Microsoft KB Archive/199501

= PRB: LNK1555 Errors, and Near Versus Far Data =

Article ID: 199501

Article Last Modified on 10/24/2003

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APPLIES TO


 * Microsoft Visual C++ 4.0 Cross Development Edition for Macintosh
 * Microsoft Visual C++ 4.0 Cross Development Edition for Macintosh
 * Microsoft Visual C++ 4.0 Cross Development Edition for Macintosh

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This article was previously published under Q199501



SYMPTOMS
When linking a 68KB application, a LNK1555 error may occur, as in the following:

obj\M68K\NOSPSANE\strgtold.obj: error LNK1555: Fixup overflow. Data symbol __ctype is > 32k from a5 and was referenced by a 16-bit fixup.



CAUSE
This problem is due to exceeding the 32KB limit for near data.

Global data for Macintosh 68KB applications is usually referenced using a 16-bit offset of the A5 register. This puts a limit of 32KB on the total size of global data supported by this mechanism, known as the Near Data Model. The remaining 32KB accessible from A5 contains the 68KB Macintosh Jump Table, used to dispatch inter-segment function calls.

The PowerMac run-time model does not share this limitation.



RESOLUTION
It is possible to exceed the 32KB global data limit by using the Far Data Model. However, the Near Data Model is slightly more efficient. If efficiency is an issue, you may want to use the data_seg or __declspec pragma to specify whether near or far data is used for specific variables.

When you run out of near data, try one the following:


 * Use the Global Threshold (/Gt) switch to force near data to the far heap. The linker option /GT1 will generate the least amount of near data.
 * Use the data_seg or __declspec pragma to move specific variables to the far model.
 * Use Llibcs.lib and Lsanes.lib instead of Libcs.lib and Sanes.lib. You may have to turn on the Link option Ignore all default libraries in order to prevent the small model Libcs.lib from being linked in.
 * When using third-party libraries, check for far model alternative libraries or request far model libraries from the vendor.
 * Use the /VERBOSE switch to see linker messages in detail.
 * Re-create workspace/project files.



STATUS
This is not a bug. This is the intended behavior of the Macintosh 68KB run-time architecture.



MORE INFORMATION
The optimal Global Threshold is one that allows all, or up to 32KB, of global data to be near.

After encountering the LNK1555 error, set the Global Threshold to 1 and rebuild the application. If the build is successful, try increasing the threshold until it fails again. The last successful threshold will provide the optimal amount of near data.

Further optimization is possible by using the pragmas previously mentioned to move frequently accessed variables into near data.

