Thread

The Linux kernel utilizes the task_struct structure, which can be viewed here to manage threads.






struct task_struct {
    ...
    pid_t pid;
    pid_t tgid;
    ...
}

Every thread in a Thread Group maps to a task structure in the kernel.
The pid variable correspond to a thread ID, and tgid, meaning Thread Group ID corresponds to a process ID.

User layer System call Variable mapped in Kernel
task_struct structure

Thread ID pid_t gettid(void) pid_t pid

Process ID pid_t getpid(void) pid_t tgid

User layer	System call	Variable mapped in Kernel `task_struct` structure
Thread ID	pid_t gettid(void)	pid_t pid
Process ID	pid_t getpid(void)	pid_t tgid

Use the following command to retrieve the information about threads, where LWP represents the thread ID and NLWP indicates the Number of LWPs. Additionally, the process systemd+ is multi-thread, with four threads having thread IDs 1195, 1358, 1359,and 1360.






$ ps -eLF -L
F S UID          PID    PPID     LWP  C NLWP PRI  NI ADDR SZ WCHAN  STIME TTY          TIME CMD
4 S syslog      1295       1    1295  0    4  80   0 - 55647 -      14:43 ?        00:00:00 /usr/sbin/rsyslogd -n -iNONE
1 S syslog      1295       1    1358  0    4  80   0 - 55647 -      14:43 ?        00:00:00 /usr/sbin/rsyslogd -n -iNONE
1 S syslog      1295       1    1359  0    4  80   0 - 55647 -      14:43 ?        00:00:00 /usr/sbin/rsyslogd -n -iNONE
1 S syslog      1295       1    1360  0    4  80   0 - 55647 -      14:43 ?        00:00:00 /usr/sbin/rsyslogd -n -iNONE

Use the following command to search for all threads of a given process ID. procfs creates a subdirectory for each thread in the /proc/PID/task path.








$ ll /proc/1295/task
total 0
dr-xr-xr-x 6 syslog syslog 0  3月 20 22:44 ./
dr-xr-xr-x 9 syslog syslog 0  3月 20 14:43 ../
dr-xr-xr-x 7 syslog syslog 0  3月 20 22:44 1295/
dr-xr-xr-x 7 syslog syslog 0  3月 20 22:44 1358/
dr-xr-xr-x 7 syslog syslog 0  3月 20 22:44 1359/
dr-xr-xr-x 7 syslog syslog 0  3月 20 22:44 1360/

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