2. System calls (functions provided by the kernel)
GETPIDSection: Linux Programmer's Manual (2)
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NAMEgetpid, getppid - get process identification
DESCRIPTIONgetpid() 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).
ERRORSThese functions are always successful.
CONFORMING TOPOSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
NOTESIf 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 differencesFrom 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.
SEE ALSOclone(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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