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sigaltstack(2)                   System Calls                   sigaltstack(2)



NAME
       sigaltstack - set or get signal alternate stack context

SYNOPSIS
       #include <signal.h>

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

DESCRIPTION
       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  sigalt-
       stack(). 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 fol-
       lowing values:

       SS_ONSTACK      The thread is currently executing on the alternate sig-
                       nal  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.



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

ERRORS
       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 MIN-
                       SIGSTKSZ.



       EPERM           An attempt was made to modify an active stack.



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


       tab()    allbox;    cw(2.750000i)|     cw(2.750000i)     lw(2.750000i)|
       lw(2.750000i).   ATTRIBUTE TYPEATTRIBUTE VALUE Interface StabilityStan-
       dard MT-LevelAsync-Signal-Safe


SEE ALSO
       getcontext(2), mmap(2), sigaction(2), ucontext.h(3HEAD), attributes(5),
       standards(5)

NOTES
       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 oper-
       ating 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");




SunOS 5.10                        1 Nov 2003                    sigaltstack(2)