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

       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

       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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       latest version of this page, can be found at

Linux                             2019-03-06                         GETPID(2)