at a fixed address, when compiled with --apcs /norwpi

at a fixed address relative to the static base (sb) register (r9), when compiled with --apcs /rwpi.

When compiled with --apcs /norwpi, read/write static data is addressed in a position-dependent fashion. This is the compiler default. Code from these variants is single-threaded because it uses read/write static data.

For example, library c_a__un has position-dependent data (see Library naming conventions).

When compiled with --apcs /rwpi, read/write static data is addressed in a position-independent fashion using offsets from the static base register sb. Code from these variants is reentrant, and can be multiply threaded if each thread uses a different static base value.

For example, library c_a__ue has position-independent data (see Library naming conventions).

The default floating-point arithmetic libraries fz_* and fj_* do not use static data and are always reentrant. However, the f_* and g_* libraries do use static data.

All statically-initialized data in the C libraries is read-only.

All writable static data is zero initialized.

Most C library functions use no writable static data and are reentrant whether built with default build options (--apcs /norwpi) or reentrant build options (--apcs /rwpi).

Some functions have static data implicit in their definitions. You must not use these in a reentrant application unless you build it with --apcs /rwpi and the caller uses different values in sb.