DragonFly On-Line Manual Pages

SIGALTSTACK(2)           DragonFly System Calls Manual          SIGALTSTACK(2)

NAME
     sigaltstack - set and/or get signal stack context

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <signal.h>

     typedef struct sigaltstack {
             void    *ss_sp;
             size_t  ss_size;
             int     ss_flags;
     } stack_t;
     int
     sigaltstack(const stack_t * restrict ss, stack_t * restrict oss);

DESCRIPTION
     Sigaltstack() allows users to define an alternate 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 EPERM.  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 alternate 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.

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 alternate stack area.
     The following code fragment is typically used to allocate an alternate
     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 alternate stack
     feature.  In computing an alternate 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.

RETURN VALUES
     The sigaltstack() function returns the value 0 if successful; otherwise
     the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS
     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]           The ss argument is not a null pointer, and the
                        ss_flags member pointed to by ss contains flags other
                        than SS_DISABLE.

     [ENOMEM]           Size of alternate stack area is less than or equal to

     [EPERM]            An attempt was made to modify an active stack.
                        MINSIGSTKSZ.

SEE ALSO
     sigaction(2), setjmp(3)

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

DragonFly 5.5-DEVELOPMENT        June 12, 2019       DragonFly 5.5-DEVELOPMENT