sigaction的使用
sigaction的原型如下：
int sigaction ( int signo, const struct sigaction * restrict act,
struct sigaction * restrict oact) ;
SIGACTION(2) -- 2010-06-16 -- Linux -- Linux Programmer's Manual
NAME
sigaction - examine and change a signal action
SYNOPSIS
#include
int sigaction(int signum, const struct sigaction *act,
struct sigaction *oldact);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
sigaction(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE ||
_POSIX_SOURCE
DESCRIPTION
The sigaction() system call is used to change the action
taken by a process on receipt of a specific signal. (See
signal(7) for an overview of signals.)
signum specifies the signal and can be any valid signal
except SIGKILL and SIGSTOP.
If act is non-NULL, the new action for signal signum is
installed from act. If oldact is non-NULL, the previous
action is saved in oldact.
The sigaction structure is defined as something like:
struct sigaction {
void (*sa_handler)(int);
void (*sa_sigaction)(int, siginfo_t *, void *);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
};
On some architectures a union is involved: do not assign to
both sa_handler and sa_sigaction.
The sa_restorer element is obsolete and should not be used.
POSIX does not specify a sa_restorer element.
sa_handler specifies the action to be associated with signum
and may be SIG_DFL for the default action, SIG_IGN to ignore
this signal, or a pointer to a signal handling function.
This function receives the signal number as its only
argument.
If SA_SIGINFO is specified in sa_flags, then sa_sigaction
(instead of sa_handler) specifies the signal-handling
function for signum. This function receives the signal
number as its first argument, a pointer to a siginfo_t as
its second argument and a pointer to a ucontext_t (cast to
void *) as its third argument.
sa_mask specifies a mask of signals which should be blocked
(i.e., added to the signal mask of the thread in which the
signal handler is invoked) during execution of the signal
handler. In addition, the signal which triggered the
handler will be blocked, unless the SA_NODEFER flag is used.
sa_flags specifies a set of flags which modify the behavior
of the signal. It is formed by the bitwise OR of zero or
more of the following:
SA_NOCLDSTOP
If signum is SIGCHLD, do not receive notification
when child processes stop (i.e., when they receive
one of SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU) or
resume (i.e., they receive SIGCONT) (see wait(2)).
This flag is only meaningful when establishing a
handler for SIGCHLD.
SA_NOCLDWAIT (since Linux 2.6)
If signum is SIGCHLD, do not transform children
into zombies when they terminate. See also
waitpid(2). This flag is only meaningful when
establishing a handler for SIGCHLD, or when setting
that signal's disposition to SIG_DFL.
If the SA_NOCLDWAIT flag is set when establishing a
handler for SIGCHLD, POSIX.1 leaves it unspecified
whether a SIGCHLD signal is generated when a child
process terminates. On Linux, a SIGCHLD signal is
generated in this case; on some other
implementations, it is not.
SA_NODEFER
Do not prevent the signal from being received from
within its own signal handler. This flag is only
meaningful when establishing a signal handler.
SA_NOMASK is an obsolete, nonstandard synonym for
this flag.
SA_ONSTACK
Call the signal handler on an alternate signal
stack provided by sigaltstack(2). If an alternate
stack is not available, the default stack will be
used. This flag is only meaningful when
establishing a signal handler.
SA_RESETHAND
Restore the signal action to the default state once
the signal handler has been called. This flag is
only meaningful when establishing a signal handler.
SA_ONESHOT is an obsolete, nonstandard synonym for
this flag.
SA_RESTART
Provide behavior compatible with BSD signal
semantics by making certain system calls
restartable across signals. This flag is only
meaningful when establishing a signal handler. See
signal(7) for a discussion of system call
restarting.
SA_SIGINFO (since Linux 2.2)
The signal handler takes 3 arguments, not one. In
this case, sa_sigaction should be set instead of
sa_handler. This flag is only meaningful when
establishing a signal handler.
The siginfo_t argument to sa_sigaction is a struct with the
following elements:
siginfo_t {
int si_signo; /* Signal number */
int si_errno; /* An errno value */
int si_code; /* Signal code */
int si_trapno; /* Trap number that caused
hardware-generated signal
(unused on most architectures) */
pid_t si_pid; /* Sending process ID */
uid_t si_uid; /* Real user ID of sending process */
int si_status; /* Exit value or signal */
clock_t si_utime; /* User time consumed */
clock_t si_stime; /* System time consumed */
sigval_t si_value; /* Signal value */
int si_int; /* POSIX.1b signal */
void *si_ptr; /* POSIX.1b signal */
int si_overrun; /* Timer overrun count; POSIX.1b timers */
int si_timerid; /* Timer ID; POSIX.1b timers */
void *si_addr; /* Memory location which caused fault */
long si_band; /* Band event (was int in
glibc 2.3.2 and earlier) */
int si_fd; /* File descriptor */
short si_addr_lsb; /* Least significant bit of address
(since kernel 2.6.32) */
}
si_signo, si_errno and si_code are defined for all signals.
