gettid

gettid is a Linux-specific system call that returns the thread ID (TID) of the calling thread. The TID is a kernel-assigned identifier that uniquely identifies a thread within the system, and it is distinct from the process ID (PID) and the thread group ID (TGID) used by the process as a whole.

In Linux, a process can consist of multiple threads that share the same TGID (and thus the value returned by getpid), but each thread has its own TID. The main thread’s TID is typically equal to the process’s PID, while other threads have different TIDs. TIDs are unique only for the lifetime of the thread and may be recycled after a thread terminates.

Calling gettid in C is done through a system call, commonly via syscall(SYS_gettid). For example, you can obtain the current thread’s TID with:

#include <unistd.h>

#include <sys/syscall.h>

pid_t tid = syscall(SYS_gettid);

Some C libraries may provide a wrapper around this system call, but gettid is not part of the POSIX standard and is not portable to non-Linux systems.

Portability and usage considerations: Because gettid is Linux-specific, code that relies on it is not portable to other Unix-like systems (such as BSD or macOS) without alternative mechanisms. It is mainly useful for kernel-level debugging, tracing, or when interfacing with tools and interfaces that expect a kernel thread ID. For user-space thread identification within a process, pthread_self or thread-local storage often suffices, but those identifiers do not necessarily correlate with the kernel’s TID. Reading /proc/self/task can show the TIDs of all threads in the process.