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SIGALTSTACK(2) Linux Programmer's Manual SIGALTSTACK(2)
sigaltstack - set and/or get signal stack context
#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
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 struc‐
ture 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, sigalt‐
stack() 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
automatically aligns the address given in ss.ss_sp to a suit‐
able address boundary for the underlying hardware architec‐
ture.
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
signal 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
sigaltstack(). The old_ss.ss_sp and old_ss.ss_size fields return the
starting address and size of that stack. The old_ss.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.
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
obtain the current settings for the alternate signal stack without
changing them.
sigaltstack() returns 0 on success, or -1 on failure with errno set
to indicate the error.
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 MINSTKSZ.
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).
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────┼───────────────┼─────────┤
│sigaltstack() │ Thread safety │ MT-Safe │
└──────────────┴───────────────┴─────────┘
POSIX.1-2001, POSIX.1-2008, SUSv2, SVr4.
The SS_AUTODISARM flag is a Linux extension.
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
growing heap, or it reaches a limit established by a call to
setrlimit(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
downward. 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 unpredictable 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
parent's alternate signal stack settings.
sigaltstack() supersedes the older sigstack() call. For backward
compatibility, 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.
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);
}
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 implementations, 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.
execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3),
signal(7)
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-11-08 SIGALTSTACK(2)
Pages that refer to this page: execve(2), getrlimit(2), sigaction(2), sigreturn(2), syscalls(2), getcontext(3), makecontext(3), pthread_create(3), sigvec(3), pthreads(7), signal(7)
Copyright and license for this manual page
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