Q58437: Accessing Child Process Exit Code from Parent Process

Article: Q58437
Product(s): See article
Version(s): 5.00 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickc s_quickasm | mspl13_c
Last Modified: 9-FEB-1990

Question:

I am using the spawn() function and I want to be able to find out
whether or not the child program terminated normally or by entry of
CTRL+C. Is there anyway I can get the system exit code from the child
process?

Response:

There is no C run-time function that will return the system exit code
from a child process. However, Interrupt 21h SubFunction 4Dh can be
used to return it.

Immediately after a child process terminates, the child exit code and
the system exit code are in the AL and AH registers respectively.

Example:

                    _________________________
                    |           |           |
      AX Register   |     |     |     |     |
                    |           |           |
                    -------------------------
                         AH     |    AL
                         /            \
                        /              \
         System Exit Code               Child Exit Code

When the spawn() family of functions is called with a mode flag of
P_WAIT, only the child exit code is returned. To read the system exit
code, a call to Interrupt 21h SubFunction 4Dh is needed.

It is important to get and store the return codes immediately upon
returning from the child process, since another function may modify
them.

The following code samples demonstrate how to get the exit code from
the child process within the parent process with C Version 5.10 and
QuickC Versions 2.00 and 2.01.

Code Example 1
--------------

/* Call this prog1.c  */
#include <dos.h>
#include <stdio.h>
#include <process.h>

union REGS inregs, outregs, tempreg;
int retcode;
unsigned char syscode;

void main (void)
{
   printf ("In program I\n");
   retcode = spawnl (P_WAIT, "sp2.exe", "sp2", NULL);

   /* Call int 21h function 4Dh to obtain exit codes */

   inregs.h.ah = 0x4d;
   intdos (&inregs, &outregs);

   /* System Exit Code will be in AH. */

   syscode = outregs.h.ah;

   printf ("Child exit code: %d\n", retcode);
   printf ("Child process ");
   switch (syscode)
   {
      case 0 : printf ("terminated normally\n");
               break;
      case 1 : printf ("exit with a Control-C\n");
               break;
      case 2 : printf ("exit with a hard error\n");
               break;
      case 3 : printf ("exit as a TSR program\n");
               break;
   }
}

Code Example 2
--------------

/* Call this sp2.c */
#include <stdio.h>

void main (void)
{
   printf ("In program II\n");
   exit (77);
}

Since QuickC 2.00 and QuickAssembler 2.01 have the feature of using
inline assembly, the AX register can be access directly without using
any interrupts. The following line of code can be used in place of the
interrupt call:

_asm mov syscode, ah

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