Section: Linux Programmer's Manual (2)
getpid, getppid - get process identification
returns the process ID (PID) of the calling process.
(This is often used by
routines that generate unique temporary filenames.)
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
or, if that process has already terminated,
the ID of the process to which this process has been reparented (either
or a "subreaper" process defined via the
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
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
and the discussion of the
C library/kernel differences
From glibc version 2.3.4 up to and including version 2.24,
the glibc wrapper function for
with the goal of avoiding additional system calls when a process calls
Normally this caching was invisible,
but its correct operation relied on support in the wrapper functions for
if an application bypassed the glibc wrappers for these system calls by using
then a call to
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
could return the wrong value even when invoking
via the glibc wrapper function.
(For a discussion of one such case, see BUGS in
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:
always invoke the actual system call, rather than returning a cached value.
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