info gcc

The following are a set of changes to ISO/IEC 9899:1999 (aka C99) that document the exact semantics of the language extension.

5.1.2 Execution environments
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Within either execution environment, a thread is a flow of control within a program. It is implementation defined whether or not there may be more than one thread associated with a program. It is implementation defined how threads beyond the first are created, the name and type of the function called at thread startup, and how threads may be terminated. However, objects with thread storage duration shall be initialized before thread startup.
6.2.4 Storage durations of objects
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An object whose identifier is declared with the storage-class specifier __thread has thread storage duration. Its lifetime is the entire execution of the thread, and its stored value is initialized only once, prior to thread startup.
6.4.1 Keywords
Add __thread.
6.7.1 Storage-class specifiers
Add __thread to the list of storage class specifiers in paragraph 1.

Change paragraph 2 to

With the exception of __thread, at most one storage-class specifier may be given […]. The __thread specifier may be used alone, or immediately following extern or static.

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The declaration of an identifier for a variable that has block scope that specifies __thread shall also specify either extern or static.

The __thread specifier shall be used only with variables.