sigaltstack
NAME
sigaltstack - set and/or get signal stack context
SYNOPSIS
#include <signal.h>
I int sigaltstack(const stack_t * ss , stack_t * oss );
DESCRIPTION
R 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
R 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 oss 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:
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,
ss.ss_flags is set to zero, and ss.ss_sp and
ss.ss_size specify the starting address and 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.
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.
To disable an existing stack, specify ss.ss_flags
as SS_DISABLE.
In this case, the remaining fields
in ss are ignored.
If oss is not NULL, then it is used to return information about
the alternate signal stack which was in effect prior to the
call to
R sigaltstack ().
The oss.ss_sp and oss.ss_size fields return the starting
address and size of that stack.
The oss.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.
RETURN VALUE
R sigaltstack ()
returns 0 on success, or -1 on failure with
errno set to indicate the error.
ERRORS
EFAULT
Either ss or oss 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
a non-zero value other than
R SS_DISABLE .
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).
CONFORMING TO
SUSv2, SVr4, POSIX.1-2001.
NOTES
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
downwards.
R 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.
R sigaltstack ()
supersedes the older
R sigstack ()
call.
For backwards compatibility, glibc also provides
R sigstack ().
All new applications should be written using
R sigaltstack ().
History
4.2BSD had a R 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.EXAMPLE
The following code segment demonstrates the use of
R 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 */;
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