#pragma init (f1[, f2…,fn])
Causes the implementation to call functions f1 to fn (initialization functions) before it calls main(). Such functions are expected to be of type void and to accept no arguments. They are called while constructing the memory image of the program at the start of execution. Initializers in a shared object are executed during the operation that brings the shared object into memory, either at program startup or some dynamic loading operation, such as dlopen(). The only ordering of calls to initialization functions is the order in which they were processed by the link editors, both static and dynamic.
Take extra precautions when an initialization function affects the global-program state. For example, unless an interface explicitly states what happens when you use a system-library initialization-function, you should capture and restore any global state information, such as the value of errno, that the system-library initialization-function may change.
Such functions are called once for every time they appear in a #pragma init directive.