getpid, getppid - get process identification
#include
<sys/types.h>
#include <unistd.h>
pid_t
getpid(void);
pid_t getppid(void);
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).
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_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 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 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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