(si_errno is generally unused on Linux.) The rest of the
struct may be a union, so that one should only read the
fields that are meaningful for the given signal:
* Signals sent with kill(2) and sigqueue(2) fill in si_pid
and si_uid. In addition, signals sent with sigqueue(2)
fill in si_int and si_ptr with the values specified by the
sender the signal; see sigqueue(2) for more details.
* Signals sent by POSIX.1b timers (since Linux 2.6) fill in
si_overrun and si_timerid. The si_timerid field is an
internal ID used by the kernel to identify the timer; it
is not the same as the timer ID returned by
timer_create(2). The si_overrun field is the timer
overrun count; this is the same information as is obtained
by a call to timer_getoverrun(2). These fields are
nonstandard Linux extensions.
* Signals sent for message queue notification (see the
description of SIGEV_SIGNAL in mq_notify(3)) fill in
si_int/si_ptr, with the sigev_value supplied to
mq_notify(3); si_pid, with the process ID of the message
sender; and si_uid, with the real user ID of the message
sender.
* SIGCHLD fills in si_pid, si_uid, si_status, si_utime and
si_stime, providing information about the child. The
si_pid field is the process ID of the child; si_uid is the
child's real user ID. The si_status field contains the
exit status of the child (if si_code is CLD_EXITED), or
the signal number that caused the process to change state.
The si_utime and si_stime contain the user and system CPU
time used by the child process; these fields do not
include the times used by waited-for children (unlike
getrusage(2) and time(2)). In kernels up to 2.6, and
since 2.6.27, these fields report CPU time in units of
sysconf(_SC_CLK_TCK). In 2.6 kernels before 2.6.27, a bug
meant that these fields reported time in units of the
(configurable) system jiffy (see time(7)).
* SIGILL, SIGFPE, SIGSEGV, SIGBUS, and SIGTRAP fill in
si_addr with the address of the fault. On some
architectures, these signals also fill in the si_trapno
filed. Some suberrors of SIGBUS, in particular
BUS_MCEERR_AO and BUS_MCEERR_AR, also fill in si_addr_lsb.
This field indicates the least significant bit of the
reported address and therefore the extent of the
corruption. For example, if a full page was corrupted,
si_addr_lsb contains log2(sysconf(_SC_PAGESIZE)).
BUS_MCERR_* and si_addr_lsb are Linux-specific extensions.
* SIGPOLL/SIGIO fills in si_band and si_fd. The si_band
event is a bit mask containing the same values as are
filled in the revents field by poll(2). The si_fd field
indicates the file descriptor for which the I/O event
occurred.
si_code is a value (not a bit mask) indicating why this
signal was sent. The following list shows the values which
can be placed in si_code for any signal, along with reason
that the signal was generated.
SI_USER kill(2) or raise(3)
SI_KERNEL Sent by the kernel.
SI_QUEUE sigqueue(2)
SI_TIMER POSIX timer expired
SI_MESGQ POSIX message queue state changed (since
Linux 2.6.6); see mq_notify(3)
SI_ASYNCIO AIO completed
SI_SIGIO queued SIGIO
SI_TKILL tkill(2) or tgkill(2) (since Linux
2.4.19)
The following values can be placed in si_code for a SIGILL
signal:
ILL_ILLOPC illegal opcode
ILL_ILLOPN illegal operand
ILL_ILLADR illegal addressing mode
ILL_ILLTRP illegal trap
ILL_PRVOPC privileged opcode
ILL_PRVREG privileged register
ILL_COPROC coprocessor error
ILL_BADSTK internal stack error
The following values can be placed in si_code for a SIGFPE
signal:
FPE_INTDIV integer divide by zero
FPE_INTOVF integer overflow
FPE_FLTDIV floating-point divide by zero
FPE_FLTOVF floating-point overflow
FPE_FLTUND floating-point underflow
FPE_FLTRES floating-point inexact result
FPE_FLTINV floating-point invalid operation
FPE_FLTSUB subscript out of range
The following values can be placed in si_code for a SIGSEGV
signal:
SEGV_MAPERR address not mapped to object
SEGV_ACCERR invalid permissions for mapped object
The following values can be placed in si_code for a SIGBUS
signal:
BUS_ADRALN invalid address alignment
BUS_ADRERR nonexistent physical address
BUS_OBJERR object-specific hardware error
BUS_MCEERR_AR (since Linux 2.6.32)
Hardware memory error consumed on a
machine check; action required.
