5.3.9.5 Persistent Type Qualifier

The __persistent type qualifier is used to indicate that objects with static storage duration should not be cleared by the runtime startup code, by having them stored in a different area of memory to other objects.

By default, C objects with static storage duration that are not explicitly initialized are cleared on startup. This is consistent with the definition of the C language. However, there are occasions where it is desirable for the content of some objects to be preserved across a Reset. The __persistent type qualifier can be used with these objects to indicate this requirement.

Any object that needs to be persistent cannot be assigned an initial value when defined. A warning will be issued if you use the __persistent qualifier with an object that is assigned an initial value, and for that object, the qualifier will be ignored.

void test(void)
{
  static __persistent int intvar;  /* locals must be static to be persistent */
  // ...
}