sigaction, rt_sigaction - examine and change a signal action

#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)):

    _POSIX_C_SOURCE

    _POSIX_C_SOURCE >= 199309L

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.

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

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);
}

kill(1), kill(2), pause(2), pidfd_send_signal(2), restart_syscall(2), seccomp(2), sigaltstack(2), signal(2), signalfd(2), sigpending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), wait(2), killpg(3), raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)