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sigreturn(2) [bsd man page]

SIGRETURN(2)							System Calls Manual						      SIGRETURN(2)

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. Addition- ally, 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 */ }; 4.3 Berkeley Distribution June 30, 1985 SIGRETURN(2)

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

sigreturn, rt_sigreturn - return from signal handler and cleanup stack frame SYNOPSIS
int sigreturn(...); DESCRIPTION
If the Linux kernel determines that an unblocked signal is pending for a process, then, at the next transition back to user mode in that process (e.g., upon return from a system call or when the process is rescheduled onto the CPU), it creates a new frame on the user-space stack where it saves various pieces of process context (processor status word, registers, signal mask, and signal stack settings). The kernel also arranges that, during the transition back to user mode, the signal handler is called, and that, upon return from the han- dler, control passes to a piece of user-space code commonly called the "signal trampoline". The signal trampoline code in turn calls sigreturn(). This sigreturn() call undoes everything that was done--changing the process's signal mask, switching signal stacks (see sigaltstack(2))--in order to invoke the signal handler. Using the information that was earlier saved on the user-space stack sigreturn() restores the process's signal mask, switches stacks, and restores the process's context (processor flags and registers, including the stack pointer and instruction pointer), so that the process resumes execution at the point where it was interrupted by the signal. RETURN VALUE
sigreturn() never returns. CONFORMING TO
Many UNIX-type systems have a sigreturn() system call or near equivalent. However, this call is not specified in POSIX, and details of its behavior vary across systems. NOTES
sigreturn() exists only to allow the implementation of signal handlers. It should never be called directly. (Indeed, a simple sigreturn() wrapper in the GNU C library simply returns -1, with errno set to ENOSYS.) Details of the arguments (if any) passed to sigreturn() vary depending on the architecture. (On some architectures, such as x86-64, sigreturn() takes no arguments, since all of the information that it requires is available in the stack frame that was previously created by the kernel on the user-space stack.) Once upon a time, UNIX systems placed the signal trampoline code onto the user stack. Nowadays, pages of the user stack are protected so as to disallow code execution. Thus, on contemporary Linux systems, depending on the architecture, the signal trampoline code lives either in the vdso(7) or in the C library. In the latter case, the C library's sigaction(2) wrapper function informs the kernel of the location of the trampoline code by placing its address in the sa_restorer field of the sigaction structure, and sets the SA_RESTORER flag in the sa_flags field. The saved process context information is placed in a ucontext_t structure (see <sys/ucontext.h>). That structure is visible within the signal handler as the third argument of a handler established via sigaction(2) with the SA_SIGINFO flag. On some other UNIX systems, the operation of the signal trampoline differs a little. In particular, on some systems, upon transitioning back to user mode, the kernel passes control to the trampoline (rather than the signal handler), and the trampoline code calls the signal handler (and then calls sigreturn() once the handler returns). C library/kernel differences The original Linux system call was named sigreturn(). However, with the addition of real-time signals in Linux 2.2, a new system call, rt_sigreturn() was added to support an enlarged sigset_t type. The GNU C library hides these details from us, transparently employing rt_sigreturn() when the kernel provides it. SEE ALSO
kill(2), restart_syscall(2), sigaltstack(2), signal(2), getcontext(3), signal(7), vdso(7) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at Linux 2017-09-15 SIGRETURN(2)

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