sigaltstack(2) - Linux manual page (original) (raw)
sigaltstack(2) System Calls Manual sigaltstack(2)
NAME top
sigaltstack - set and/or get signal stack contextLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <signal.h>**
**int sigaltstack(const stack_t *_Nullable restrict** _ss_**,**
**stack_t *_Nullable restrict** _oldss_**);**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_SOURCEDESCRIPTION top
**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)](../man2/sigaction.2.html)) 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)](../man2/sigaction.2.html), 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 _oldss_ 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 _stackt_ 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.ssflags_
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)](../man3/swapcontext.3.html). 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.sssp_
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.sssp_ to a suitable address boundary for the underlying
hardware architecture.
_ss.sssize_
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.ssflags_ as **SS_DISABLE**.
In this case, the kernel ignores any other flags in _ss.ssflags_
and the remaining fields in _ss_.
If _oldss_ 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 _oldss.sssp_ and _oldss.sssize_ fields
return the starting address and size of that stack. The
_oldss.ssflags_ 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)](../man3/swapcontext.3.html). 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 _oldss_ as a non-NULL value, one can
obtain the current settings for the alternate signal stack without
changing them.RETURN VALUE top
**sigaltstack**() returns 0 on success, or -1 on failure with _[errno](../man3/errno.3.html)_
set to indicate the error.ERRORS top
**EFAULT** Either _ss_ or _oldss_ is not NULL and points to an area
outside of the process's address space.
**EINVAL** _ss_ is not NULL and the _ssflags_ field contains an invalid
flag.
**ENOMEM** The specified size of the new alternate signal stack
_ss.sssize_ 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).ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ **Interface** │ **Attribute** │ **Value** │
├──────────────────────────────────────┼───────────────┼─────────┤
│ **sigaltstack**() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘STANDARDS top
POSIX.1-2008.
**SS_AUTODISARM** is a Linux extension.HISTORY top
POSIX.1-2001, SUSv2, SVr4.NOTES top
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)](../man2/execve.2.html) removes any existing alternate
signal stack. A child process created via [fork(2)](../man2/fork.2.html) inherits a copy
of its parent's alternate signal stack settings. The same is also
true for a child process created using [clone(2)](../man2/clone.2.html), 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.
BUGS top
In Linux 2.2 and earlier, the only flag that could be specified in
_ss.saflags_ 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.ssflags==SSONSTACK_ with the same meaning as
_ss.ssflags==0_ (i.e., the inclusion of **SS_ONSTACK** in _ss.ssflags_
is a no-op). On other implementations, and according to POSIX.1,
**SS_ONSTACK** appears only as a reported flag in _oldss.ssflags_. On
Linux, there is no need ever to specify **SS_ONSTACK** in _ss.ssflags_,
and indeed doing so should be avoided on portability grounds:
various other systems give an error if **SS_ONSTACK** is specified in
_ss.ssflags_.EXAMPLES top
The following code segment demonstrates the use of **sigaltstack**()
(and [sigaction(2)](../man2/sigaction.2.html)) 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 top
[execve(2)](../man2/execve.2.html), [setrlimit(2)](../man2/setrlimit.2.html), [sigaction(2)](../man2/sigaction.2.html), [siglongjmp(3)](../man3/siglongjmp.3.html),
[sigsetjmp(3)](../man3/sigsetjmp.3.html), [signal(7)](../man7/signal.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 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)