'\" t
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.\" aeb, various minor fixes
.TH SIGALTSTACK 2 2017-11-08 "Linux" "Linux Programmer's Manual"
.SH NAME
sigaltstack \- set and/or get signal stack context
.SH SYNOPSIS
.B #include <signal.h>
.PP
.BI "int sigaltstack(const stack_t *" ss ", stack_t *" old_ss );
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR sigaltstack ():
.ad l
.RS 4
.PD 0
_XOPEN_SOURCE\ >=\ 500
.\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
    || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE
.PD
.RE
.ad
.SH DESCRIPTION
.BR 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
.BR sigaction (2))
requested it.
.PP
The normal sequence of events for using an alternate signal stack
is the following:
.TP 3
1.
Allocate an area of memory to be used for the alternate
signal stack.
.TP
2.
Use
.BR sigaltstack ()
to inform the system of the existence and
location of the alternate signal stack.
.TP
3.
When establishing a signal handler using
.BR sigaction (2),
inform the system that the signal handler should be executed
on the alternate signal stack by
specifying the \fBSA_ONSTACK\fP flag.
.PP
The \fIss\fP argument is used to specify a new
alternate signal stack, while the \fIold_ss\fP 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.
.PP
The
.I stack_t
type used to type the arguments of this function is defined as follows:
.PP
.in +4n
.EX
typedef struct {
    void  *ss_sp;     /* Base address of stack */
    int    ss_flags;  /* Flags */
    size_t ss_size;   /* Number of bytes in stack */
} stack_t;
.EE
.in
.PP
To establish a new alternate signal stack,
the fields of this structure are set as follows:
.TP
.I ss.ss_flags
This field contains either 0, or the following flag:
.RS
.TP
.BR SS_AUTODISARM " (since Linux 4.7)"
.\" commit 2a74213838104a41588d86fd5e8d344972891ace
.\" See tools/testing/selftests/sigaltstack/sas.c in kernel sources
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.
.IP
This flag was added in order make it safe
to switch away from the signal handler with
.BR 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,
.BR sigaltstack ()
fails with the error
.BR EINVAL
when this flag is supplied.
.RE
.TP
.I 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 \fIss.ss_sp\fP
to a suitable address boundary for the underlying hardware architecture.
.TP
.I ss.ss_size
This field specifies the size of the stack.
The constant \fBSIGSTKSZ\fP is defined to be large enough
to cover the usual size requirements for an alternate signal stack,
and the constant \fBMINSIGSTKSZ\fP defines the minimum
size required to execute a signal handler.
.PP
To disable an existing stack, specify \fIss.ss_flags\fP
as \fBSS_DISABLE\fP.
In this case, the kernel ignores any other flags in
.IR ss.ss_flags
and the remaining fields
in \fIss\fP.
.PP
If \fIold_ss\fP is not NULL, then it is used to return information about
the alternate signal stack which was in effect prior to the
call to
.BR sigaltstack ().
The \fIold_ss.ss_sp\fP and \fIold_ss.ss_size\fP fields return the starting
address and size of that stack.
The \fIold_ss.ss_flags\fP may return either of the following values:
.TP
.B 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.)
.TP
.B SS_DISABLE
The alternate signal stack is currently disabled.
.IP
Alternatively, this value is returned if the process is currently
executing on an alternate signal stack that was established using the
.B SS_AUTODISARM
flag.
In this case, it is safe to switch away from the signal handler with
.BR swapcontext (3).
It is also possible to set up a different alternative signal stack
using a further call to
.BR sigaltstack ().
.\" FIXME Was it intended that one can set up a different alternative
.\" signal stack in this scenario? (In passing, if one does this, the
.\" sigaltstack(NULL, &old_ss) now returns old_ss.ss_flags==SS_AUTODISARM
.\" rather than old_ss.ss_flags==SS_DISABLE. The API design here seems
.\" confusing...
.TP
.B SS_AUTODISARM
The alternate signal stack has been marked to be autodisarmed
as described above.
.PP
By specifying
.I ss
as NULL, and
.I old_ss
as a non-NULL value, one can obtain the current settings for
the alternate signal stack without changing them.
.SH RETURN VALUE
.BR sigaltstack ()
returns 0 on success, or \-1 on failure with
\fIerrno\fP set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
Either \fIss\fP or \fIold_ss\fP is not NULL and points to an area
outside of the process's address space.
.TP
.B EINVAL
\fIss\fP is not NULL and the \fIss_flags\fP field contains
an invalid flag.
.TP
.B ENOMEM
The specified size of the new alternate signal stack
.I ss.ss_size
was less than
.BR MINSTKSZ .
.TP
.B 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).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR sigaltstack ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SUSv2, SVr4.
.PP
The
.B SS_AUTODISARM
flag is a Linux extension.
.SH NOTES
The most common usage of an alternate signal stack is to handle the
.B SIGSEGV
signal that is generated if the space available for the
normal process stack is exhausted: in this case, a signal handler for
.B SIGSEGV
cannot be invoked on the process stack; if we wish to handle it,
we must use an alternate signal stack.
.PP
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 \fBsetrlimit(RLIMIT_STACK, &rlim)\fP.
If the standard stack is exhausted, the kernel sends
the process a \fBSIGSEGV\fP signal.
In these circumstances the only way to catch this signal is
on an alternate signal stack.
.PP
On most hardware architectures supported by Linux, stacks grow
downward.
.BR sigaltstack ()
automatically takes account
of the direction of stack growth.
.PP
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.
.PP
A successful call to
.BR execve (2)
removes any existing alternate
signal stack.
A child process created via
.BR fork (2)
inherits a copy of its parent's alternate signal stack settings.
.PP
.BR sigaltstack ()
supersedes the older
.BR sigstack ()
call.
For backward compatibility, glibc also provides
.BR sigstack ().
All new applications should be written using
.BR sigaltstack ().
.SS History
4.2BSD had a
.BR 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.
.SH EXAMPLE
The following code segment demonstrates the use of
.BR sigaltstack ()
(and
.BR sigaction (2))
to install an alternate signal stack that is employed by a handler
for the
.BR SIGSEGV
signal:
.PP
.in +4n
.EX
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);
}
.EE
.in
.SH BUGS
In Linux 2.2 and earlier, the only flag that could be specified
in
.I ss.sa_flags
was
.BR SS_DISABLE .
In the lead up to the release of the Linux 2.4 kernel,
.\" Linux 2.3.40
.\" After quite a bit of web and mail archive searching,
.\" I could not find the patch on any mailing list, and I
.\" could find no place where the rationale for this change
.\" explained -- mtk
a change was made to allow
.BR sigaltstack ()
to allow
.I ss.ss_flags==SS_ONSTACK
with the same meaning as
.IR "ss.ss_flags==0"
(i.e., the inclusion of
.B SS_ONSTACK
in
.I ss.ss_flags
is a no-op).
On other implementations, and according to POSIX.1,
.B SS_ONSTACK
appears only as a reported flag in
.IR old_ss.ss_flags .
On Linux, there is no need ever to specify
.B SS_ONSTACK
in
.IR ss.ss_flags ,
and indeed doing so should be avoided on portability grounds:
various other systems
.\" See the source code of Illumos and FreeBSD, for example.
give an error if
.B SS_ONSTACK
is specified in
.IR ss.ss_flags .
.SH SEE ALSO
.BR execve (2),
.BR setrlimit (2),
.BR sigaction (2),
.BR siglongjmp (3),
.BR sigsetjmp (3),
.BR signal (7)
.SH COLOPHON
This page is part of release 4.16 of the Linux
.I 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/.