getpid, getppid - get process identification
Standard C library (
libc,
-lc)
#include <unistd.h>
pid_t getpid(void);
pid_t getppid(void);
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).
These functions are always successful.
POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
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).
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.
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.
clone(2),
fork(2),
gettid(2),
kill(2),
exec(3),
mkstemp(3),
tempnam(3),
tmpfile(3),
tmpnam(3),
credentials(7),
pid_namespaces(7)