|SIGRETURN(2)||Linux Programmer's Manual||SIGRETURN(2)|
The kernel also arranges that, during the transition back to user mode, the signal handler is called, and that, upon return from the handler, 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.
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).