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SIGALTSTACK(2)                System Calls Manual               SIGALTSTACK(2)

     sigaltstack - set and/or get signal stack context

     Standard C Library (libc, -lc)

     #include <signal.h>

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

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

     sigaltstack() allows users to define an alternative stack on which
     signals are to be processed.  If ss is non-zero, it specifies a pointer
     to and the size of a signal stack 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 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 signal stack for the duration of the signal handler's execution.

     If SS_DISABLE is set in ss_flags, ss_sp and ss_size are ignored and the
     signal stack will be disabled.  Trying to disable an active stack will
     cause sigaltstack to return -1 with errno set to EINVAL.  A disabled
     stack will cause all signals to be taken on the regular user stack.  If
     the stack is later re-enabled then all signals that were specified to be
     processed on an alternative stack will resume doing so.

     If oss is non-zero, the current signal stack state is returned.  The
     ss_flags field will contain the value SS_ONSTACK if the process 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 alternative stack
     area.  The following code fragment is typically used to allocate an
     alternative stack.

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

     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 alternative
     stack feature.  In computing an alternative 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.

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

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

     [EFAULT]           Either ss or oss points to memory that is not a valid
                        part of the process address space.

     [EINVAL]           An attempt was made to disable an active stack.

     [ENOMEM]           Size of alternative stack area is less than

     sigaction(2), setjmp(3), signal(7)

     The sigaltstack() function conforms to X/Open Portability Guide Issue 4,
     Version 2 ("XPG4.2").

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

NetBSD 10.99                   October 31, 2021                   NetBSD 10.99