getpid
Section: System Calls (2)
Updated: 202-0-08
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NAME
getpid, getppid - get process identification
LIBRARY
Standard C library
(
libc,~
-lc)
SYNOPSIS
#include <unistd.h>
pid_t getpid(void);
pid_t getppid(void);
DESCRIPTION
getpid()
returns the process ID (PID) of the calling process.
(This is often used by
routines that generate unique temporary filenames.)
getppid()
returns the process ID of the parent of the calling process.
This will be either the ID of the process that created this process using
fork(),
or, if that process has already terminated,
the ID of the process to which this process has been reparented (either
init(1)
or a "subreaper" process defined via the
prctl(2)
PR_SET_CHILD_SUBREAPER
operation).
ERRORS
These functions are always successful.
VERSIONS
On Alpha, instead of a pair of
getpid()
and
getppid()
system calls, a single
getxpid()
system call is provided, which returns a pair of PID and parent PID.
The glibc
getpid()
and
getppid()
wrapper functions transparently deal with this.
See
syscall(2)
for details regarding register mapping.
STANDARDS
POSIX.-2024.
HISTORY
4.3BSD,
SVr4,
POSIX.-1988.
C library/kernel differences
From glibc 2.3.4 up to and including glibc 2.24,
the glibc wrapper function for
getpid()
cached PIDs,
with the goal of avoiding additional system calls when a process calls
getpid()
repeatedly.
Normally this caching was invisible,
but its correct operation relied on support in the wrapper functions for
fork(2),
vfork(2),
and
clone(2):
if an application bypassed the glibc wrappers for these system calls by using
syscall(2),
then a call to
getpid()
in the child would return the wrong value
(to be precise: it would return the PID of the parent process).
In addition, there were cases where
getpid()
could return the wrong value even when invoking
clone(2)
via the glibc wrapper function.
(For a discussion of one such case, see BUGS in
clone(2).)
Furthermore, the complexity of the caching code had been
the source of a few bugs within glibc over the years.
Because of the aforementioned problems,
since glibc 2.25, the PID cache is removed:
calls to
getpid()
always invoke the actual system call, rather than returning a cached value.
NOTES
If the caller's parent is in a different PID namespace (see
pid_namespaces(7)),
getppid()
returns 0.
From a kernel perspective,
the PID (which is shared by all of the threads in a multithreaded process)
is sometimes also known as the thread group ID (TGID).
This contrasts with the kernel thread ID (TID),
which is unique for each thread.
For further details, see
gettid(2)
and the discussion of the
CLONE_THREAD
flag in
clone(2).
SEE ALSO
clone(2),
fork(2),
gettid(2),
kill(2),
exec(3),
mkstemp(3),
tempnam(3),
tmpfile(3),
tmpnam(3),
credentials(7),
pid_namespaces(7)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- ERRORS
-
- VERSIONS
-
- STANDARDS
-
- HISTORY
-
- C library/kernel differences
-
- NOTES
-
- SEE ALSO
-
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