strace/linux/i386/arch_sigreturn.c

static void
arch_sigreturn(struct tcb *tcp)
{
	/*
	 * On i386, sigcontext is followed on stack by struct fpstate
	 * and after it an additional u32 extramask which holds
	 * upper half of the mask.
	 */
	struct {
		uint32_t struct_sigcontext_padding1[20];
		uint32_t oldmask;
		uint32_t struct_sigcontext_padding2;
		uint32_t struct_fpstate_padding[156];
		uint32_t extramask;
	} frame;

	if (umove(tcp, *i386_esp_ptr, &frame) < 0) {
		tprintf("{mask=%#lx}", (unsigned long) *i386_esp_ptr);
	} else {
		uint32_t mask[2] = { frame.oldmask, frame.extramask };
		tprintsigmask_addr("{mask=", mask);
		tprints("}");
	}
}