The sigaltstack() function allows a thread to define and examine the state of an alternate stack area on which signals are processed. If ss is non-zero, it specifies a pointer to and the size
of a stack area on which to deliver signals, and informs the system whether the thread is currently executing on that stack. When a signal's action indicates its handler should execute on the alternate signal stack (specified with a sigaction(2) call), the system checks whether the thread chosen to execute the signal handler is currently executing on that stack. If the thread is not currently executing on the signal stack, the system arranges a switch to the alternate signal stack
for the duration of the signal handler's execution.
The stack_t structure includes the following members:
If ss is not NULL, it points to a structure specifying the alternate signal stack that will take effect upon successful return from sigaltstack(). The ss_sp and ss_size members specify
the new base and size of the stack, which is automatically adjusted for direction of growth and alignment. The ss_flags member specifies the new stack state and may be set to the following:
- The stack is to be disabled and ss_sp and ss_size are ignored. If SS_DISABLE is not set, the stack will be enabled.
If oss is not NULL, it points to a structure specifying the alternate signal stack that was in effect prior to the call to sigaltstack(). The ss_sp and ss_size members specify
the base and size of that stack. The ss_flags member specifies the stack's state, and may contain the following values:
- The thread is currently executing on the alternate signal stack. Attempts to modify the alternate signal stack while the thread is executing on it will fail.
- The alternate signal stack is currently disabled.