Return from signal handler and cleanup stack frame
int sigreturn(unsigned long __unused);
When the Linux kernel creates the stack frame for a signal handler, a call to sigreturn() is inserted into the stack frame so that upon return from the signal handler, sigreturn() will be called.
This sigreturn() call undoes everything that was done–changing the process's signal mask, switching stacks (see sigaltstack(2))–in order to invoke the signal handler: it restores the process's signal mask, switches stacks, and restores the process's context (registers, processor flags), so that the process directly resumes execution at the point where it was interrupted by the signal.
sigreturn() never returns.
sigreturn() is specific to Linux and should not be used in programs intended to be portable.
The sigreturn() call is used by the kernel to implement signal handlers. It should never be called directly. Better yet, the specific use of the __unused argument varies depending on the architecture.
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