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diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index b27bec4..e37a66b 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -432,6 +432,7 @@ extensions, accepted by GCC in C89 mode and in C++. * Target Builtins:: Built-in functions specific to particular targets. * Pragmas:: Pragmas accepted by GCC. * Unnamed Fields:: Unnamed struct/union fields within structs/unions. +* Thread-Local:: Per-thread variables. @end menu @node Statement Exprs @@ -6165,6 +6166,55 @@ It is ambiguous which @code{a} is being referred to with @samp{foo.a}. Such constructs are not supported and must be avoided. In the future, such constructs may be detected and treated as compilation errors. +@node Thread-Local +@section Thread-Local Storage +@cindex Thread-Local Storage +@cindex TLS +@cindex __thread + +Thread-local storage (TLS) is a mechanism by which variables are +allocated such that there is one instance of the variable per extant +thread. The run-time model GCC uses to implement this originates +in the IA-64 processor-specific ABI, but has since been migrated +to other processors as well. It requires significant support from +the linker (@command{ld}), dynamic linker (@command{ld.so}), and +system libraries (@file{libc.so} and @file{libpthread.so}), so it +is not supported everywhere. + +At the user level, the extension is visible with a new storage +class keyword: @code{__thread}. For example: + +@example +__thread int i; +extern __thread struct state s; +static __thread char *p; +@end example + +The @code{__thread} specifier may be used alone, with the @code{extern} +or @code{static} specifiers, but with no other storage class specifier. +When used with @code{extern} or @code{static}, @code{__thread} must appear +immediately after the other storage class specifier. + +The @code{__thread} specifier may be applied to any global, file-scoped +static, function-scoped static, or class-scoped static variable. It may +not be applied to function-scoped automatic or class-scoped member variables. + +When the address-of operator is applied to a thread-local variable, it is +evaluated at run-time and returns the address of the current thread's +instance of that variable. An address so obtained may be used by any +thread. When a thread terminates, any pointers to thread-local variables +in that thread become invalid. + +No static initialization may refer to the address of a thread-local variable. + +In C++, a thread-local variable may not be initialized by a static +constructor. + +See @uref{http://people.redhat.com/drepper/tls.pdf, +ELF Handling For Thread-Local Storage} for a detailed explanation of +the four thread-local storage addressing models, and how the run-time +is expected to function. + @node C++ Extensions @chapter Extensions to the C++ Language @cindex extensions, C++ language |