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 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_names- paces(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 get- tid(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. Nor- mally 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 func- tion. (For a discussion of one such case, see BUGS in clone(2).) Fur- thermore, 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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Linux 2017-11-26 GETPID(2)