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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

/GT (Support Fiber-Safe Thread-Local Storage)

Supports fiber safety for data allocated using static thread-local storage, that is, data allocated with __declspec(thread).


Data declared with __declspec(thread) is referenced through a thread-local storage (TLS) array. The TLS array is an array of addresses that the system maintains for each thread. Each address in this array gives the location of thread-local storage data.

A fiber is a lightweight object that consists of a stack and a register context and can be scheduled on various threads. A fiber can run on any thread. Because a fiber may get swapped out and restarted later on a different thread, the address of the TLS array must not be cached or optimized as a common subexpression across a function call (see the /Og (Global Optimizations) option for details). /GT prevents such optimizations.

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see How to: Open Project Property Pages.

  2. Click the C/C++ folder.

  3. Click the Optimization property page.

  4. Modify the Enable Fiber-safe Optimizations property.

To set this compiler option programmatically

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