1.c

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#define NUM_THREADS     5

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   if (tid == 0){
       sleep(100);
   }
   printf("Hello World! It's me, thread #%ld!\n", tid);
   pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   int rc;
   long t;
   for(t=0; t<NUM_THREADS; t++){
      printf("In main: creating thread %ld\n", t);
      rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
      if (rc){
         printf("ERROR; return code from pthread_create() is %d\n", rc);
         return -1;
      }
   }

   /* Last thing that main() should do */
   pthread_exit(NULL);
}

gdb session

 % gdb ./a.out         
GNU gdb (GDB) 8.2.1
Copyright (C) 2018 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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Type "show copying" and "show warranty" for details.
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<http://www.gnu.org/software/gdb/bugs/>.
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    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./a.out...done.
(gdb) b 8
Breakpoint 1 at 0x1185: file 1.c, line 9.
(gdb) r
Starting program: ./a.out 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
In main: creating thread 0
[New Thread 0x7fa6cb71c700 (LWP 4677)]
In main: creating thread 1
[New Thread 0x7fa6caf1b700 (LWP 4678)]
In main: creating thread 2
[New Thread 0x7fa6ca71a700 (LWP 4679)]
[Switching to Thread 0x7fa6cb71c700 (LWP 4677)]

Thread 2 "a.out" hit Breakpoint 1, PrintHello (threadid=0x0) at 1.c:9
9	   tid = (long)threadid;
(gdb) del br
Delete all breakpoints? (y or n) y
(gdb) p tid
$1 = 0
(gdb) thread
[Current thread is 2 (Thread 0x7fa6cb71c700 (LWP 4677))]
(gdb) s
In main: creating thread 3
Hello World! It's me, thread #2!
[New Thread 0x7fa6c9f19700 (LWP 5539)]
Hello World! It's me, thread #1!
Hello World! It's me, thread #3!
In main: creating thread 4
[New Thread 0x7fa6c9718700 (LWP 5540)]
Hello World! It's me, thread #4!
10	   if (tid == 0){
(gdb) thread
[Current thread is 2 (Thread 0x7fa6cb71c700 (LWP 4677))]
(gdb) s
11	       sleep(100);
(gdb) s
[Thread 0x7fa6caf1b700 (LWP 4678) exited]
[Thread 0x7fa6c9f19700 (LWP 5539) exited]
[Thread 0x7fa6ca71a700 (LWP 4679) exited]
[Thread 0x7fa6c9718700 (LWP 5540) exited]
[Thread 0x7fa6cb71d740 (LWP 4662) exited]
13	   printf("Hello World! It's me, thread #%ld!\n", tid);
(gdb) thread
[Current thread is 2 (Thread 0x7fa6cb71c700 (LWP 4677))]
(gdb) c
Continuing.
Hello World! It's me, thread #0!
[Inferior 1 (process 4662) exited normally]
(gdb)