SIGALTSTACK(2) Linux Programmer's Manual SIGALTSTACK(2)
NAME
sigaltstack - set and/or get signal stack context
SYNOPSIS
#include<signal.h>
intsigaltstack(conststack_t*ss,stack_t*oss);
DESCRIPTION
sigaltstack allows a process to define a new alternate signal stack
and/or retrieve the state of an existing alternate signal stack. An
alternate signal stack is used during the execution of a signal handler
if the establishment of that handler (see sigaction(2)) requested it.
The normal sequence of events for using an alternate signal stack is
the following:
1. Allocate an area of memory to be used for the alternate signal
stack.
2. Use sigaltstack to inform the system of the existence and loca-
tion of the alternate signal stack.
3. When establishing a signal handler using sigaction, inform the
system that the signal handler should be executed on the alter-
nate signal stack by specifying the SA_ONSTACK flag.
The ss argument is used to specify a new alternate signal stack, while
the oss argument is used to retrieve information about the currently
established signal stack. If we are interested in performing just one
of these tasks then the other argument can be specified as NULL. Each
of these arguments is a structure of the following type:
typedef struct {
void *ss_sp; /* Base address of stack */
int ss_flags; /* Flags */
size_t ss_size; /* Number of bytes in stack */
} stack_t;
To establish a new alternate signal stack, ss.ss_flags is set to zero,
and ss.sp_sp and ss.ss_size specify the starting address and size of
the stack. The constant SIGSTKSZ is defined to be large enough to
cover the usual size requirements for an alternate signal stack, and
the constant MINSIGSTKSZ defines the minimum size required to execute a
signal handler.
To disable an existing stack, specify ss.ss_flags as SS_DISABLE. In
this case, the remaining fields in ss are ignored.
If oss is not NULL, then it is used to return information about the
alternate signal stack which was in effect prior to the call to sigalt-
stack. The oss.ss_sp and oss.ss_size fields return the starting
address and size of that stack. The oss.ss_flags may return either of
the following values:
SS_ONSTACK
The process is currently executing on the alternate signal
stack. (Note that it is not possible to change the alternate
signal stack if the process is currently executing on it.)
SS_DISABLE
The alternate signal stack is currently disabled.
RETURNVALUE
sigaltstack returns 0 on success, or -1 on failure with errno set to
indicate the error.
ERRORS
ENOMEM The specified size of the new alternate signal stack
(ss.ss_size) was less than MINSTKSZ.
EFAULT Either ss or oss is not NULL and points to an area outside of
the process's address space.
EPERM An attempt was made to change the alternate signal stack while
it was active (i.e., the process was already executing on the
current alternate signal stack).
EINVAL ss is not NULL and the ss_flags field contains a non-zero value
other than SS_DISABLE.
NOTES
The following code segment demonstrates the use of sigaltstack:
stack_t ss;
ss.ss_sp = malloc(SIGSTKSZ);
if (ss.ss_sp == NULL)
/* Handle error */;
ss.ss_size = SIGSTKSZ;
ss.ss_flags = 0;
if (sigaltstack(&ss, NULL) == -1)
/* Handle error */;
Establishing an alternate signal stack is useful if a process expects
that it may exhaust its standard stack. This may occur, for example,
because the stack grows so large that it encounters the upwardly grow-
ing heap, or it reaches a limit established by a call to setr-
limit(RLIMIT_STACK,&rlim). If the standard stack is exhausted, the
kernel sends the process a SIGSEGV signal. In these circumstances the
only way to catch this signal is on an alternate signal stack.
On most hardware architectures supported by Linux, stacks grow down-
wards. sigaltstack automatically takes account of the direction of
stack growth.
Functions called from a signal handler executing on an alternate signal
stack will also use the alternate signal stack. (This also applies to
any handlers invoked for other signals while the process is executing
on the alternate signal stack.) Unlike the standard stack, the system
does not automatically extend the alternate signal stack. Exceeding
the allocated size of the alternate signal stack will lead to unpre-
dictable results.
A successful call to execve removes any existing alternate signal
stack.
sigaltstack supersedes the older sigstack call. For backwards compati-
bility, glibc also provides sigstack. All new applications should be
written using sigaltstack.
HISTORY
BSD 4.2 had a sigstack() system call. It used a slightly different
struct, and had as major disadvantage that the caller had to know the
direction of stack growth.
CONFORMINGTO
SUSv2, SVr4, POSIX 1003.1-2001.
SEEALSO
sigaction(2), setrlimit(2), execve(2), sigsetjmp(3), siglongjmp(3),
signal(7)
Linux 2.4 2001-09-27 SIGALTSTACK(2)
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