Detailed explanation of several ways to obtain the PID (TID, LWP) of Linux threads

Detailed explanation of several ways to obtain the PID (TID, LWP) of Linux threads

In Linux C/C++, thread-level operations are usually performed through the pthread library.

In the pthread library there are functions:

pthread_t pthread_self(void);

It returns a variable of type pthread_t, which refers to the "ID" of the thread that called the pthread_self function.

How do you understand this "ID"?

This "ID" is the unique identifier within the process defined by the pthread library for each thread and is maintained by the pthread library.

Since each process has its own independent memory space, the scope of this "ID" is process-level rather than system-level (the kernel does not recognize it).

In fact, the pthread library also creates threads through system calls provided by the kernel (such as clone), and the kernel creates a system-wide unique "ID" for each thread to uniquely identify the thread.

This system's globally unique "ID" is called thread PID (process ID), or TID (thread ID), and is also called LWP (lightweight process = thread).

How to view the system-wide unique "ID" of a thread in the kernel? It can be roughly divided into the following methods.

Test code:

main.c

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

void *start_routine(void *arg) {
 char msg[32] = "";
 snprintf(msg, sizeof(msg)-1, "thd%d: i am thd%d\n", *((int *)arg), *((int *)arg));
 while (1) {
 write(1, msg, strlen(msg));
 sleep(1);
 }
}

int main() {

 int th1 = 1;
 pthread_t tid1;
 pthread_create(&tid1, NULL, start_routine, &th1);

 int th2 = 2;
 pthread_t tid2;
 pthread_create(&tid2, NULL, start_routine, &th2);
 
 int th3 = 3;
 pthread_t tid3;
 pthread_create(&tid3, NULL, start_routine, &th3);

 const char *msg = "main: i am main\n";
 while (1) {
 write(1, msg, strlen(msg));
 sleep(1);
 }

 return 0;
}

In the main thread, three threads are created through the pthread library, which continuously output the information "i am xxx".

Running output:

[test1280@localhost 20190227]$ gcc -o main main.c -lpthread
[test1280@localhost 20190227]$ ./main
main: i am main
thd2: i am thd2
thd3: i am thd3
thd1: i am thd1
thd2: i am thd2
…

Method 1: ps -Lf $pid

[test1280@localhost ~]$ ps -Lf 11029
UID PID PPID LWP C NLWP STIME TTY STAT TIME CMD
test1280 11029 9374 11029 0 4 10:58 pts/0 Sl+ 0:00 ./main
test1280 11029 9374 11030 0 4 10:58 pts/0 Sl+ 0:00 ./main
test1280 11029 9374 11031 0 4 10:58 pts/0 Sl+ 0:00 ./main
test1280 11029 9374 11032 0 4 10:58 pts/0 Sl+ 0:00 ./main

11209 is the PID of the process to be observed.

The output shows that this process contains 4 threads, their PIDs are all 11209, and their PPIDs are all 9374. LWP is the thread ID we are looking for.

We noticed that there is a thread whose LWP is consistent with the PID of the process, and that thread is the main thread.

-L Show threads, possibly with LWP and NLWP columns
-f does full-format listing.

Method 2: pstree -p $pid

[test1280@localhost ~]$ pstree -p 11029
main(11029)─┬─{main}(11030)
   ├─{main}(11031)
   └─{main}(11032)

Method 3: top -Hp $pid

[test1280@localhost ~]$ top -Hp 11029

The process PID is specified in top. The output contains four threads. The PID field can be used to obtain the PID (TID/LWP) of each thread.

man top
-H:Threads toggle
Starts top with the last remembered 'H' state reversed.
When this toggle is On, all individual threads will be displayed.
Otherwise, top displays a summation of all threads in a process.
-p: Monitor PIDs

Method 4: ls -l /proc/$pid/task/

[test1280@localhost ~]$ ls -l /proc/11029/task/
total 0
dr-xr-xr-x. 6 test1280 test1280 0 Feb 27 10:58 11029
dr-xr-xr-x. 6 test1280 test1280 0 Feb 27 10:58 11030
dr-xr-xr-x. 6 test1280 test1280 0 Feb 27 10:58 11031
dr-xr-xr-x. 6 test1280 test1280 0 Feb 27 10:58 11032

Method 5: pidstat -t -p $pid

[test1280@localhost ~]$ pidstat -t -p 11029
Linux 2.6.32-642.el6.x86_64 (localhost.localdomain) 02/27/2019 _x86_64_ (4 CPUs)

11:20:39 AM TGID TID %usr %system %guest %CPU CPU Command
11:20:39 AM 11029 - 0.00 0.00 0.00 0.00 1 main
11:20:39 AM - 11029 0.00 0.00 0.00 0.00 1 |__main
11:20:39 AM - 11030 0.00 0.00 0.00 0.00 1 |__main
11:20:39 AM - 11031 0.00 0.00 0.00 0.00 0 |__main
11:20:39 AM - 11032 0.00 0.00 0.00 0.00 3 |__main

TGID is the thread group ID. The TID of the main thread is equal to the thread group ID of the main thread, which is equal to the process ID of the process where the main thread is located.

man pidstat
-t Also display statistics for threads associated with selected tasks.
 This option adds the following values ​​to the reports:
 TGID:The identification number of the thread group leader.
 TID:The identification number of the thread being monitored.

