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
is 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)). 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 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 in kernels 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.
