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GETPID(2) Linux Programmer's Manual GETPID(2)
NAME
getpid, getppid - get process identification
SYNOPSIS
#include <unistd.h>
pid_t getpid(void);
pid_t getppid(void);
DESCRIPTION
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).
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 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 re‐
turn 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.
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)
Linux 2021-03-22 GETPID(2)