GETPID(2) (2020-11-01) GETPID(2)
NAME
getpid, getppid - get process identification
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
pid_t getpid(void);
pid_t getppid(void);
DESCRIPTION
getpid() returns the process ID (PID) of the calling pro-
cess. (This is often used by routines that generate unique
temporary filenames.)
getppid() returns the process ID of the parent of the call-
ing 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 pro-
cess 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.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
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).
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. Normally this caching was invis-
ible, 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
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GETPID(2) (2020-11-01) GETPID(2)
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).) 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.
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 map-
ping.
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)
COLOPHON
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about
reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
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