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:

int *ss_sp long ss_size int ss_flags

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:

SS_DISABLE

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:

SS_ONSTACK

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.

SS_DISABLE

The alternate signal stack is currently disabled.

Upon successful completion, 0 is return. Otherwise, -1 is returned and errno is set to indicate the error.

The sigaltstack() function will fail if:

EFAULT

The ss or oss argument points to an illegal address.

EINVAL

The ss argument is not a null pointer, and the ss_flags member pointed to by ss contains flags other than SS_DISABLE.

ENOMEM

The size of the alternate stack area is less than MINSIGSTKSZ.

EPERM

An attempt was made to modify an active stack.

See attributes(5) for descriptions of the following attributes:

getcontext(2), mmap(2), sigaction(2), ucontext(3HEAD)

The value SIGSTKSZ is defined to be the number of bytes that would be used to cover the usual case when allocating an alternate stack area. The value MINSIGSTKSZ is defined to be the minimum stack size for a signal handler. In computing an alternate stack size, a program should add that amount to its stack requirements to allow for the operating system overhead.

The following code fragment is typically used to allocate an alternate stack with an adjacent red zone (an unmapped page) to guard against stack overflow, as with default stacks:

#include <signal.h> #include <sys/mman.h> stack_t sigstk; sigstk.ss_sp = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0); if (sigstk.ss_sp == MAP_FAILED) /* error return */; sigstk.ss_size = SIGSTKSZ; sigstk.ss_flags = 0; if (sigaltstack(&sigstk, NULL) < 0) perror("sigaltstack");