getpid(2) - Linux manual page (original) (raw)

getpid(2) System Calls Manual getpid(2)

NAME top

   getpid, getppid - get process identification

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <unistd.h>**

   **pid_t getpid(void);**
   **pid_t getppid(void);**

DESCRIPTION top

   **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)](../man1/init.1.html) or a "subreaper" process defined via
   the [prctl(2)](../man2/prctl.2.html) **PR_SET_CHILD_SUBREAPER** operation).

ERRORS top

   These functions are always successful.

VERSIONS top

   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)](../man2/syscall.2.html)
   for details regarding register mapping.

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001, 4.3BSD, SVr4.

C library/kernel differences 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.

NOTES top

   If the caller's parent is in a different PID namespace (see
   [pid_namespaces(7)](../man7/pid%5Fnamespaces.7.html)), **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)](../man2/gettid.2.html) and the discussion of the **CLONE_THREAD** flag in [clone(2)](../man2/clone.2.html).

SEE ALSO top

   [clone(2)](../man2/clone.2.html), [fork(2)](../man2/fork.2.html), [gettid(2)](../man2/gettid.2.html), [kill(2)](../man2/kill.2.html), [exec(3)](../man3/exec.3.html), [mkstemp(3)](../man3/mkstemp.3.html),
   [tempnam(3)](../man3/tempnam.3.html), [tmpfile(3)](../man3/tmpfile.3.html), [tmpnam(3)](../man3/tmpnam.3.html), [credentials(7)](../man7/credentials.7.html),
   [pid_namespaces(7)](../man7/pid%5Fnamespaces.7.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 getpid(2)

Pages that refer to this page:strace(1), capget(2), clone(2), fcntl(2), gettid(2), PR_SET_CHILD_SUBREAPER(2const), sched_setaffinity(2), sched_setscheduler(2), syscalls(2), id_t(3type), libcap(3), pmnotifyerr(3), pmwebtimerregister(3), raise(3), lloadd.conf(5), slapd.conf(5), slapd-config(5), credentials(7), fanotify(7), pid_namespaces(7), pthreads(7), signal-safety(7), lloadd(8), slapd(8)