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SIGRETURN(2) System Calls Manual SIGRETURN(2)

return from signal

#include <signal.h>
sigreturn(struct sigcontext *scp);

sigreturn() allows users to atomically unmask, switch stacks, and return from a signal context. The processes signal mask and stack status are restored from the context. The system call does not return; the users stack pointer, frame pointer, argument pointer, and processor status longword are restored from the context. Execution resumes at the specified pc. This system call is used by the trampoline code and longjmp(3) when returning from a signal to the previously executing program.
Note that sigcontext contains machine dependent information.

This system call is not available in 4.2BSD hence it should not be used if backward compatibility is needed.

If successful, the system call does not return. Otherwise, a value of -1 is returned and errno is set to indicate the error.

sigreturn() will fail and the process context will remain unchanged if one of the following occurs.
scp points to memory that is not a valid part of the process address space.
The process status longword is invalid or would improperly raise the privilege level of the process.

sigaction(2), setjmp(3)

The sigreturn() function call appeared in 4.3BSD.
September 9, 2014 OpenBSD-5.8