'\" t
-.\" Copyright (c) 2001, Michael Kerrisk (mtk.manpages@gmail.com)
+.\" Copyright (c) 2001, 2017 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
-.\" the use of the information contained herein.
+.\" the use of the information contained herein. The author(s) may not
+.\" have taken the same level of care in the production of this manual,
+.\" which is licensed free of charge, as they might when working
+.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" aeb, various minor fixes
-.TH SIGALTSTACK 2 2007-07-26 "Linux" "Linux Programmer's Manual"
+.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>
-.sp
-.BI "int sigaltstack(const stack_t *" ss ", stack_t *" oss );
-.sp
+.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
-.sp
+.PP
.BR sigaltstack ():
-_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
-.\" .br
-.\" .BR sigstack ():
-.\" _BSD_SOURCE || _XOPEN_SOURCE >= 500
+.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
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
inform the system that the signal handler should be executed
on the alternate signal stack by
specifying the \fBSA_ONSTACK\fP flag.
-.P
+.PP
The \fIss\fP argument is used to specify a new
-alternate signal stack, while the \fIoss\fP argument
+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.
-Each of these arguments is a structure of the following type:
-.sp
+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
-.nf
+.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;
-.fi
+.EE
.in
-
+.PP
To establish a new alternate signal stack,
-\fIss.ss_flags\fP is set to zero, and \fIss.ss_sp\fP and
-\fIss.ss_size\fP specify the starting address and size of
-the 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.
-
-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.
-
+.PP
To disable an existing stack, specify \fIss.ss_flags\fP
as \fBSS_DISABLE\fP.
-In this case, the remaining fields
-in \fIss\fP are ignored.
-
-If \fIoss\fP is not NULL, then it is used to return information about
+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 \fIoss.ss_sp\fP and \fIoss.ss_size\fP fields return the starting
+The \fIold_ss.ss_sp\fP and \fIold_ss.ss_size\fP fields return the starting
address and size of that stack.
-The \fIoss.ss_flags\fP may return either of the following values:
+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.
.TP
.B SS_DISABLE
The alternate signal stack is currently disabled.
-.SH "RETURN VALUE"
+.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 \fIoss\fP is not NULL and points to an area
+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
-a nonzero value other than
-.BR SS_DISABLE .
+an invalid flag.
.TP
.B ENOMEM
The specified size of the new alternate signal stack
-(\fIss.ss_size\fP) was less than \fBMINSTKSZ\fP.
+.I ss.ss_size
+was less than
+.BR MINSIGSTKSZ .
.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 "CONFORMING TO"
-SUSv2, SVr4, POSIX.1-2001.
+.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
.B SIGSEGV
cannot be invoked on the process stack; if we wish to handle it,
we must use an alternate signal stack.
-.P
+.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
the process a \fBSIGSEGV\fP signal.
In these circumstances the only way to catch this signal is
on an alternate signal stack.
-.P
+.PP
On most hardware architectures supported by Linux, stacks grow
-downwards.
+downward.
.BR sigaltstack ()
automatically takes account
of the direction of stack growth.
-.P
+.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
automatically extend the alternate signal stack.
Exceeding the allocated size of the alternate signal stack will
lead to unpredictable results.
-.P
+.PP
A successful call to
.BR execve (2)
removes any existing alternate
signal stack.
-.P
+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 backwards compatibility, glibc also provides
+For backward compatibility, glibc also provides
.BR sigstack ().
All new applications should be written using
.BR sigaltstack ().
had to know the direction of stack growth.
.SH EXAMPLE
The following code segment demonstrates the use of
-.BR sigaltstack ():
-
+.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
-.nf
+.EX
stack_t ss;
ss.ss_sp = malloc(SIGSTKSZ);
-if (ss.ss_sp == NULL)
- /* Handle error */;
+if (ss.ss_sp == NULL) {
+ perror("malloc");
+ exit(EXIT_FAILURE);
+}
+
ss.ss_size = SIGSTKSZ;
ss.ss_flags = 0;
-if (sigaltstack(&ss, NULL) == \-1)
- /* Handle error */;
-.fi
+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 "SEE ALSO"
+.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),
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