set_tid_address(2) System Calls Manual set_tid_address(2)
NAME
set_tid_address - set pointer to thread ID
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
pid_t syscall(SYS_set_tid_address, int *tidptr);
Note: glibc provides no wrapper for set_tid_address(), necessitating
the use of syscall(2).
DESCRIPTION
For each thread, the kernel maintains two attributes (addresses) called
set_child_tid and clear_child_tid. These two attributes contain the
value NULL by default.
set_child_tid
If a thread is started using clone(2) with the CLONE_CHILD_SET-
TID flag, set_child_tid is set to the value passed in the ctid
argument of that system call.
When set_child_tid is set, the very first thing the new thread
does is to write its thread ID at this address.
clear_child_tid
If a thread is started using clone(2) with the
CLONE_CHILD_CLEARTID flag, clear_child_tid is set to the value
passed in the ctid argument of that system call.
The system call set_tid_address() sets the clear_child_tid value for
the calling thread to tidptr.
When a thread whose clear_child_tid is not NULL terminates, then, if
the thread is sharing memory with other threads, then 0 is written at
the address specified in clear_child_tid and the kernel performs the
following operation:
futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
The effect of this operation is to wake a single thread that is per-
forming a futex wait on the memory location. Errors from the futex
wake operation are ignored.
RETURN VALUE
set_tid_address() always returns the caller's thread ID.
ERRORS
set_tid_address() always succeeds.
VERSIONS
This call is present since Linux 2.5.48. Details as given here are
valid since Linux 2.5.49.
STANDARDS
This system call is Linux-specific.
SEE ALSO
clone(2), futex(2), gettid(2)
Linux man-pages 6.03 2022-10-30 set_tid_address(2)
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