Name
set_tid_address - set pointer to thread IDLibrary
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 andclear_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_SETTID 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 performing 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.Standards
Linux.History
Linux 2.5.48.Details as given here are valid since Linux 2.5.49.