sigaltstack(2)

set or get signal alternate stack context

From the original Amiga UNIX 2.1 manual. Some things it mentions may not be in this release.

NAME

sigaltstack - set or get signal alternate stack context

SYNOPSIS

#include  <signal.h>  int  sigaltstack(const  stack_t   *ss,
stack_t *oss);

DESCRIPTION

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 pro-
cess 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  sys-
tem  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 alter-
nate signal stack for the duration of the  signal  handler's
execution.  The structure sigaltstack 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 return
from sigaltstack.  The ss_sp and ss_size fields specify  the
new  base  and  size  of  the  stack, which is automatically
adjusted  for  direction  of  growth  and  alignment.    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 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 fields specify the base and size of that stack. The ss_flags field specifies the stack's state, and may contain the following values:

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_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 address space.

EINVAL An attempt was made to disable an active stack or the ss_flags field specifies invalid flags.

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

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 operating system overhead.

The following code fragment is typically used to allocate an alternate stack.

if ((sigstk.ss_sp = (char *)malloc(SIGSTKSZ)) == NULL)
     /* error return */;

sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, (stack_t *)0) < 0)
     perror("sigaltstack");

DIAGNOSTICS