How to Pass a Far Pointer from Parent to Spawned Child

ID: Q40774

6.00 6.00a 6.00ax 7.00 | 6.00 6.00a | 1.00 1.50

MS-DOS                 | OS/2       | WINDOWS

The information in this article applies to:

• Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
• Microsoft C for OS/2, versions 6.0 and 6.0a
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0 and 1.5

SUMMARY

The best way to pass a pointer from a parent process to a child is to convert the pointer to an ASCII string using sprintf(), spawn the child with the string on the command line, and convert the string back to a pointer with sscanf(). Below is a sample program that does this. Note that the program and the child process must be compiled separately. Be sure to pass a far (segment and offset) pointer rather than a near (offset only) pointer. Attempting to pass a near pointer will not work.

MORE INFORMATION

It is also possible to leave the pointer in an agreed-upon memory area. Sometimes unused interrupt vectors are used for this purpose under DOS. Microsoft does not recommend this method because it could cause conflicts with other software using the same memory. The method described above is much safer.

Sample Code

Compile the following two programs separately. Be sure to name the child process MAIN2.EXE, or change the name in the spawnlp() call in the parent process to match your name.

Parent Process

/* Compile options needed: /AL
*/ 

#include <process.h>
#include <stdlib.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
char buffer[80];            /* buffer to be passed to child */ 
char ascptr[20];            /* holds ASCII version of ptr.  */ 

void main(void)

int ret;

                            /* initial string */ 
    strcpy(buffer, "Main1");
    printf("Beginning main1...\nString is:  %s\n", buffer);

                            /* convert address to ASCII */ 
    sprintf(ascptr, "%p", buffer);
    printf("ASCII version of pointer is: %p\nSpawning main2...\n\n",
            ascptr);

                            /* spawn, passing ASCII address on
                               command line */ 
    ret = spawnlp(P_WAIT, "main2.exe", "main2", ascptr, NULL);
    printf("\nBack to main1--spawnlp returned %d\n", ret);

                            /* Show that string is changed */ 
    printf("String is now:  %s\nAll done", buffer);
    exit(0);

/* end of program number one*/ 

Child Process

/* Compile options needed: /AL

*/ 

#include <process.h>
#include <stdlib.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

char far *ptrstuff;

void main(int argc, char **argv)

int ch;

    printf("In main2:  argv[1] is %s\n", argv[1]);
    if (argc != 2) {        /* error in argument list */ 
        printf("Error in argument list to main2\n");
        exit(1);
    }

                            /* retrieve pointer value from string */ 
    sscanf(argv[1], "%p", &ptrstuff);
    printf("\nPointer is actually set to:  %p\n", ptrstuff);

                            /* check to see that it's right */ 
    printf("Pointer points to: %s\n", ptrstuff);

                            /* change string (CAREFULLY!!!!!) */ 
    printf("Warning:  modifying the string incorrectly could hang "
           "your system...\n");
    printf("\007Should I modify this string? ");
    ch = getchar();
    if (toupper(ch) == 'Y')  {
        strcpy(ptrstuff, "2nd main");
        printf("Changed string to: %s\n", ptrstuff);
    }
    else printf("String not changed....\n");

    printf("Exiting main2...\n");
    exit(0);

/* end of child process */ 

Last Reviewed: July 18, 1997