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What is the difference between PID and TID?
The standard answer would be that PID is for processes while TID is for threads. However, I have seen that some commands use them interchangeably. For example, htop has a column for PIDs, in which PIDs for threads of the same process are shown (with different values). So when does a PID represent a thread or a process?
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The only method seems to open /proc/TID/status , read the file into a buffer, scan line-by-line for Tgid , and then parse the string as an unsigned integer. I'm hoping that there is a syscall I missed that returns the tgid/pid given the pid, even if the id is returned indirectly within a some data structure.
TID (Thread Identifier) It is an integer that serves to identify threads. In serial programming, the TID and the PID are the same, as there is only one thread. In multi-threaded environments, each thread has its own TID. All the threads from the same process have the same PID.
pid : The is the process ID (PID) of the process you call the Process. pid method in. ppid : The PID of the parent process (the process that spawned the current one). For example, if you run ruby test.
The application summary, by Tid, displays an output of all the threads that were running on the system during the time of trace collection and their CPU consumption.
It is complicated: pid is process identifier; tid is thread identifier.
But as it happens, the kernel doesn't make a real distinction between them: threads are just like processes but they share some things (memory, fds...) with other instances of the same group.
So, a tid is actually the identifier of the schedulable object in the kernel (thread), while the pid is the identifier of the group of schedulable objects that share memory and fds (process).
But to make things more interesting, when a process has only one thread (the initial situation and in the good old times the only one) the pid and the tid are always the same. So any function that works with a tid will automatically work with a pid.
It is worth noting that many functions/system calls/command line utilities documented to work with pid actually use tids. But if the effect is process-wide you will simply not notice the difference.
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Actually, each thread in a Linux process is Light Weight Process (LWP). So, people may call thread as a process... But there is surely a difference. Each thread in a process has a different thread ID (TID) and share the same process ID (PID).
If you are working with pthread library functions, then these functions don't use these TIDs because these are kernel/OS level thread IDs.
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