sigaltstack(2) - NetBSD Manual Pages

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SIGALTSTACK(2)            NetBSD Programmer's Manual            SIGALTSTACK(2)


NAME

     sigaltstack - set and/or get signal stack context


SYNOPSIS

     #include <sys/types.h>
     #include <signal.h>

     struct sigaltstack {
             char   *ss_sp;
             int     ss_size;
             int     ss_flags;
     };

     int
     sigaltstack(const struct sigaltstack *ss, struct sigaltstack *oss);


DESCRIPTION

     Sigaltstack() allows users to define an alternative stack on which sig-
     nals 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 sig-
     nal'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 cur-
     rently 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 pro-
     cessed 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 cur-
     rently on a signal stack and SS_DISABLE if the signal stack is currently
     disabled.


NOTES

     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 al-
     ternative stack.

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

     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 pro-
     tected 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.


RETURN VALUES

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


ERRORS

     Sigstack() 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 or equal to
                   MINSIGSTKSZ.


SEE ALSO

     sigaction(2),  setjmp(3)


HISTORY

     The predecessor to sigaltstack, the sigstack() system call, appeared in
     4.2BSD.

4.2 Berkeley Distribution        June 4, 1993                                2