BUS_MCEERR_AO (since Linux 2.6.32)
Hardware memory error detected in process
but not consumed; action optional.
The following values can be placed in si_code for a SIGTRAP
signal:
TRAP_BRKPT process breakpoint
TRAP_TRACE process trace trap
TRAP_BRANCH (since Linux 2.4)
process taken branch trap
TRAP_HWBKPT (since Linux 2.4)
hardware breakpoint/watchpoint
The following values can be placed in si_code for a SIGCHLD
signal:
CLD_EXITED child has exited
CLD_KILLED child was killed
CLD_DUMPED child terminated abnormally
CLD_TRAPPED traced child has trapped
CLD_STOPPED child has stopped
CLD_CONTINUED stopped child has continued (since Linux
2.6.9)
The following values can be placed in si_code for a SIGPOLL
signal:
POLL_IN data input available
POLL_OUT output buffers available
POLL_MSG input message available
POLL_ERR I/O error
POLL_PRI high priority input available
POLL_HUP device disconnected
RETURN VALUE
sigaction() returns 0 on success and -1 on error.
ERRORS
EFAULT
act or oldact points to memory which is not a valid
part of the process address space.
EINVAL
An invalid signal was specified. This will also be
generated if an attempt is made to change the action
for SIGKILL or SIGSTOP, which cannot be caught or
ignored.
CONFORMING TO
POSIX.1-2001, SVr4.
NOTES
A child created via fork(2) inherits a copy of its parent's
signal dispositions. During an execve(2), the dispositions
of handled signals are reset to the default; the
dispositions of ignored signals are left unchanged.
According to POSIX, the behavior of a process is undefined
after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that
was not generated by kill(2) or raise(3). Integer division
by zero has undefined result. On some architectures it will
generate a SIGFPE signal. (Also dividing the most negative
integer by -1 may generate SIGFPE.) Ignoring this signal
might lead to an endless loop.
POSIX.1-1990 disallowed setting the action for SIGCHLD to
SIG_IGN. POSIX.1-2001 allows this possibility, so that
ignoring SIGCHLD can be used to prevent the creation of
zombies (see wait(2)). Nevertheless, the historical BSD and
System V behaviors for ignoring SIGCHLD differ, so that the
only completely portable method of ensuring that terminated
children do not become zombies is to catch the SIGCHLD
signal and perform a wait(2) or similar.
POSIX.1-1990 only specified SA_NOCLDSTOP. POSIX.1-2001
added SA_NOCLDWAIT, SA_RESETHAND, SA_NODEFER, and
SA_SIGINFO. Use of these latter values in sa_flags may be
less portable in applications intended for older Unix
implementations.
The SA_RESETHAND flag is compatible with the SVr4 flag of
the same name.
The SA_NODEFER flag is compatible with the SVr4 flag of the
same name under kernels 1.3.9 and newer. On older kernels
the Linux implementation allowed the receipt of any signal,
not just the one we are installing (effectively overriding
any sa_mask settings).
sigaction() can be called with a NULL second argument to
query the current signal handler. It can also be used to
check whether a given signal is valid for the current
machine by calling it with NULL second and third arguments.
It is not possible to block SIGKILL or SIGSTOP (by
specifying them in sa_mask). Attempts to do so are silently
ignored.
See sigsetops(3) for details on manipulating signal sets.
See signal(7) for a list of the async-signal-safe functions
that can be safely called inside from inside a signal
handler.
Undocumented
Before the introduction of SA_SIGINFO it was also possible
to get some additional information, namely by using a
sa_handler with second argument of type struct sigcontext.
See the relevant kernel sources for details. This use is
obsolete now.
BUGS
In kernels up to and including 2.6.13, specifying SA_NODEFER
in sa_flags prevents not only the delivered signal from
being masked during execution of the handler, but also the
signals specified in sa_mask. This bug was fixed in kernel
2.6.14.
EXAMPLE
See mprotect(2).
