Linux cli command set_tid_address

➡ A Linux man page (short for manual page) is a form of software documentation found on Linux and Unix-like operating systems. This man-page explains the command set_tid_address and provides detailed information about the command set_tid_address, system calls, library functions, and other aspects of the system, including usage, options, and examples of _. You can access this man page by typing man followed by the set_tid_address.

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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_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.

SEE ALSO

clone(2), futex(2), gettid(2)

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