack_message(symbol);
      s := dbms_pipe.send_message('stock_service');
      IF s <> 0 THEN
        raise_application_error(-20000, 'Error:'||to_char(s)||
          ' sending on pipe');
      END IF;
      s := dbms_pipe.receive_message(dbms_pipe.unique_session_name);
      IF s <> 0 THEN
        raise_application_error(-20000, 'Error:'||to_char(s)||
          ' receiving on pipe');
      END IF;
      dbms_pipe.unpack_message(errormsg);
      IF errormsg <> 'SUCCESS' THEN
        raise_application_error(-20000, errormsg);
      END IF;
      dbms_pipe.unpack_message(price);
      RETURN price;
    END;

You would typically only grant execute on dbms_stock_service to the stock service application server, and would only grant execute on stock_request to those users allowed to use the service.

See Also:

Chapter 2, "DBMS_ALERT"  


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