sigstack - set or query alternative stack for signal

Syntax	#include <signal.h> int sigstack (struct sigstack *ss, struct sigstack *oss);
Description	sigstack() can be used to define an alternative stack, called a signal stack, in which the signals are processed. If the action of a signal indicates that the processing routine is to be executed in a signal stack (specified by a sigaction() call), the system checks whether the process is currently being executed in this stack. If the process is not being executed in the signal stack, the system switches over to the signal stack until the signal-handling routine terminates. A signal stack is specified by a sigstack structure which contains the following elements: char *ss_sp; / * pointer of signal stack */ int ss_onstack; / * current status */ ss_sp is the start address of the stack. If the ss_onstack field is non-zero, the signal stack is to be activated. If ss is not a null pointer, sigstack() sets the status of the signal stack to the value in the sigstack structure to which ss points. The length of the stack must be at least SIGSTKSZ bytes. If ss_onstack is non-zero, the system assumes that the process is being executed in the signal stack. If ss is a null pointer, the status of the signal stack remains unchanged. If oss is not a null pointer, the current status of the signal stack is saved in the sigstack structure to which oss points.
Return val.	0 -1	if successful. if an error occurs. errno is set to indicate the error.
Errors	sigstack() will fail if: EPERM       an attempt was made to modify an active stack
Notes	Signal stacks are not automatically enlarged like normal stacks. Therefore, an overflow of the signal stack can cause unexpected results. Portable applications should use sigaltstack() instead of sigstack(). Programs should not terminate a signal-handling routine with longjmp() if it is executed in a stack that was set up with sigstack(). In certain circumstances this stack can become unusable. You are therefore advised to use the functions siglongjmp(), setcontext() or swapcontext() in this case.