getpid(2) — manpages-dev — Debian unstable — Debian Manpages (original) (raw)

NAME¶

getpid, getppid - get process identification

LIBRARY¶

Standard C library (libc, -lc)

SYNOPSIS¶

#include <unistd.h>

pid_t getpid(void); pid_t getppid(void);

DESCRIPTION¶

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 (eitherinit(1) or a "subreaper" process defined via theprctl(2) PR_SET_CHILD_SUBREAPER operation).

ERRORS¶

These functions are always successful.

VERSIONS¶

On Alpha, instead of a pair of getpid() andgetppid() system calls, a single getxpid() system call is provided, which returns a pair of PID and parent PID. The glibcgetpid() and getppid() wrapper functions transparently deal with this. See syscall(2) for details regarding register mapping.

STANDARDS¶

POSIX.1-2024.

HISTORY¶

4.3BSD, SVr4, POSIX.1-1988.

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), andclone(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 wheregetpid() could return the wrong value even when invokingclone(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 2.25, the PID cache is removed: calls to getpid() always invoke the actual system call, rather than returning a cached value.

NOTES¶

If the caller's parent is in a different PID namespace (seepid_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).