sigstack(2)

NAME

sigstack() — set and/or get alternate signal stack context

SYNOPSIS

#include <signal.h> 

int sigstack( 
     struct sigstack *ss, 
     struct sigstack *oss 
); 

DESCRIPTION

The sigstack() function allows the calling process to indicate to the system an area of its address space to be used for processing signals received by the process.

The sigstack() function requires the application to have knowledge of the underlying system's stack architecture. To ensure portability, use sigaltstack() instead of sigstack() when writing or rewriting applications.

If the ss argument is not a null pointer, it must point to a sigstack structure. The length of the application-supplied stack must be at least SIGSTKSZ bytes. If the alternate signal stack overflows, the resulting behavior is undefined. (See APPLICATION USAGE below.)

The value of the ss_onstack member indicates whether the process wants the system to use an alternate signal stack when delivering signals.
The value of the ss_sp member indicates the desired location of the alternate signal stack area in the process' address space.
If the ss argument is a null pointer, the current alternate signal stack context is not changed.

If the oss argument is not a null pointer, it points to a sigstack structure in which the current alternate signal stack context is placed. The value stored in the ss_onstack member of oss will be non- zero if the process is currently executing on the alternate signal stack. If the oss argument is a null pointer, the current alternate signal stack context is not returned.

When a signal's action indicates its handler should execute on the alternate signal stack (specified by calling sigaction()), the implementation checks to see if the process is currently executing on that stack. If the process is not currently executing on the alternate signal stack, the system arranges a switch to the alternate signal stack for the duration of the signal handler's execution.

After a successful call to one of the exec() functions, there are no alternate signal stacks in the new process image.

The correct use of sigstack() is hardware dependent, and therefore is not portable between different HP-UX implementations. sigspace() is portable between different HP-UX implementations and should be used when the application does not need to know where the signal stack is located (see sigspace(2)). sigstack() is provided for compatibility with other systems that provide this functionality. Users should note that there is no guarantee that functionality similar to this is even possible on some architectures.

The sigstack() function is scheduled to be withdrawn from a future version of the Single UNIX Specification. The sigaltstack() function is the recommended replacement. Refer to the sigaltstack(2) reference page.

RETURN VALUE

The sigstack() function returns the following:

0: Success.
-1: Failure: errno is set to indicate the error.

ERRORS

The sigstack() function will fail if:

[EPERM]: An attempt was made to modify an active stack or an active alternate stack.
[EFAULT]: Either of ss or oss is not a null pointer and points outside the allocated address space of the process. This error is not reliably detected.

WARNINGS

Do not use sigstack() in conjunction with sigspace().

The direction of stack growth is not indicated in the historical definition of struct sigstack. The only way to portably establish a stack pointer is for the application to determine stack growth direction, or to allocate a block of storage and set the stack pointer to the middle. An application can assume that the size of the signal stack is SIGSTKSZ. To specify a signal stack size other than SIGSTKSZ, use the sigaltstack() function.

Leaving the context of a service routine abnormally, such as by longjmp() (see setjmp(3C)), might remove the guarantee that the ordinary execution of the program does not extend into the guaranteed space. It might also cause the program to lose forever its ability to automatically increase the stack size, causing the program to be limited to the guaranteed space.

In HP-UX release 11.i, use of sigstack() in threads created with process contention scope could result in undefined behavior. This function will continue to be reliable in threads created with system contention scope.

Obsolescent Interfaces

sigstack() is to be obsoleted at a future date.

APPLICATION USAGE

A portable application, when being written or rewritten, should use sigaltstack() instead of sigstack().

On some implementations, stack space is automatically extended as needed. On those implementations, automatic extension is typically not available for an alternate stack. If a signal stack overflows, the resulting behavior of the process is undefined.

The direction of stack growth is not indicated in the historical definition of struct sigstack. The only way to portably establish a stack pointer is for the application to determine stack growth direction, or to allocate a block of storage and set the stack pointer to the middle. The implementation may assume that the size of the signal stack is SIGSTKSZ as found in <signal.h>. An implementation that would like to specify a signal stack size other than SIGSTKSZ should use sigaltstack().

Programs should not use longjmp() to leave a signal handler that is running on a stack established with sigstack(). Doing so may disable future use of the signal stack. For abnormal exit from a signal handler, siglongjmp(), setcontext(), or swapcontext() may be used. These functions fully support switching from one stack to another.

Threads Considerations

Each thread (with system contention scope) may define an alternate signal handling stack.

AUTHOR

sigstack() was developed by HP and the University of California, Berkeley.

CHANGE HISTORY

First released in Issue 4, Version 2.