GETPID(2) Linux Programmer's Manual GETPID(2)
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
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_names‐
paces(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 get‐
tid(2) and the discussion of the CLONE_THREAD flag in clone(2).
C library/kernel differences
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. Nor‐
mally 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 func‐
tion. (For a discussion of one such case, see BUGS in clone(2).) Fur‐
thermore, 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.
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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Linux 2017-11-26 GETPID(2)