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.

The following values can be placed in si_code for a SIGILL signal:

The following values can be placed in si_code for a SIGFPE signal:

The following values can be placed in si_code for a SIGSEGV signal:

The following values can be placed in si_code for a SIGBUS signal:

The following values can be placed in si_code for a SIGTRAP signal:

The following values can be placed in si_code for a SIGCHLD signal:

The following values can be placed in si_code for a SIGIO/SIGPOLL signal:

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

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.

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.

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);
}