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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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sigaltstack(2) System Calls Manual sigaltstack(2)
sigaltstack - set and/or get signal stack context
Standard C library (libc, -lc)
#include <signal.h>
int sigaltstack(const stack_t *_Nullable restrict ss,
stack_t *_Nullable restrict 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 <= 2.19: */ _BSD_SOURCE
sigaltstack() allows a thread 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 to 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 automatically 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 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 thread is currently executing on the alternate signal
stack. (Note that it is not possible to change the
alternate signal stack if the thread is currently
executing on it.)
SS_DISABLE
The alternate signal stack is currently disabled.
Alternatively, this value is returned if the thread is
currently executing on an alternate signal stack that was
established 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 MINSIGSTKSZ.
EPERM An attempt was made to change the alternate signal stack
while it was active (i.e., the thread 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.1-2008.
SS_AUTODISARM is a Linux extension.
POSIX.1-2001, SUSv2, SVr4.
The most common usage of an alternate signal stack is to handle
the SIGSEGV signal that is generated if the space available for
the standard stack is exhausted: in this case, a signal handler
for SIGSEGV cannot be invoked on the standard stack; if we wish
to handle it, we must use an alternate signal stack.
Establishing an alternate signal stack is useful if a thread
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 thread 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
thread 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. The same
is also true for a child process created using clone(2), unless
the clone flags include CLONE_VM and do not include CLONE_VFORK,
in which case any alternate signal stack that was established in
the parent is disabled in the child process.
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.
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.
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);
}
execve(2), setrlimit(2), sigaction(2), siglongjmp(3),
sigsetjmp(3), signal(7)
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Linux man-pages 6.9.1 2024-05-02 sigaltstack(2)
Pages that refer to this page: clone(2), execve(2), getrlimit(2), sigaction(2), sigreturn(2), syscalls(2), getcontext(3), makecontext(3), pthread_create(3), sigvec(3), pthreads(7), signal(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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