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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
structure are set as follows:
ss.ss_flags
This field is set to zero.
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 remaining fields in ss are ignored.
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.
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.1-2009, 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
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():
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 */;
In the lead up to the development of the Linux 2.4 kernel, someone
got confused and allowed the kernel to accept SS_ONSTACK in
ss.ss_flags, which results in behavior that is the same as when
ss_flags is 0. On other implementations, and according to POSIX.1,
SS_ONSTACK appears only as a reported flag in old_ss.ss_flags. There
is no need ever to specify this flag in ss.ss_flags (and indeed,
doing so decreases portability, since some implementations 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 4.12 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 2017-07-13 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)