> How can I detect whether my program is running under a multitasker or if
 > SHARE support is present etc?

Ralf Brown's interrupt list - INT 2Fh. There are calls there to detect the
presence of SHARE.  Either that or attempt to lock a file - if file SHARE
support isn't present, you'll get a "bad system call" return
code.

Multitaskers are another ballgame, but it's all been done before. The
following is from Bob Stout's C_ECHO SNIPPETs at tasker.c & tasker.h -
it's reasonably complete. The whole SNIPPETs file may prove useful - AFAIK,
the latest issue is still 0494, usually available as SNIP0494.xxx.

--- tasker.txt ---
                    Multi-Tasker Detection Routines
                             by David Gibbs
                          FidoNet: 1:115/439.0
             Internet: David.Gibbs{at}f439.n115.z1.fidonet.org

The following is a set of C routines that will enable a programmer to
detect a mutli-tasking environment and release the time slice when
desired.  Currently DESQview, Windows, & OS/2 are the environments
supported.

Routines consist of two functions, two global int variables, one global
structure, and a table of character pointers.

void t_get_os(); This routines detects the operating environment, sets
on the appropriate bits in the t_os_type field, and sets the t_os field
to the dominant environment.

void t_slice(); This routine will release the remainder of the current
tasks time slice in the manner appropriate to the dominant envionment.

The following fields & structures are available...

int t_os_type; is a bit mapped integer that indicates the presence of
various operating envionments.  If Bit 0 is on, DOS is present, Bit 1 =
OS2, bit 2 = DESQview, bit 3 = Windows standard, bit 4 = Windows 386
Enh.  These bits can be tested by using logical operations with the
symbolic constants is_DOS, is_OS2, is_DV, is_WINS, and is_WIN3.

int t_os; represents the dominant environment.  The dominant envionment
is defined as the multi-tasking system that takes precidence.  For
instance, you can run Windows *UNDER* DESQview, but DESQview would be
dominant, the same goes true for OS/2 & Windows.  This value can be
tested by comparing to the symbolic constants: DOS, OS2, DV, WINS, and
WIN3.

struct t_os_ver ts_os_ver[]; indicates the versions of the various
environments present.  Major & minor versions are found in the
structure members 'maj' and 'min'.  The structure is subscripted, so you
can access the version of envionments using the symbolic constants use
in 't_os'.

const char *t_os_name[]; contains the names of the environments
detectable.  These too are subscripted and can be accessed using the
symbolic constants above.

A sample program that uses these routines follows:

#include 
#include "tasker.h"

void main() {
        get_os();

        printf("%s %d.%d detected",t_os_name[t_os],
                                   t_os_ver[t_os].maj,
                                   t_os_ver[t_os].min);

        while(!kbhit()) {
                printf("Hit a key!\r\n");
                t_slice();
        }
}


Special thanks go to Geoffery Booher (1:2270/233) for assistance with
Windows & OS/2 detection & Time slicing.

This routine is released to the public as CommentWare - If you use it,
please send me a comment as to what you thought of it... oh yeah, you
might think of giving me credit for the routines.

Also, if you can think of a enhancement or correction, please let me
know.  I can be reached at the above mentioned email addresses.

Copyrights: DESQview by Quarterdeck Office Systems
            Windows by Microsoft
            OS/2 by IBM
            TurboC++ by Borland

--- tasker.h ---
/*
**  Tasker.H
**
**  public domain by David Gibbs
*/

#ifndef DG_TASKER
#define DG_TASKER

struct ts_os_ver {
      int maj;
      int min;
};

#define TOT_OS  5

#define DOS     0
#define OS2     1
#define DV      2
#define WINS    3
#define WIN3    4

                        /*   76543210  */
#define is_DOS  0x01    /* b'00000001' */
#define is_OS2  0x02    /* b'00000010' */
#define is_DV   0x04    /* b'00000100' */
#define is_WINS 0x08    /* b'00001000' */
#define is_WIN3 0x10    /* b'00010000' */


extern int t_os_type;
extern int t_os;

extern const char *t_os_name[TOT_OS];

extern struct ts_os_ver t_os_ver[TOT_OS];


/* Function prototypes */

int get_os();
void t_slice();

#endif /* DG_TASKER */

--- tasker.c ---
/*
**  Tasker.C
**
**  public domain by David Gibbs
*/

struct ts_os_ver t_os_ver[TOT_OS];
int t_os_type;
int t_os;

const char *t_os_name[TOT_OS] = {
      "DOS",
      "OS/2 DOS",
      "DESQview",
      "Windows Std",
      "Windows 386"
      };

int get_os (void)
{
      union REGS t_regs;

      t_os_type = 0;
      t_os = 0;

      /* test for DOS or OS/2 */

      if (_osmajor < 10)
      {
            t_os_ver[DOS].maj = _osmajor;
            t_os_ver[DOS].min = _osminor;
            t_os_type = t_os_type | is_DOS;
      }
      else
      {
            t_os_type = t_os_type | is_OS2;
            t_os_ver[OS2].maj = _osmajor/10;
            t_os_ver[OS2].min = _osminor;
      }

      /* test for Windows */

      t_regs.x.ax = 0x4680;
      int86(0x2F, &t_regs, &t_regs);

      if (t_regs.x.ax == 0x0000)
      {
            t_os_ver[WINS].maj = 3;
            t_os_ver[WINS].min = 0;
            t_os_type = t_os_type | is_WINS;
      }
      else
      {
            t_regs.x.ax = 0x1600 ;
            int86(0x2F, &t_regs, &t_regs);

            switch (t_regs.h.al)
            {
            case 0x00 :
            case 0x80 :
            case 0x01 :
            case 0xFF :
                  break;

            default   :
                  t_os_type = t_os_type | is_WIN3;
                  t_os_ver[WIN3].maj = t_regs.h.al;
                  t_os_ver[WIN3].min = t_regs.h.ah;
                  break ;
            }  /* endswitch  */
      } /* endif */

      /* Test for DESQview */

      t_regs.x.cx = 0x4445;     /* load incorrect date */
      t_regs.x.dx = 0x5351;
      t_regs.x.ax = 0x2B01;     /*  DV set up call     */

      intdos(&t_regs, &t_regs);
      if (t_regs.h.al != 0xFF)
      {
            t_os_type = t_os_type | is_DV;
            t_os_ver[DV].maj = t_regs.h.bh;
            t_os_ver[DV].min = t_regs.h.bl;
      }

      if(t_os_type & is_DOS)
            t_os = DOS;

      if(t_os_type & is_WINS)
            t_os = WINS;

      if(t_os_type & is_WIN3)
            t_os = WIN3;

      if(t_os_type & is_DV)
            t_os = DV;

      if(t_os_type & is_OS2)
            t_os = OS2;

      return(t_os-1);

}

void t_slice(void)
{
      union REGS t_regs;
    
      switch (t_os)
      {
      case DOS  :
            break;

      case OS2  :
      case WIN3 :
      case WINS :
            t_regs.x.ax = 0x1680;
            int86(0x2f,&t_regs,&t_regs);
            break;

      case DV   :
            t_regs.x.ax = 0x1000;
            int86(0x15,&t_regs,&t_regs);
            break;
      } /* switch(t_os) */
}

--- Squish/386 v1.11