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Editorial Team
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Asked: 2026-05-27T23:55:35+00:00

I am reading from a thread local variable in my code like this, //

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I am reading from a thread local variable in my code like this,

// tid_local is declared as __thread int tid_local;
long tid = tid_local

Looking around the dissassembled code, I saw something like this, which I suspect is the instruction which assigns tid by reading tid_local.

movslq %fs:0xfffffffffffffffc,%rbx

Now my question is if this can really be the instruction which is doing this, that is, reading from the local thread variable and if gcc always uses the fs segment for storing thread local variables. How is this supposed to work?

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1 Answer

  1. Editorial Team

    Yes, this could well be the right instruction. From the gcc manual:

    -mtls-direct-seg-refs

    -mno-tls-direct-seg-refs

    Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is legal depends on the operating system, and whether it maps the segment to cover the entire TLS area.

    edit Here is an excellent link suggested by @janneb in the comments: http://www.akkadia.org/drepper/tls.pdf

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