SEE ALSO
kill(1), kill(2), killpg(2), pause(2), sigaltstack(2),
signal(2), signalfd(2), sigpending(2), sigprocmask(2),
sigqueue(2), sigsuspend(2), wait(2), raise(3),
siginterrupt(3), sigsetops(3), sigvec(3), core(5), signal(7)
COLOPHON
This page is part of release 3.28 of the Linux man-pages
project. A description of the project, and information
about reporting bugs, can be found at
MPROTECT(2) -- 2008-08-06 -- Linux -- Linux Programmer's Manual
NAME
mprotect - set protection on a region of memory
SYNOPSIS
#include
int mprotect(const void *addr, size_t len, int prot);
DESCRIPTION
mprotect() changes protection for the calling process's
memory page(s) containing any part of the address range in
the interval [addr, addr+len-1]. addr must be aligned to a
page boundary.
If the calling process tries to access memory in a manner
that violates the protection, then the kernel generates a
SIGSEGV signal for the process.
prot is either PROT_NONE or a bitwise-or of the other values
in the following list:
PROT_NONE The memory cannot be accessed at all.
PROT_READ The memory can be read.
PROT_WRITE The memory can be modified.
PROT_EXEC The memory can be executed.
RETURN VALUE
On success, mprotect() returns zero. On error, -1 is
returned, and errno is set appropriately.
ERRORS
EACCES
The memory cannot be given the specified access. This
can happen, for example, if you mmap(2) a file to which
you have read-only access, then ask mprotect() to mark
it PROT_WRITE.
EINVAL
addr is not a valid pointer, or not a multiple of the
system page size.
ENOMEM
Internal kernel structures could not be allocated.
ENOMEM
Addresses in the range [addr, addr+len] are invalid for
the address space of the process, or specify one or
more pages that are not mapped. (Before kernel 2.4.19,
the error EFAULT was incorrectly produced for these
cases.)
CONFORMING TO
SVr4, POSIX.1-2001. POSIX says that the behavior of
mprotect() is unspecified if it is applied to a region of
memory that was not obtained via mmap(2).
NOTES
On Linux it is always permissible to call mprotect() on any
address in a process's address space (except for the kernel
vsyscall area). In particular it can be used to change
existing code mappings to be writable.
Whether PROT_EXEC has any effect different from PROT_READ is
architecture- and kernel version-dependent. On some
hardware architectures (e.g., i386), PROT_WRITE implies
PROT_READ.
POSIX.1-2001 says that an implementation may permit access
other than that specified in prot, but at a minimum can only
allow write access if PROT_WRITE has been set, and must not
allow any access if PROT_NONE has been set.
EXAMPLE
The program below allocates four pages of memory, makes the
third of these pages read-only, and then executes a loop
that walks upward through the allocated region modifying
bytes.
An example of what we might see when running the program is
the following:
$ ./a.out
Start of region: 0x804c000
Got SIGSEGV at address: 0x804e000
Program source
#include
#include
#include
#include
#include
#include
#include
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
char *buffer;
static void
handler(int sig, siginfo_t *si, void *unused)
{
printf("Got SIGSEGV at address: 0x%lx\n",
(long) si->si_addr);
exit(EXIT_FAILURE);
}
int
main(int argc, char *argv[])
{
char *p;
int pagesize;
struct sigaction sa;
sa.sa_flags = SA_SIGINFO;
sigemptyset(&sa.sa_mask);
sa.sa_sigaction = handler;
if (sigaction(SIGSEGV, &sa, NULL) == -1)
handle_error("sigaction");
pagesize = sysconf(_SC_PAGE_SIZE);
if (pagesize == -1)
handle_error("sysconf");
/* Allocate a buffer aligned on a page boundary;
initial protection is PROT_READ | PROT_WRITE */
buffer = memalign(pagesize, 4 * pagesize);
if (buffer == NULL)
handle_error("memalign");
printf("Start of region: 0x%lx\n", (long) buffer);
if (mprotect(buffer + pagesize * 2, pagesize,
PROT_NONE) == -1)
handle_error("mprotect");
for (p = buffer ; ; )
*(p++) = 'a';
printf("Loop completed\n"); /* Should never happen */
exit(EXIT_SUCCESS);
}
SEE ALSO
mmap(2), sysconf(3)
COLOPHON
This page is part of release 3.28 of the Linux man-pages
project. A description of the project, and information
about reporting bugs, can be found at
把以上的具体实例运行调试一下就知道其用法了！
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