SIGRETURN(2)	    UNIX Programmer's Manual	     SIGRETURN(2)

     sigreturn - return from signal

     #include <signal.h>

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

     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 speci-
     fied pc.  This system call is used by the trampoline code,
     and longjmp(3) when returning from a signal to the previ-
     ously executing program.

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

     The definition of the sigcontext structure is machine depen-
     dent (the structure cited above is that for a VAX running
     4.3BSD); no program should depend on its internal structure.
     Setjmp(3) may be used to build sigcontext structures in a
     machine independent manner.

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

     Sigreturn will fail and the process context will remain
     unchanged if one of the following occurs.

     [EFAULT]	    Scp points to memory that is not a valid part
		    of the process address space.

Printed 11/26/99	  June 30, 1985                         1

SIGRETURN(2)	    UNIX Programmer's Manual	     SIGRETURN(2)

     [EINVAL]	    The process status longword is invalid or
		    would improperly raise the privilege level of
		    the process.

     sigvec(2), setjmp(3)

     On the PDP-11 the field sc_ap (argument pointer) does not
     exist and the field sc_fp (frame pointer) is the PDP-11
     register r5.  Additionally, three new fields sc_r0, sc_r1
     and sc_ovno are present on the PDP-11 which hold register
     values r0 and r1 and the text overlay number to restore (see

     struct  sigcontext {
	     int   sc_onstack; /* sigstack state to restore */
	     long  sc_mask;    /* signal mask to restore */
	     int   sc_sp;      /* sp to restore */
	     int   sc_fp;      /* fp to restore */
	     int   sc_r1;      /* r1 to restore */
	     int   sc_r0;      /* r0 to restore */
	     int   sc_pc;      /* pc to restore */
	     int   sc_ps;      /* psl to restore */
	     int   sc_ovno     /* overlay to restore */

Printed 11/26/99	  June 30, 1985                         2

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