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authorFlorian Weimer <fweimer@redhat.com>2018-12-14 21:11:09 +0100
committerFlorian Weimer <fweimer@redhat.com>2018-12-14 21:11:09 +0100
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treeab45a511cca066be7ce447d80b58cf401424a1c5 /manual
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manual: Document thread/task IDs for Linux
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Diffstat (limited to 'manual')
-rw-r--r--manual/process.texi60
-rw-r--r--manual/signal.texi4
2 files changed, 46 insertions, 18 deletions
diff --git a/manual/process.texi b/manual/process.texi
index b82b91f..652b000 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 +171,40 @@ 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
+@dfn{thread ID}. The thread ID of the initial (main) thread is the
+same as the process ID of the entire process. Thread IDs for
+subsequently created threads are distinct. They are allocated from
+the same numbering space as process IDs. 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 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 9577ff0..8b3a52e 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.