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sigaltstackset and/or get signal stack context

#include <signal.h>

typedef struct sigaltstack {
	void	*ss_sp;
	size_t	 ss_size;
	int	 ss_flags;
} stack_t;

sigaltstack(const stack_t *ss, stack_t *oss);

() allows users to define an alternate stack on which signals delivered to this thread are to be processed. If ss is non-zero and SS_DISABLE is set in ss_flags, the signal stack will be disabled. A disabled stack will cause all signals to be taken on the regular user stack. Trying to disable an active stack will cause sigaltstack() to return -1 with errno set to EPERM.

Otherwise, ss_sp specifies a pointer to a space to be used as the signal stack and ss_size specifies the size of that space. When a signal's action indicates its handler should execute on the signal stack (specified with a sigaction(2) call), the system checks to see if the thread is currently executing on that stack. If the thread is not currently executing on the signal stack, the system arranges a switch to the signal stack for the duration of the signal handler's execution.

If oss is non-zero, the current signal stack state is returned. The ss_flags field will contain the value SS_ONSTACK if the thread is currently on a signal stack and SS_DISABLE if the signal stack is currently disabled.

The value SIGSTKSZ is defined to be the number of bytes/chars that would be used to cover the usual case when allocating an alternate stack area. The following code fragment is typically used to allocate an alternate stack.

if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
	/* error return */
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, NULL) == -1)

An alternative approach is provided for programs with signal handlers that require a specific amount of stack space other than the default size. The value MINSIGSTKSZ is defined to be the number of bytes/chars that is required by the operating system to implement the alternate stack feature. In computing an alternate stack size, programs should add MINSIGSTKSZ to their stack requirements to allow for the operating system overhead.

Signal stacks are automatically adjusted for the direction of stack growth and alignment requirements. Signal stacks may or may not be protected by the hardware and are not “grown” automatically as is done for the normal stack. If the stack overflows and this space is not protected, unpredictable results may occur.

On OpenBSD some additional restrictions prevent dangerous address space modifications. The proposed space at ss_sp is verified to be contiguously mapped for read-write permissions (no execute) and incapable of syscall entry (see msyscall(2)). If those conditions are met, a page-aligned inner region will be freshly mapped (all zero) with MAP_STACK (see mmap(2)), destroying the pre-existing data in the region. Once the sigaltstack is disabled, the MAP_STACK attribute remains on the memory, so it is best to deallocate the memory via a method that results in munmap(2).

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

sigaltstack() will fail and the signal stack context will remain unchanged if one of the following occurs.

Either ss or oss points to memory that is not a valid part of the process address space.
The ss_flags member pointed to by the ss argument contains flags other than SS_DISABLE.
The memory region is not acceptable for use as a stack; see above.
Size of alternate stack area is less than or equal to MINSIGSTKSZ.
An attempt was made to disable an active stack.

sigaction(2), setjmp(3)

The sigaltstack() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The predecessor to sigaltstack(), the sigstack() system call, appeared in 4.2BSD.

October 19, 2022 OpenBSD-7.5