How to Execute Programs Stored in a Database

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How to Execute Programs Stored in a Database

Introduction

Empress RDBMS can store any kind of information inside its database. This includes not only all types of data one could imagine, but also a programming logic which can drive, for example, knowledge-based or rule-based systems.

Empress database can serve as a repository for programs, shell scripts, executables of all kinds. Then, those executables can be invoked via Empress PSM technology.

This technical note explains the basic steps one needs to do in order to acquire this functionality. In future Empress versions (V8.60+), invoking an executable stored in the database will be an automatic process. Hence, a built-in functionality will be provided.

Example

In this technical note we present an example of how to store a simple "Hello World" C program in the database and execute it from an SQL interface.

At the end of this exercise we would be able to do the following:


SELECT executeBLOB(executable_name, executable_object)
   FROM execRepositoryTable
   WHERE where_condition

Hello World!

where module_executeBLOB is Empress PSM function which reads the content of the executable_object (e.g. "Hello World" executable) in table execRepositoryTable and executes it.


# A simple "Hello World" C program
#include <stdio.h>
int main()
{
        printf("Hello World!\n");
        return(0);
}

The executable hello could be created in the following way:


cc -o hello hello.c

Let us create a database execRepositoryDB which will serve as a repository for different kind of executables/shell scripts.


empmkdb execRepositoryDB

Create table execRepositoryTable with the following attributes:

exec_id - Identificator of the executable
exec_name - Name of the executable
exec_blob - Object containing an executable


CREATE TABLE execRepositoryTable (exec_id integer, exec_name char, exec_object bulk);

Insert "ls" executable from the system into the table execRepositoryTable


INSERT INTO TABLE execRepositoryTable VALUES (1, "ls", DIRECT_FROM '/bin/ls');

Insert "hello" executable in the table execRepositoryTable


INSERT INTO TABLE execRepositoryTable VALUES (2, "hello", DIRECT_FROM './hello');

Pseudo Code for an Empress PSM function is given (the full source code is given in the appendix):


function executeBLOB(executable_name, executable_object)
{
        Write contents of the executable_object to a temporary file
        Change "execute" permissions on the file
        Use "system" command to execute the file
        Remove the temporary file
        Return execution status
}

APPENDIX

The following script can be executed with EMPRESS V8.60 installed on Intel x86 machine running Linux OS Release 2.0 with libc.so.6 (ELF) in order to acquire the above described functionality. The script is simplified in order to show the basic idea. Other platforms (e.g. MS Windows) would require changes.

#!/bin/sh

cat > ./hello.c <<EOM
#include <stdio.h>
int main()
{
        printf("Hello World!\n");
        return (0);
}
EOM

cc -o hello hello.c

cat > ./module_executeBLOB.c <<EOM

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include  <usrfns.h>

char    *executeBLOB( char *name, gen_binary *blob )
{
        FILE    *fp;
        char    *rc;
        char    fname[32] = "./";
        int     i;

        rc = mspsm_malloc (128);
        strcat(fname, name);

        if ((fp = fopen ( fname, "w")) == NULL)
        {
                snprintf(rc, 128, "Failed to open file %s", fname);
                return (rc);
        }

        for (i = 0; i < blob->num_segments; i++)
        {
                if (fwrite (blob->segment[i].data, blob->segment[i].data_len, 1, fp) < 1)
                {
                        fclose (fp);
                        snprintf(rc, 128, "Failed to write data to file %s", fname);
                        return (rc);
                }
        }

        chmod(fname, S_IXUSR);
        fclose (fp);

        system(fname);
        remove(fname);
        strcpy(rc, "Success");
        return (rc);
}
EOM

emppsmcc -o module_executeBLOB.dll module_executeBLOB.c


empmkdb execRepositoryDB

empbatch execRepositoryDB <<EOM
CREATE MODULE module_executeBLOB
       FUNCTION executeBLOB( GENERIC CHAR, GENERIC BINARY)
       RETURNS GENERIC CHAR
       EXTERNAL NAME executeBLOB;
END MODULE

UPDATE MODULE module_executeBLOB
FROM './module_executeBLOB.dll';


CREATE TABLE execRepositoryTable (exec_id integer, exec_name char, exec_object bulk);
INSERT INTO TABLE execRepositoryTable VALUES (1, "ls", DIRECT_FROM '/bin/ls');
INSERT INTO TABLE execRepositoryTable VALUES (2, "hello", DIRECT_FROM './hello');
EOM

rm ./hello

empcmd execRepositoryDB "SELECT executeBLOB(exec_name, exec_object) FROM execRepositoryTable WHERE exec_id=2;"