Q44927: How Does _osmode Get Set?

Article: Q44927
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# G890523-18896 | mspl13_c
Last Modified: 21-AUG-1989

The Microsoft C Run-time Library variable _osmode is set to one (1)
when running under OS/2 and to zero (0) when running under DOS. A C
program can look at this variable and easily determine whether it's
running under DOS or OS/2, without calling DosGetMachineMode.

To declare the variable, use the following line:

   #include <stdlib.h>

An examination of the start-up code (included with the compiler) shows
how the value of this variable is determined. The DOS start-up code
just initializes the variable to zero. The OS/2 code initializes it by
making a call to the OS/2 API DosGetMachineMode, which sets it to zero
for DOS or one for OS/2. Thus, you don't need to make the call.

If you compile for DOS (/Lr or /Lc), the DOS start-up code is included
because the linker links your program with the appropriate DOS library
(xLIBCyR, where x is the memory model and y is the math option). By
the same token, when you compile with /Lp, the linker links the proper
library for OS/2 (xLIBCyP). If you don't specify one of these options,
your code is linked with xLIBCy.

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