GETPID(2)                   Linux Programmer's Manual                  GETPID(2)

       getpid, getppid - get process identification

       #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

       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.

       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.

       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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Linux                              2021-03-22                          GETPID(2)