SIGALTSTACK(2) Linux Programmer's Manual SIGALTSTACK(2)
NAME
sigaltstack - set and/or get signal stack context
SYNOPSIS
#include <signal.h>
int sigaltstack(const stack_t *ss, stack_t *old_ss);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sigaltstack():
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
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 location
of the alternate signal stack.
3. When establishing a signal handler using sigaction(2), inform the
system that the signal handler should be executed on the alternate
signal stack by specifying the SA_ONSTACK flag.
The ss argument is used to specify a new alternate signal stack, while
the old_ss 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.
The stack_t type used to type the arguments of this function is defined
as follows:
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, the fields of this structure
are set as follows:
ss.ss_flags
This field contains either 0, or the following flag:
SS_AUTODISARM (since Linux 4.7)
Clear the alternate signal stack settings on entry to the
signal handler. When the signal handler returns, the
previous alternate signal stack settings are restored.
This flag was added in order make it safe to switch away
from the signal handler with swapcontext(3). Without
this flag, a subsequently handled signal will corrupt the
state of the switched-away signal handler. On kernels
where this flag is not supported, sigaltstack() fails
with the error EINVAL when this flag is supplied.
ss.ss_sp
This field specifies the starting address of the stack. When a
signal handler is invoked on the alternate stack, the kernel au-
tomatically aligns the address given in ss.ss_sp to a suitable
address boundary for the underlying hardware architecture.
ss.ss_size
This field specifies the 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 sig-
nal handler.
To disable an existing stack, specify ss.ss_flags as SS_DISABLE. In
this case, the kernel ignores any other flags in ss.ss_flags and the
remaining fields in ss.
If old_ss 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 old_ss.ss_sp and old_ss.ss_size fields return the start-
ing address and size of that stack. The old_ss.ss_flags may return ei-
ther 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.
Alternatively, this value is returned if the process is cur-
rently executing on an alternate signal stack that was estab-
lished using the SS_AUTODISARM flag. In this case, it is safe
to switch away from the signal handler with swapcontext(3). It
is also possible to set up a different alternative signal stack
using a further call to sigaltstack().
SS_AUTODISARM
The alternate signal stack has been marked to be autodisarmed as
described above.
By specifying ss as NULL, and old_ss as a non-NULL value, one can ob-
tain the current settings for the alternate signal stack without chang-
ing them.
RETURN VALUE
sigaltstack() returns 0 on success, or -1 on failure with errno set to
indicate the error.
ERRORS
EFAULT Either ss or old_ss is not NULL and points to an area outside of
the process's address space.
EINVAL ss is not NULL and the ss_flags field contains an invalid flag.
ENOMEM The specified size of the new alternate signal stack ss.ss_size
was less than MINSIGSTKSZ.
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).
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
+--------------+---------------+---------+
|Interface | Attribute | Value |
+--------------+---------------+---------+
|sigaltstack() | Thread safety | MT-Safe |
+--------------+---------------+---------+
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SUSv2, SVr4.
The SS_AUTODISARM flag is a Linux extension.
NOTES
The most common usage of an alternate signal stack is to handle the
SIGSEGV signal that is generated if the space available for the normal
process stack is exhausted: in this case, a signal handler for SIGSEGV
cannot be invoked on the process stack; if we wish to handle it, we
must use an alternate signal stack.
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-
ward. 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(2) removes any existing alternate signal
stack. A child process created via fork(2) inherits a copy of its par-
ent's alternate signal stack settings.
sigaltstack() supersedes the older sigstack() call. For backward com-
patibility, glibc also provides sigstack(). All new applications
should be written using sigaltstack().
History
4.2BSD had a sigstack() system call. It used a slightly different
struct, and had the major disadvantage that the caller had to know the
direction of stack growth.
BUGS
In Linux 2.2 and earlier, the only flag that could be specified in
ss.sa_flags was SS_DISABLE. In the lead up to the release of the Linux
2.4 kernel, a change was made to allow sigaltstack() to allow
ss.ss_flags==SS_ONSTACK with the same meaning as ss.ss_flags==0 (i.e.,
the inclusion of SS_ONSTACK in ss.ss_flags is a no-op). On other im-
plementations, and according to POSIX.1, SS_ONSTACK appears only as a
reported flag in old_ss.ss_flags. On Linux, there is no need ever to
specify SS_ONSTACK in ss.ss_flags, and indeed doing so should be
avoided on portability grounds: various other systems give an error if
SS_ONSTACK is specified in ss.ss_flags.
EXAMPLES
The following code segment demonstrates the use of sigaltstack() (and
sigaction(2)) to install an alternate signal stack that is employed by
a handler for the SIGSEGV signal:
stack_t ss;
ss.ss_sp = malloc(SIGSTKSZ);
if (ss.ss_sp == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
ss.ss_size = SIGSTKSZ;
ss.ss_flags = 0;
if (sigaltstack(&ss, NULL) == -1) {
perror("sigaltstack");
exit(EXIT_FAILURE);
}
sa.sa_flags = SA_ONSTACK;
sa.sa_handler = handler(); /* Address of a signal handler */
sigemptyset(&sa.sa_mask);
if (sigaction(SIGSEGV, &sa, NULL) == -1) {
perror("sigaction");
exit(EXIT_FAILURE);
}
SEE ALSO
execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3),
signal(7)
COLOPHON
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latest version of this page, can be found at
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Linux 2020-06-09 SIGALTSTACK(2)