Ubuntu Manpage: sigaction, rt_sigaction - examine and change a signal action

NAME

       sigaction, rt_sigaction - examine and change a signal action

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <signal.h>

       int sigaction(int signum,
                     const struct sigaction *_Nullable restrict act,
                     struct sigaction *_Nullable restrict oldact);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       sigaction():
           _POSIX_C_SOURCE

       siginfo_t:
           _POSIX_C_SOURCE >= 199309L

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 field is not intended for application use. (POSIX does not specify a
sa_restorer field.) Some further details of the purpose of this field can be found in
sigreturn(2).

sa_handler specifies the action to be associated with signum and can be one of the
following:

• SIG_DFL for the default action.

• SIG_IGN to ignore this signal.

• 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 three
arguments, as described below.

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 meaningful only 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 meaningful only 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 add the signal to the thread's signal mask while the handler is executing,
unless the signal is specified in act.sa_mask. Consequently, a further instance of
the signal may be delivered to the thread while it is executing the handler. This
flag is meaningful only 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 meaningful only when establishing a signal handler.

SA_RESETHAND
Restore the signal action to the default upon entry to the signal handler. This
flag is meaningful only 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 meaningful only when establishing a
signal handler. See signal(7) for a discussion of system call restarting.

SA_RESTORER
Not intended for application use. This flag is used by C libraries to indicate
that the sa_restorer field contains the address of a "signal trampoline". See
sigreturn(2) for more details.

SA_SIGINFO (since Linux 2.2)
The signal handler takes three arguments, not one. In this case, sa_sigaction
should be set instead of sa_handler. This flag is meaningful only when
establishing a signal handler.

SA_UNSUPPORTED (since Linux 5.11)
Used to dynamically probe for flag bit support.

If an attempt to register a handler succeeds with this flag set in act->sa_flags
alongside other flags that are potentially unsupported by the kernel, and an
immediately subsequent sigaction() call specifying the same signal number and with
a non-NULL oldact argument yields SA_UNSUPPORTED clear in oldact->sa_flags, then
oldact->sa_flags may be used as a bitmask describing which of the potentially
unsupported flags are, in fact, supported. See the section "Dynamically probing
for flag bit support" below for more details.

SA_EXPOSE_TAGBITS (since Linux 5.11)
Normally, when delivering a signal, an architecture-specific set of tag bits are
cleared from the si_addr field of siginfo_t. If this flag is set, an architecture-
specific subset of the tag bits will be preserved in si_addr.

Programs that need to be compatible with Linux versions older than 5.11 must use
SA_UNSUPPORTED to probe for support.

The siginfo_t argument to a SA_SIGINFO handler
When the SA_SIGINFO flag is specified in act.sa_flags, the signal handler address is
passed via the act.sa_sigaction field. This handler takes three arguments, as follows:

void
handler(int sig, siginfo_t *info, void *ucontext)
{
...
}

These three arguments are as follows

sig The number of the signal that caused invocation of the handler.

info A pointer to a siginfo_t, which is a structure containing further information about
the signal, as described below.

ucontext
This is a pointer to a ucontext_t structure, cast to void *. The structure pointed
to by this field contains signal context information that was saved on the user-
space stack by the kernel; for details, see sigreturn(2). Further information
about the ucontext_t structure can be found in getcontext(3) and signal(7).
Commonly, the handler function doesn't make any use of the third argument.

The siginfo_t data type is a structure with the following fields:

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 */
union sigval 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 Linux 2.6.32) */
void *si_lower; /* Lower bound when address violation
occurred (since Linux 3.19) */
void *si_upper; /* Upper bound when address violation
occurred (since Linux 3.19) */
int si_pkey; /* Protection key on PTE that caused
fault (since Linux 4.6) */
void *si_call_addr; /* Address of system call instruction
(since Linux 3.5) */
int si_syscall; /* Number of attempted system call
(since Linux 3.5) */
unsigned int si_arch; /* Architecture of attempted system call
(since Linux 3.5) */
}

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 read only the fields
that are meaningful for the given signal:

• Signals sent with kill(2) and sigqueue(3) fill in si_pid and si_uid. In addition,
signals sent with sigqueue(3) fill in si_int and si_ptr with the values specified by
the sender of the signal; see sigqueue(3) 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 times(2)). Up to Linux 2.6, and since Linux 2.6.27, these
fields report CPU time in units of sysconf(_SC_CLK_TCK). In Linux 2.6 kernels before
Linux 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 field.

