'\" t
-.\" Copyright (c) 2001, Michael Kerrisk (mtk-manpages@gmx.net)
+.\" Copyright (c) 2001, 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.\" manual page may be incorrect or out-of-date.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 2001-09-27 "Linux 2.4" "Linux Programmer's Manual"
+.TH SIGALTSTACK 2 2017-07-13 "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 );
+.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
-\fBsigaltstack\fP() allows a process to define a new alternate
+.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
+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
+.TP 3
1.
Allocate an area of memory to be used for the alternate
signal stack.
.TP
2.
-Use \fBsigaltstack\fP() to inform the system of the existence and
+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 \fBsigaction\fP(),
+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.
.P
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
-.RS
+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:
+
+.in +4n
.nf
typedef struct {
void *ss_sp; /* Base address of stack */
size_t ss_size; /* Number of bytes in stack */
} stack_t;
.fi
-.RE
+.in
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 is set to zero.
+.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 bundary 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
+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
+.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 \fBsigaltstack\fP().
-The \fIoss.ss_sp\fP and \fIoss.ss_size\fP fields return the starting
+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 \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. (Note that it is not possible
+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.
-.SH "RETURN VALUE"
-\fBsigaltstack\fP() returns 0 on success, or \-1 on failure with
+.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 \fPss_flags\fP field contains
-a non-zero value other than SS_DISABLE.
+\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
-(\fIss.ss_size\fP) was less than \fBMINSTKSZ\fP.
+.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.1-2009, SUSv2, SVr4.
.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,
+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.
-
-The following code segment demonstrates the use of \fBsigaltstack\fP():
-
-.RS
-.nf
-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 */;
-.fi
-.RE
.P
Establishing an alternate signal stack is useful if a process
expects that it may exhaust its standard stack.
on an alternate signal stack.
.P
On most hardware architectures supported by Linux, stacks grow
-downwards. \fBsigaltstack\fP() automatically takes account
+downward.
+.BR sigaltstack ()
+automatically takes account
of the direction of stack growth.
.P
Functions called from a signal handler executing on an alternate
Exceeding the allocated size of the alternate signal stack will
lead to unpredictable results.
.P
-A successful call to \fBexecve\fP() removes any existing alternate
+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.
.P
-\fBsigaltstack\fP() supersedes the older \fBsigstack\fP() call.
-For backwards compatibility, glibc also provides \fBsigstack\fP().
-All new applications should be written using \fBsigaltstack\fP().
-.SH HISTORY
-4.2BSD had a \fIsigstack\fP() system call. It used a slightly
+.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 "CONFORMING TO"
-SUSv2, SVr4, POSIX 1003.1-2001.
-.SH "SEE ALSO"
+.SH EXAMPLE
+The following code segment demonstrates the use of
+.BR sigaltstack ():
+
+.in +4n
+.nf
+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 */;
+.fi
+.in
+.SH BUGS
+In the lead up to the development of the Linux 2.4 kernel,
+.\" Linux 2.3.40
+someone got confused and allowed the kernel to accept
+.B SS_ONSTACK
+in
+.IR ss.ss_flags ,
+which results in behavior that is the same as when
+.I ss_flags
+is 0.
+On other implementations, and according to POSIX.1,
+.B SS_ONSTACK
+appears only as a reported flag in
+.IR old_ss.ss_flags .
+There is no need ever to specify this flag in
+.IR ss.ss_flags
+(and indeed, doing so decreases portability,
+since some implementations
+.\" 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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