sigaltstack - set or get signal alternate stack context
int sigaltstack(const stack_t *ss, stack_t *oss);
sigaltstack allows users to define an alternate stack area on which
signals are to be processed. If ss is non-zero, it specifies a pointer
to, and the size of a stack area on which to deliver signals, and tells
the system if the process 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 to
see if the process is currently executing on that stack. If the process
is not currently executing on the signal stack, the system arranges a
switch to the alternate signal stack for the duration of the signal
The structure stack_t includes the following members.
If ss is not NULL, it points to a structure specifying the alternate
signal stack that will take effect upon return from sigaltstack. The
ss_sp and ss_size fields specify the new base and size of the stack. The
ss_sp field should be adjusted for the direction of growth (see example
below). The ss_flags field 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
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 fields specify the base and size of that stack. The
ss_flags field specifies the stack's state, and may contain the following
SS_ONSTACK The process is currently executing on the alternate signal
stack. Attempts to modify the alternate signal stack while
the process is executing on it will fail. SS_ONSTACK
cannot be modified by users.
SS_DISABLE The alternate signal stack is currently disabled.
sigaltstack fails if any of the following is true:
EFAULT Either ss or oss points outside the process's allocated
EINVAL If ss is non-null, and the ss_flags field pointed to by ss
contains invalid flags.
EPERM If an attempt was made to modify an active stack.
ENOMEM The size of the alternate stack area is less than
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
The stack grows downward from high to lower addresses. The following
code fragment is typically used to allocate an alternate stack.
if ((sigstk.ss_sp = (char *)malloc(SIGSTKSZ)) == NULL)
/* error return */;
sigstk.ss_sp += SIGSTKSZ - 1;
/* adjust ss_sp to point to base of stack */
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, (stack_t *)0) < 0)
getcontext(2), sigaction(2), sigsetjmp(3C), ucontext(5).
On success, sigaltstack returns zero. On failure, it returns -1 and sets
errno to indicate the error.
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