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)). When SIGTRAP is delivered in
response to a ptrace(2) event (PTRACE_EVENT_foo), si_addr is not populated, but si_pid
and si_uid are populated with the respective process ID and user ID responsible for
delivering the trap. In the case of seccomp(2), the tracee will be shown as delivering
the event. BUS_MCEERR_* and si_addr_lsb are Linux-specific extensions.

The SEGV_BNDERR suberror of SIGSEGV populates si_lower and si_upper.

The SEGV_PKUERR suberror of SIGSEGV populates si_pkey.

• SIGIO/SIGPOLL (the two names are synonyms on Linux) 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; for further details, see the description of F_SETSIG in fcntl(2).

• SIGSYS, generated (since Linux 3.5) when a seccomp filter returns SECCOMP_RET_TRAP,
fills in si_call_addr, si_syscall, si_arch, si_errno, and other fields as described in
seccomp(2).

The si_code field
The si_code field inside the siginfo_t argument that is passed to a SA_SIGINFO signal
handler is a value (not a bit mask) indicating why this signal was sent. For a ptrace(2)
event, si_code will contain SIGTRAP and have the ptrace event in the high byte:

(SIGTRAP | PTRACE_EVENT_foo << 8).

For a non-ptrace(2) event, the values that can appear in si_code are described in the
remainder of this section. Since glibc 2.20, the definitions of most of these symbols are
obtained from <signal.h> by defining feature test macros (before including any header
file) as follows:

• _XOPEN_SOURCE with the value 500 or greater;

• _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED; or

• _POSIX_C_SOURCE with the value 200809L or greater.

For the TRAP_* constants, the symbol definitions are provided only in the first two cases.
Before glibc 2.20, no feature test macros were required to obtain these symbols.

For a regular signal, the following list shows the values which can be placed in si_code
for any signal, along with the reason that the signal was generated.

SI_USER
kill(2).

SI_KERNEL
Sent by the kernel.

SI_QUEUE
sigqueue(3).

SI_TIMER
POSIX timer expired.

SI_MESGQ (since Linux 2.6.6)
POSIX message queue state changed; see mq_notify(3).

SI_ASYNCIO
AIO completed.

SI_SIGIO
Queued SIGIO (only up to Linux 2.2; from Linux 2.4 onward SIGIO/SIGPOLL fills
in si_code as described below).

SI_TKILL (since Linux 2.4.19)
tkill(2) or tgkill(2).

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.

SEGV_BNDERR (since Linux 3.19)
Failed address bound checks.

SEGV_PKUERR (since Linux 4.6)
Access was denied by memory protection keys. See pkeys(7). The protection key
which applied to this access is available via si_pkey.

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, IA64 only)
Process taken branch trap.

TRAP_HWBKPT (since Linux 2.4, IA64 only)
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 (since Linux 2.6.9)
Stopped child has continued.

The following values can be placed in si_code for a SIGIO/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.

The following value can be placed in si_code for a SIGSYS signal:

SYS_SECCOMP (since Linux 3.5)
Triggered by a seccomp(2) filter rule.

Dynamically probing for flag bit support
The sigaction() call on Linux accepts unknown bits set in act->sa_flags without error.
The behavior of the kernel starting with Linux 5.11 is that a second sigaction() will
clear unknown bits from oldact->sa_flags. However, historically, a second sigaction()
call would typically leave those bits set in oldact->sa_flags.

This means that support for new flags cannot be detected simply by testing for a flag in
sa_flags, and a program must test that SA_UNSUPPORTED has been cleared before relying on
the contents of sa_flags.

