#include <signal.h> 

 struct sigcontext {  int sc_onstack;  int sc_mask;  int sc_sp;  int sc_fp;  int sc_ap;  int sc_pc;  int sc_ps;  }; 

 sigreturn(scp);  struct sigcontext *scp; 
 DESCRIPTION
 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.
 NOTES
This system call is not available in 4.2BSD,
hence it should not be used if backward compatibility is needed.
 "RETURN VALUE
If successful, the system call does not return.
Otherwise, a value of -1 is returned and 
 errno is set to indicate the error.
 ERRORS
 Sigreturn will fail and the process context will remain unchanged
if one of the following occurs.
 15
[EFAULT]
 Scp points to memory that is not a valid part of the process
address space.

[EINVAL]
The process status longword is invalid or would improperly
raise the privilege level of the process.
 "SEE ALSO"
sigvec(2), setjmp(3)