Spawning All programs that we execute from DOS prompt

Spawning All programs that we execute from DOS prompt can be thought of as children of COMMAND.COM. Thus, the program that we execute is a child process, whereas COMMAND.COM running in memory is its parent. The process of a parent process giving birth to a child process is known as 'spawning'. If the spawned program so desires, it may in turn spawn children of its own, which then execute and return control to their parent. Who is the parent of COMMAND.COM? COMMAND.COM itself. We can trace the ancestors of our program using the field Parent Process ID (PID) present at offset 0x16 in the Program Segment Prefix (PSP). To trace this ancestry our program should first locate its PSP, extract the parent process ID from it and then use this to find PSP of the parent. This process can be repeated till we reach COMMAND.COM (process ID of COMMAND.COM is its own PSP), the father of all processes. Here is a program which achieves this...

/* SPAWN.C */

#include "dos.h"

unsigned oldpsp, newpsp, far *eb_seg, i ;

char far *eb_ptr ;

main( )

{

oldpsp = _psp ;

while ( 1 )

{

printf ( "\n" ) ;

printname ( oldpsp ) ;

printf ( " spawned by " ) ;

newpsp = * ( ( unsigned far * ) MK_FP ( oldpsp, 0x16 ) ) ;

if ( * ( ( unsigned * ) MK_FP ( newpsp, 0x16 ) ) == newpsp )

break ;

else

oldpsp = newpsp ;

printname ( newpsp ) ;

}

printf ( "%-20s (%04X)", "COMMAND.COM", newpsp ) ;

}

printname ( unsigned lpsp )

{

char drive[5], dir[68], name[13], ext[5] ;

eb_seg = ( unsigned far * ) MK_FP ( lpsp, 0x2C ) ;

eb_ptr = MK_FP ( *eb_seg, 0 ) ;

i = 0 ;

while ( 1 )

{

if ( eb_ptr[i] == 0 )

{

if ( eb_ptr[i + 1] == 0 && eb_ptr[i + 2] == 1 )

{

i += 4 ;

break ;

}

}

i++ ;

}

fnsplit ( eb_ptr + i, drive, dir, name, ext ) ;

strcat ( name, ext ) ;

printf ( "%-20s (%04X)", name, oldpsp ) ;

}

On running the program from within TC the output obtained is shown below. SPWAN.EXE (58A9) spawned by TC.EXE (0672) TC.EXE (0672) spawned by COMMAND.COM (05B8). The program simply copies its own process ID in the variable oldpsp and then uses it to extract its own filename from its environment block. This is done by the function printname( ). The value in oldpsp is then used to retrieve the parent's PID in newpsp. From there the program loops reporting the values of oldpsp, newpsp and the corresponding file names until the program reaches COMMAND.COM.

The printname( ) function first locates the environment block of the program and then extracts the file name from the environment block. The fnsplit( ) function has been used to eliminate the path present prior to the file name. Do not run the program from command line since it would give you only one level of ancestry.

Data Structures

Choosing the data structures to be used for information retrieval. For problems of information retrieval, consider the size, number, and location of the records along with the type and structure of the keys while choosing the data structures to be used. For small records, high-speed internal memory will be used, and binary search trees will likely prove adequate. For information retrieval from disk files, methods employing multiway branching, such as trees, B-trees , and hash tables, will usually be superior. Tries are particularly suited to applications where the keys are structured as a sequence of symbols and where the set of keys is relatively dense in the set of all possible keys. For other applications, methods that treat the key as a single unit will often prove superior. B-trees, together with various generalization and extensions, can be usefully applied to many problems concerned with external information retrieval.