Method 6: Source code acquisition

main.c

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/syscall.h>

pid_t gettid() {
 return syscall(SYS_gettid);
}

void *start_routine(void *arg) {
 pid_t pid = gettid();
 pthread_t tid = pthread_self();
 printf("thd%d: pid=%d, tid=%lu\n", *((int *)arg), pid, tid);

 char msg[32] = "";
 snprintf(msg, sizeof(msg)-1, "thd%d: i am thd%d\n", *((int *)arg), *((int *)arg));
 while (1) {
 write(1, msg, strlen(msg));
 sleep(1);
 }
}

int main() {

 pid_t pid = gettid();
 pthread_t tid = pthread_self();
 printf("main: pid=%d, tid=%lu\n", pid, tid);

 int th1 = 1;
 pthread_t tid1;
 pthread_create(&tid1, NULL, start_routine, &th1);

 int th2 = 2;
 pthread_t tid2;
 pthread_create(&tid2, NULL, start_routine, &th2);
 
 int th3 = 3;
 pthread_t tid3;
 pthread_create(&tid3, NULL, start_routine, &th3);

 const char *msg = "main: i am main\n";
 while (1) {
 write(1, msg, strlen(msg));
 sleep(1);
 }

 return 0;
}

The syscall(SYS_gettid) system call returns a pid_t type value, which is the ID of the thread in the kernel.

[test1280@localhost 20190227]$ gcc -o main main.c -lpthread
[test1280@localhost 20190227]$ ./main
main: pid=11278, tid=140429854775040
main: i am main
thd3: pid=11281, tid=140429833787136
thd3: i am thd3
thd2: pid=11280, tid=140429844276992
thd2: i am thd2
thd1: pid=11279, tid=140429854766848
thd1: i am thd1
…

What is the value of a thread's PID (TID, LWP)?

The PID of many command parameters actually refers to the ID of the thread in the kernel, such as taskset, strace and other commands.

For example, the taskset command can bind a process to a specified CPU core.

If the process is in multi-threaded mode, directly using taskset will only bind the main thread, and other threads cannot be bound and take effect.

example:

# Bind the 11282 process to the CPU core 0 [test1280@localhost ~]$ ps -Lf 11282
UID PID PPID LWP C NLWP STIME TTY STAT TIME CMD
test1280 11282 9374 11282 0 4 11:33 pts/0 Sl+ 0:00 ./main
test1280 11282 9374 11283 0 4 11:33 pts/0 Sl+ 0:00 ./main
test1280 11282 9374 11284 0 4 11:33 pts/0 Sl+ 0:00 ./main
test1280 11282 9374 11285 0 4 11:33 pts/0 Sl+ 0:00 ./main
[test1280@localhost ~]$ taskset -pc 0 11282
pid 11282's current affinity list: 0-3
pid 11282's new affinity list: 0

# Check whether other threads are really bound to CPU core 0 [test1280@localhost ~]$ taskset -pc 11283
pid 11283's current affinity list: 0-3
[test1280@localhost ~]$ taskset -pc 11284
pid 11284's current affinity list: 0-3
[test1280@localhost ~]$ taskset -pc 11285
pid 11285's current affinity list: 0-3
[test1280@localhost ~]$ taskset -pc 11282
pid 11282's current affinity list: 0
# At this time, only the main thread is actually bound to the CPU core 0. # Bind the other four threads to the CPU core 0 [test1280@localhost ~]$ taskset -pc 0 11283
pid 11283's current affinity list: 0-3
pid 11283's new affinity list: 0
[test1280@localhost ~]$ taskset -pc 0 11284
pid 11284's current affinity list: 0-3
pid 11284's new affinity list: 0
[test1280@localhost ~]$ taskset -pc 0 11285
pid 11285's current affinity list: 0-3
pid 11285's new affinity list: 0
# At this point, all threads of the process with process PID=11282 will run only in CPU core 0

Similarly, strace can specify the thread PID and track the system calls and signals executed by a thread.

This is the end of this article about several ways to obtain the PID (TID, LWP) of a Linux thread. For more information about how to obtain the PID of a Linux thread, please search for previous articles on 123WORDPRESS.COM or continue to browse the following related articles. I hope you will support 123WORDPRESS.COM in the future!

You may also be interested in:
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  • Linux finds the full path of the startup program based on the process number PID
  • Detailed explanation of the value range of pid under Linux system
  • How to find the corresponding process name and directory by PID number in Linux
  • Detailed explanation of pid files in the /var/run/ directory under Linux and their functions
  • Linux pidof command usage summary
  • Linux gets the process name and process pid based on pid (get pid in C language)

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