How to get thread id of a pthread in linux c program

Question

In a Linux C program  how do I print the thread id of a thread created by the pthread library  For example like how we can get pid of a process by getpid

User · Answer

As noted in other answers  pthreads does not define a platform-independent way to retrieve an integral thread ID   On Linux systems  you can get thread ID thus    include  lt sys types h gt  pid t tid   gettid      On many BSD-based platforms  this answer https   stackoverflow com a 21206357 316487 gives a non-portable way   However  if the reason you think you need a thread ID is to know whether you re running on the same or different thread to another thread you control  you might find some utility in this approach  static pthread t threadA      On thread A    threadA   pthread self        On thread B    pthread t threadB   pthread self    if  pthread equal threadA  threadB   printf  Thread B is same as thread A  n    else printf  Thread B is NOT same as thread A  n      If you just need to know if you re on the main thread  there are additional ways  documented in answers to this question how can I tell if pthread self is the main  first  thread in the process

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I think not only is the question not clear but most people also are not cognizant of the difference  Examine the following saying      POSIX thread IDs are not the same as the thread IDs returned by the   Linux specific gettid   system call  POSIX thread IDs are assigned   and maintained by the threading implementation  The thread ID returned   by gettid   is a number  similar to a process ID  that is assigned by   the kernel  Although each POSIX thread has a unique kernel thread ID   in the Linux NPTL threading implementation  an application generally   doesn   t need to know about the kernel IDs  and won   t be portable if it   depends on knowing them        Excerpted from  The Linux Programming Interface  A Linux and UNIX System Programming Handbook  Michael Kerrisk   IMHO  there is only one portable way that pass a structure in which define a variable holding numbers in an ascending manner e g  1 2 3     to per thread  By doing this  threads  id can be kept track  Nonetheless  int pthread equal tid1  tid2  function should be used   if  pthread equal tid1  tid2   printf  Thread 2 is same as thread 1  n    else printf  Thread 2 is NOT same as thread 1  n

User · Answer

You can use pthread self    The parent gets to know the thread id after the pthread create   is executed sucessfully  but while executing the thread if we want to access the thread id we have to use the function pthread self

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What   The person asked for Linux specific  and the equivalent of getpid     Not BSD or Apple    The answer is gettid   and returns an integral type   You will have to call it using syscall    like this   include  lt sys types h gt   include  lt unistd h gt   include  lt sys syscall h gt           pid t x   syscall   NR gettid    While this may not be portable to non-linux systems  the threadid is directly comparable and very fast to acquire   It can be printed  such as for LOGs  like a normal integer

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Platform-independent way  starting from c  11  is    include  lt thread gt   std  this thread  get id

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pthread getthreadid np wasn t on my Mac os x   pthread t is an opaque type   Don t beat your head over it   Just assign it to void  and call it good   If you need to printf use  p

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There is also another way of getting thread id  While creating threads with  int pthread create pthread t   thread  const pthread attr t   attr  void     start routine  void     void  arg     function call  the first parameter pthread t   thread is actually a thread id  that is an unsigned long int defined in bits pthreadtypes h   Also  the last argument void  arg is the argument that is passed to  void     start routine  function to be threaded    You can create a structure to pass multiple arguments and send a pointer to a structure   typedef struct thread info       pthread t thread              thread info        tinfo   malloc sizeof thread info    NUMBER OF THREADS         pthread create   amp tinfo i  thread  NULL  handler   void   amp tinfo i          void  handler void  targs        thread info  tinfo   targs         here you get the thread id with tinfo- gt thread

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This single line gives you pid   each threadid and spid    printf  before calling pthread create getpid   d getpthread self   lu tid  lu n  getpid    pthread self    syscall SYS gettid

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pid t tid   syscall SYS gettid     Linux provides such system call to allow you get id of a thread

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pthread self   function will give the thread id of current thread    pthread t pthread self void     The pthread self   function returns the Pthread handle of the calling thread  The pthread self   function does NOT return the integral thread of the calling thread  You must use pthread getthreadid np   to return an integral identifier for the thread   NOTE   pthread id np t   tid  tid   pthread getthreadid np      is significantly faster than these calls  but provides the same behavior   pthread id np t   tid  pthread t         self  self   pthread self    pthread getunique np  amp self   amp tid

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You can also write in this manner and it does the same  For eg   for int i 0 i  lt  total  i          pthread join pth i  NULL       cout  lt  lt   SUM of thread id    lt  lt  pth i   lt  lt    is    lt  lt  args i  sum  lt  lt  endl      This program sets up an array of pthread t and calculate sum on each  So it is printing the sum of each thread with thread id

[c] How to get thread id of a pthread in linux c program?

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