Since the behavior of the signal handler cannot be guaranteed unless the check passes, it
is wise to either block the affected signal while registering the handler and performing
the check in this case, or where this is not possible, for example if the signal is
synchronous, to issue the second sigaction() in the signal handler itself.

In kernels that do not support a specific flag, the kernel's behavior is as if the flag
was not set, even if the flag was set in act->sa_flags.

The flags SA_NOCLDSTOP, SA_NOCLDWAIT, SA_SIGINFO, SA_ONSTACK, SA_RESTART, SA_NODEFER,
SA_RESETHAND, and, if defined by the architecture, SA_RESTORER may not be reliably probed
for using this mechanism, because they were introduced before Linux 5.11. However, in
general, programs may assume that these flags are supported, since they have all been
supported since Linux 2.6, which was released in the year 2003.

See EXAMPLES below for a demonstration of the use of SA_UNSUPPORTED.

RETURN VALUE

       sigaction() returns 0 on success; on error, -1 is returned, and errno is set  to  indicate
       the 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.

STANDARDS

       POSIX.1-2001, POSIX.1-2008, 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 and later
allow 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 specified only SA_NOCLDSTOP. POSIX.1-2001 added SA_NOCLDSTOP, SA_NOCLDWAIT,
SA_NODEFER, SA_ONSTACK, SA_RESETHAND, SA_RESTART, 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 later. 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-safety(7) for a list of the async-signal-safe functions that can be safely
called inside from inside a signal handler.

C library/kernel differences
The glibc wrapper function for sigaction() gives an error (EINVAL) on attempts to change
the disposition of the two real-time signals used internally by the NPTL threading
implementation. See nptl(7) for details.

On architectures where the signal trampoline resides in the C library, the glibc wrapper
function for sigaction() places the address of the trampoline code in the act.sa_restorer
field and sets the SA_RESTORER flag in the act.sa_flags field. See sigreturn(2).

The original Linux system call was named sigaction(). However, with the addition of real-
time signals in Linux 2.2, the fixed-size, 32-bit sigset_t type supported by that system
call was no longer fit for purpose. Consequently, a new system call, rt_sigaction(), was
added to support an enlarged sigset_t type. The new system call takes a fourth argument,
size_t sigsetsize, which specifies the size in bytes of the signal sets in act.sa_mask and
oldact.sa_mask. This argument is currently required to have the value sizeof(sigset_t)
(or the error EINVAL results). The glibc sigaction() wrapper function hides these details
from us, transparently calling rt_sigaction() when the kernel provides it.

Undocumented
Before the introduction of SA_SIGINFO, it was also possible to get some additional
information about the signal. This was done by providing an sa_handler signal handler
with a second argument of type struct sigcontext, which is the same structure as the one
that is passed in the uc_mcontext field of the ucontext structure that is passed (via a
pointer) in the third argument of the sa_sigaction handler. See the relevant Linux kernel
sources for details. This use is obsolete now.

BUGS

       When  delivering  a  signal  with a SA_SIGINFO handler, the kernel does not always provide
       meaningful values for all of the fields of  the  siginfo_t  that  are  relevant  for  that
       signal.

       Up  to and including Linux 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 Linux 2.6.14.

EXAMPLES

       See mprotect(2).

   Probing for flag support
       The  following  example  program  exits  with  status EXIT_SUCCESS if SA_EXPOSE_TAGBITS is
       determined to be supported, and EXIT_FAILURE otherwise.

       #include <signal.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       void
       handler(int signo, siginfo_t *info, void *context)
       {
           struct sigaction oldact;

           if (sigaction(SIGSEGV, NULL, &oldact) == -1
               || (oldact.sa_flags & SA_UNSUPPORTED)
               || !(oldact.sa_flags & SA_EXPOSE_TAGBITS))
           {
               _exit(EXIT_FAILURE);
           }
           _exit(EXIT_SUCCESS);
       }

       int
       main(void)
       {
           struct sigaction act = { 0 };

           act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS;
           act.sa_sigaction = &handler;
           if (sigaction(SIGSEGV, &act, NULL) == -1) {
               perror("sigaction");
               exit(EXIT_FAILURE);
           }

           raise(SIGSEGV);
       }