Scroll to navigation

tkill(2) System Calls Manual tkill(2)


tkill, tgkill - send a signal to a thread


Standard C library (libc, -lc)


#include <signal.h>           /* Definition of SIG* constants */
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>
[[deprecated]] int syscall(SYS_tkill, pid_t tid, int sig);
#include <signal.h>
int tgkill(pid_t tgid, pid_t tid, int sig);

Note: glibc provides no wrapper for tkill(), necessitating the use of syscall(2).


tgkill() sends the signal sig to the thread with the thread ID tid in the thread group tgid. (By contrast, kill(2) can be used to send a signal only to a process (i.e., thread group) as a whole, and the signal will be delivered to an arbitrary thread within that process.)

tkill() is an obsolete predecessor to tgkill(). It allows only the target thread ID to be specified, which may result in the wrong thread being signaled if a thread terminates and its thread ID is recycled. Avoid using this system call.

These are the raw system call interfaces, meant for internal thread library use.


On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.


The RLIMIT_SIGPENDING resource limit was reached and sig is a real-time signal.
Insufficient kernel memory was available and sig is a real-time signal.
An invalid thread ID, thread group ID, or signal was specified.
Permission denied. For the required permissions, see kill(2).
No process with the specified thread ID (and thread group ID) exists.




Linux 2.4.19 / 2.5.4.
Linux 2.5.75, glibc 2.30.


See the description of CLONE_THREAD in clone(2) for an explanation of thread groups.


clone(2), gettid(2), kill(2), rt_sigqueueinfo(2)

2023-03-30 Linux man-pages 6.05.01