The tls_model attribute (IBM extension)

The tls_model attribute allows source-level control for the thread-local storage model used for a given variable. The tls_model attribute must specify one of local-exec, initial-exec, local-dynamic, or global-dynamic access method, which overrides the -qtls option for that variable. For example:

__thread int i __attribute__((tls_model("local-exec")));

The tls_model attribute allows the linker to check that the correct thread model has been used to build the application or shared library. The linker/loader behavior is as follows:

Table 1. Link time/runtime behavior for thread access models
Access method	Link-time diagnostic	Runtime diagnostic
`local-exec`	Fails if referenced symbol is imported.	Fails if module is not the main program. Fails if referenced symbol is imported (but the linker should have detected the error already).
`initial-exec`	None.	`dlopen()/load()` fails if referenced symbol is not in the module loaded at execution time.
`local-dynamic`	Fails if referenced symbol is imported.	Fails if referenced symbol is imported (but the linker should have detected the error already).
`global-dynamic`	None.	None.