SIGRETURN(2)

NAME
sigreturn - return from signal

SYNOPSIS
#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;
};

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.

The definition of the sigcontext structure is machine dependent (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.

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.

[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)

NOTES (PDP-11)
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 ld(1)).

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 */
};