ID Number: Q72418
5.10 6.00 6.00a 6.00ax 7.00 | 5.10 6.00 6.00a
MS-DOS | OS/2
Summary:
In most cases, the /ND (name data segment) compiler option should not
be used on any module that contains calls to the C run-time library,
or fatal program errors will result. The C run-time code attempts to
access data in the wrong segment, generating a protection violation
(GP fault) under OS/2 or possibly hanging the system under MS-DOS.
More Information:
The /ND switch may be used to generate a default data segment of a
particular name. This is desirable in many situations in order to
control data segment names (creating a shared segment, for instance).
When /ND is used, the compiler creates three segments (dataname,
dataname_BSS, and dataname_CONST), makes them a part of a new group
(dataname_GROUP), and performs an explicit DS load with the new
default data segment for every function defined in the module.
This can cause problems when the C run-time library is used. The
run-time library is built with the assumption that DS points to the
default data segment for the entire application (DGROUP). This allows
the C run-time to use near pointers to access its internal data
structures because it assumes that this data is based in the default
data segment. When you use /ND on a module that contains run-time
code, DS is not set to the correct segment and the run-time data will
not be accessed correctly.
Therefore, if you require the /ND switch for your application, do not
use it on a module with run-time calls. The best solution is to create
a module that defines data only, and compile it with /ND. The rest of
your modules can use "extern" references to access that data. In this
manner, you can have the benefits of naming your data segments without
causing problems with the run-time library. The sample code below
illustrates this method.
The CDLLOBJS library (C run time in a DLL) is not subject to these
restrictions. In this implementation alone, DS is loaded with the
application's default data segment (DGROUP) inside the run-time code.
Sample Code #1
--------------
/* Compile the following modules as indicated and then link the
resulting .OBJ files together (for example, LINK code data;).
*/
/* Module 1 - DATA.C */
/* Compile options needed: /c /ND MYDATA
*/
char array[] = "This is defined in the MYDATA data segment";
Sample Code #2
--------------
/* Module 2 - CODE.C */
/* Compile options needed: /c
*/
extern char _far array[];
void main(void)
{
printf("The string in MYDATA is: %Fs\n", array);
}
Additional reference words: 6.00 6.00a 6.00ax