getpid(2) System Calls Manual getpid(2)
NAME
getpid, getppid - get process identification
LIBRARY
Standard C library (libc, -lc)
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 us-
ing 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 op-
eration).
ERRORS
These functions are always successful.
STANDARDS
POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
NOTES
If the caller's parent is in a different PID namespace (see pid_name-
spaces(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 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 appli-
cation 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.
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 man-pages 6.03 2023-01-22 getpid(2)
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