getpid(2)                                               System Calls Manual                                               getpid(2)

       getpid, getppid - get process identification

       Standard C library (libc, -lc)

       #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 tempo‐
       rary filenames.)

       getppid() returns the process ID of the parent of the calling process.  This will be either the ID of the process that  cre‐
       ated  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 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 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

       Because  of the aforementioned problems, since glibc 2.25, the PID cache is removed: calls to getpid() always invoke the ac‐
       tual 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  re‐
       turns  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_name‐

Linux man-pages 6.03                                         2023-01-22                                                   getpid(2)