ULK --- Chap3 Processes: Identifying a Process

As a general rule, each execution context that can be independently scheduled must have its own

process descriptor; therefore, even lightweight processes, which share a large portion of their kernel

data structures, have their own task_struct structures.

The strict one-to-one correspondence between the process and process descriptor makes the 32 bit

address of the task_struct a useful means for the kernel to identify processes. These addresses are referred

to as process descriptor pointers. Most of the references to processes that the kernel makes are through

process descriptor pointers.

On the other hand, Unix-like operating sytems allow usres to identify processes by means of a number

called the Process ID (or PID), which is stored in the pid field of the process descriptor. PIDs are numbered

sequentially: the PID of a newly created process is normally the PID of the previously created process 

increased by one. Of course, there is an upper limit on the PID values; when the kernel reaches such limit,

it must start recycling the lower, unused PIDs. By default, the maximum PID number is 32767 (

PID_MAX_DEFAULT - 1); the system administrator may reduce this limit by writing a smaller value into

the /proc/sys/kernel/pid_max file. In 64-bit architectures, the system administrator can enlarge the

maximum number up to 4194303.

When recycling PID numbers, the kernel must manage a pidmap_array bitmap that denotes which are the

PIDs currently assigned and which are the free ones. Because a page frame contains 32768 bits, in 32-bit

architectures the pidmap_array bitmap is stored in a single page. In 64 bit architectures, however, additional

pages can be added to the bitmap when the kernel assigns a PID number too large for the current bitmap size.

These pages are never released.

Linux associates a different PID with each process or lightweight process in the system. As we shall see later

in this chapter, there is a tiny exception on multiprocessor systems. This approach allows the maximum flexibility,

because every execution context in the system can be uniquely identified.

On the other hand, Unix programmers expect threads in the same group to have a common PID. For instance,

it should be possible to send a signal specifying a PID that affects all threads in the group. In fact, the POSIX 1003.1c

standard states that all threads of a multithreaded application must have the same PID.

To comply with this standard, Linux makes use of thread groups. The identifier shared by the threads is the PID

of the thread group leader, that is, the PID of the first lightweight process in the group; it is stored in the tgid field

of the process descriptors. The getpid() system call returns the value of tgid relative to the current process instead of

the value of pid, so all the threads of a multithreaded application share the same identifier. Most processes belong

to a thread group consisting of a single number; as thread group leaders, they have the tgid field equal to the pid

field, thus the getpid() system call works as usual for this kind of process.

Later, we will show you how it is possible to derive a true process descriptor pointer efficiently from its respective

PID. Efficiency is important because many system calls such as kill() use the PID to denote the affected process.