rt_sigqueueinfo(2)

👑 rt_sigqueueinfo — the manual page, but make it cute 💖

okay so bestie, you found the rt_sigqueueinfo(2) page in the System Calls Manual — yes, section 2, the deep lore section. this is the one where the kernel lets you yeet a signal plus a lil payload of data at a process or a thread. it's giving backstage-pass energy. let's get into it 🎀

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✨ NAME ✨

rt_sigqueueinfo, rt_tgsigqueueinfo — queue a signal and data 💌

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📚 LIBRARY 📚

Standard C library (libc, -lc) — the usual suspect, no cap.

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🌸 SYNOPSIS 🌸

#include <linux/signal.h>     /* Definition of SI_* constants */
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>

int syscall(SYS_rt_sigqueueinfo, pid_t tgid,
            int sig, siginfo_t *info);
int syscall(SYS_rt_tgsigqueueinfo, pid_t tgid, pid_t tid,
            int sig, siginfo_t *info);

✋ Note: There are no glibc wrappers for these system calls; see the NOTES section. so you're raw-dogging it with syscall(2), bestie 💅

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🦋 DESCRIPTION 🦋

So. rt_sigqueueinfo() and rt_tgsigqueueinfo() are the low-level interfaces used to send a signal plus data to a process or a thread. like, kill(2) but with a lil gift bag attached 🎁

The receiver of the signal can obtain the accompanying data by establishing a signal handler with the sigaction(2) SA_SIGINFO flag. that's the whole trick. that's the vibe. iykyk.

These system calls are not intended for direct application use — they exist so that sigqueue(3) and pthread_sigqueue(3) can be implemented. they're the plumbing, the scaffolding, the backstage crew. respect them but don't date them.

rt_sigqueueinfo() sends the signal sig to the thread group with the ID tgid. and before you ask: yes, "thread group" is synonymous with "process", and tgid corresponds to the traditional UNIX process ID. periodt. 💅

The signal will be delivered to an arbitrary member of the thread group — i.e., one of the threads that is not currently blocking the signal. so like, whoever's free catches it. no assigned seating, main character roulette 👑

The info argument specifies the data to accompany the signal. it's a pointer to a structure of type siginfo_t, described in sigaction(2) (and defined by including <sigaction.h>). the caller should set the following fields in this structure:

🌈 the fields you gotta set 🌈

• si_code — This should be one of the SI_* codes in the Linux kernel source file include/asm-generic/siginfo.h. If the signal is being sent to any process other than the caller itself, the following restrictions apply:
  • The code can't be a value greater than or equal to zero. In particular, it can't be SI_USER, which is used by the kernel to indicate a signal sent by kill(2), and nor can it be SI_KERNEL, which is used to indicate a signal generated by the kernel.
  • The code can't (since Linux 2.6.39) be SI_TKILL, which is used by the kernel to indicate a signal sent using tgkill(2).
• si_pid — This should be set to a process ID, typically the process ID of the sender.
• si_uid — This should be set to a user ID, typically the real user ID of the sender.
• si_value — This field contains the user data to accompany the signal. For more information, see the description of the last (union sigval) argument of sigqueue(3).

Internally, the kernel sets the si_signo field to the value specified in sig, so that the receiver of the signal can also obtain the signal number via that field. sneaky. efficient. iconic ✨

rt_tgsigqueueinfo() is like rt_sigqueueinfo(), but sends the signal and data to the single thread specified by the combination of tgid, a thread group ID, and tid, a thread in that thread group. it's the targeted DM version instead of the group chat 🫶

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💫 RETURN VALUE 💫

On success, these system calls return 0. on error, they return -1 and errno is set to indicate the error. classic.

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💀 ERRORS 💀

• EAGAIN — The limit of signals that may be queued has been reached. (See signal(7) for further information.) the queue said "no more" 😭
• EINVAL — sig, tgid, or tid was invalid.
• EPERM — The caller does not have permission to send the signal to the target. For the required permissions, see kill(2).
• EPERM — tgid specifies a process other than the caller and info->si_code is invalid.
• ESRCH — rt_sigqueueinfo(): No thread group matching tgid was found.
• rt_tgsigqueinfo(): No thread matching tgid and tid was found.

🙈 also note: the original spells that last one rt_tgsigqueinfo() — no u after sigq. i'm keeping it exactly as written because that's the rule, bestie. character for character 💅

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🎀 STANDARDS 🎀

Linux. just Linux. that's it. that's the whole standard. 🐧

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📜 HISTORY 📜

• rt_sigqueueinfo() — Linux 2.2.
• rt_tgsigqueueinfo() — Linux 2.6.31.

a whole glow-up arc across kernel versions, we love to see it 🌱

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🧚 NOTES 🧚

Since these system calls are not intended for application use, there are no glibc wrapper functions; use syscall(2) in the unlikely case that you want to call them directly. unlikely!! she said unlikely!!! okay but if you do, you do 💅

As with kill(2), the null signal (0) can be used to check if the specified process or thread exists. free vibe check, no signal actually sent 🦋

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🔥 SEE ALSO 🔥

kill(2), pidfd_send_signal(2), sigaction(2), sigprocmask(2), tgkill(2), pthread_sigqueue(3), sigqueue(3), signal(7)

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and that's the page, bestie 💖 this was Linux man-pages 6.20, dated 2026-02-08, for your reading pleasure. go forth and queue responsibly, mwah 😘