Discussion:
[PATCH] manual: Document thread/task IDs for Linux
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Florian Weimer
2018-12-07 13:00:12 UTC
Permalink
2018-12-07 Florian Weimer <***@redhat.com>

* manual/process.texi (Process Creation Concepts): Remove
documentation of process (ID) lifetime. List more process
creation functions. Reference Process Identification section.
(Process Identification): Add information about process ID
lifetime. Describe Linux thread/task IDs.
* manual/signal.texi (Signaling Another Process): Mention that the
signal is always sent to the process.

diff --git a/manual/process.texi b/manual/process.texi
index b82b91f9f1..25c8393903 100644
--- a/manual/process.texi
+++ b/manual/process.texi
@@ -132,22 +132,19 @@ output channels of the command being executed.
This section gives an overview of processes and of the steps involved in
creating a process and making it run another program.

-@cindex process ID
-@cindex process lifetime
-Each process is named by a @dfn{process ID} number. A unique process ID
-is allocated to each process when it is created. The @dfn{lifetime} of
-a process ends when its termination is reported to its parent process;
-at that time, all of the process resources, including its process ID,
-are freed.
-
@cindex creating a process
@cindex forking a process
@cindex child process
@cindex parent process
-Processes are created with the @code{fork} system call (so the operation
-of creating a new process is sometimes called @dfn{forking} a process).
-The @dfn{child process} created by @code{fork} is a copy of the original
-@dfn{parent process}, except that it has its own process ID.
+@cindex subprocess
+A new processes is created when one of the functions
+@code{posix_spawn}, @code{fork}, or @code{vfork} is called. (The
+@code{system} and @code{popen} also create new processes internally.)
+Due to the name of the @code{fork} function, the act of creating a new
+process is sometimes called @dfn{forking} a process. Each new process
+(the @dfn{child process} or @dfn{subprocess}) is allocated a process
+ID, distinct from the process ID of the parent process. @xref{Process
+Identification}.

After forking a child process, both the parent and child processes
continue to execute normally. If you want your program to wait for a
@@ -174,11 +171,39 @@ too, instead of returning to the previous process image.
@node Process Identification
@section Process Identification

-The @code{pid_t} data type represents process IDs. You can get the
-process ID of a process by calling @code{getpid}. The function
-@code{getppid} returns the process ID of the parent of the current
-process (this is also known as the @dfn{parent process ID}). Your
-program should include the header files @file{unistd.h} and
+@cindex process ID
+Each process is named by a @dfn{process ID} number, a value of type
+@code{pid_t}. A process ID is allocated to each process when it is
+created. Process IDs are reused over time. The lifetime of a process
+ends when the parent process of the corresponding process waits on the
+process ID after the process has terminated. @xref{Process
+Completion}. (The parent process can arrange for such waiting to
+happen implicitly.) A process ID uniquely identifies a process only
+during the lifetime of the process. As a rule of thumb, this means
+that the process must still be running.
+
+Process IDs can also denote process groups and sessions.
+@xref{Job Control}.
+
+@cindex thread ID
+@cindex task ID
+@cindex thread group
+On Linux, threads created by @code{pthread_create} also receive a
+number form the process ID namespace, a @dfn{thread ID}. The thread
+ID of the initial (main) thread is the same as the process ID of the
+entire process. Process IDs and thread IDs are sometimes also
+referred to collectively as @dfn{task IDs}. In contrast to processes,
+threads are never waited for explicitly, so a thread ID becomes
+eligible for reuse as soon as a thread exits or is canceled. This is
+true even for joinable threads, not just detached threads. Threads
+are also assigned to a @dfn{thread group}. In @theglibc{}
+implementation running on Linux, the process ID is the thread group ID
+of all threads in the process.
+
+You can get the process ID of a process by calling @code{getpid}. The
+function @code{getppid} returns the process ID of the parent of the
+current process (this is also known as the @dfn{parent process ID}).
+Your program should include the header files @file{unistd.h} and
@file{sys/types.h} to use these functions.
@pindex sys/types.h
@pindex unistd.h
diff --git a/manual/signal.texi b/manual/signal.texi
index 9577ff091d..8b3a52e22a 100644
--- a/manual/signal.texi
+++ b/manual/signal.texi
@@ -2246,7 +2246,9 @@ signal:

@table @code
@item @var{pid} > 0
-The process whose identifier is @var{pid}.
+The process whose identifier is @var{pid}. (On Linux, the signal is
+sent to the entire process even if @var{pid} is a thread ID distinct
+from the process ID.)

@item @var{pid} == 0
All processes in the same process group as the sender.

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