getpid, getppid - get process identification
() returns the process ID (PID) of the calling process. (This is
often used by routines that generate unique temporary filenames.)
() 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
(), or, if that process has already terminated, the ID of the
process to which this process has been reparented (either init(1)
"subreaper" process defined via the prctl(2)
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
() 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
flag in clone(2)
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)
, and clone(2)
an application bypassed the glibc wrappers for these system calls by using
, 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)
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.
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