sigaltstack() — Set or get signal alternate stack context

Standards

Standards / Extensions	C or C++	Dependencies
XPG4.2 Single UNIX Specification, Version 3	both	POSIX(ON)

Format

#define _XOPEN_SOURCE_EXTENDED 1
#include <signal.h>

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

General description

The sigaltstack() function allows a thread to define and examine the state of an alternate stack for signal handlers. Signals that have been explicitly declared to execute on the alternate stack will be delivered on the alternate stack.

Note: To explicitly declare that a signal catcher is to run on the alternate signal stack, the SA_ONSTACK flag must be set in the sa_flags when the signal action is set using sigaction().

If ss is not a NULL pointer, it points to a stack_t structure that specifies the alternate signal stack that will take effect upon return from sigaltstack(). The ss_flags member specifies the new stack state. If it is set to SS_DISABLE, the stack is disabled and ss_sp and ss_size are ignored. Otherwise the stack will be enabled, and the ss_sp and ss_size members specify the new address and size of the stack.

AMODE 64 considerations: Storage for this stack must be above the 2GB bar. It may not be storage acquired with the __malloc24() or __malloc31() functions.

The range of addresses starting at ss_sp, up to but not including ss_sp + ss_size, is available to the implementation for use as the stack. This interface makes no assumptions regarding which end is the stack base and in which direction the stack grows as items are pushed.

If oss is not a NULL pointer, on successful completion it will point to a stack_t structure that specifies the alternate signal stack that was in effect before the call to sigaltstack(). The ss_sp and ss_size members specify the address and size of that stack. The ss_flags member specifies the stack's state, and may contain one of the following values:

SS_ONSTACK: The thread is currently executing on the alternate signal stack. Attempts to modify the alternate signal stack while the thread is executing on it fails. This flag must not be modified by threads.
SS_DISABLE: The alternate signal stack is currently disabled.

The value SIGSTKSZ is a system default specifying the number of bytes that would be used to cover the usual case when manually allocating an alternate stack area. The value MINSIGSTKSZ is defined to be the minimum stack size for a signal handler. In computing an alternate signal stack size, a program should add that amount to its stack requirements to allow for the system implementation overhead. The constants SS_ONSTACK, SS_DISABLE, SIGSTKSZ, and MINSIGSTKSZ are defined in <signal.h>.

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

Notes:

If a signal handler is enabled to run on an alternate stack, then all functions called by that signal handler must be compiled with the same linkage. For example, if the signal handler is compiled with XPLINK, then all functions it calls must also be compiled XPLINK. Since only one alternate stack can be supplied, no mixing of linkages (which would require both upward and downward-growing alternate stacks) is allowed. The type of stack created will be based on the attributes of the signal handler to be given control. If the signal handler has been compiled with XPLINK, then a downward-growing stack will be created in the alternate stack, including, in AMODE 31, using enough storage in the user stack to create a 4k read-only guard page (aligned on a 4k boundary).
If a new signal is received while a signal handler is running on an alternate stack, and that new signal specified a signal handler that also runs on the alternate stack, then both signal handlers must have been compiled with the same linkage (XPLINK versus non-XPLINK).

Returned value

If successful, sigaltstack() returns 0.

If unsuccessful, sigaltstack() returns -1 and sets errno to one of the following values:

Error Code: Description
EINVAL: 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 MINSIGSTKSZ.
EPERM: An attempt was made to modify an active stack.