getpid, getppid - get process identification
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 version 2.3.4 up to and including version 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 version 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)
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