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Diffstat (limited to 'man/man2/getpid.2')
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diff --git a/man/man2/getpid.2 b/man/man2/getpid.2 new file mode 100644 index 000000000..811270f05 --- /dev/null +++ b/man/man2/getpid.2 @@ -0,0 +1,150 @@ +.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.TH getpid 2 (date) "Linux man-pages (unreleased)" +.SH NAME +getpid, getppid \- get process identification +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.B #include <unistd.h> +.P +.B pid_t getpid(void); +.B pid_t getppid(void); +.fi +.SH DESCRIPTION +.BR getpid () +returns the process ID (PID) of the calling process. +(This is often used by +routines that generate unique temporary filenames.) +.P +.BR 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 +.BR fork (), +or, if that process has already terminated, +the ID of the process to which this process has been reparented (either +.BR init (1) +or a "subreaper" process defined via the +.BR prctl (2) +.B PR_SET_CHILD_SUBREAPER +operation). +.SH ERRORS +These functions are always successful. +.SH VERSIONS +On Alpha, instead of a pair of +.BR getpid () +and +.BR getppid () +system calls, a single +.BR getxpid () +system call is provided, which returns a pair of PID and parent PID. +The glibc +.BR getpid () +and +.BR getppid () +wrapper functions transparently deal with this. +See +.BR syscall (2) +for details regarding register mapping. +.SH STANDARDS +POSIX.1-2008. +.SH HISTORY +POSIX.1-2001, 4.3BSD, SVr4. +.SS C library/kernel differences +From glibc 2.3.4 up to and including glibc 2.24, +the glibc wrapper function for +.BR getpid () +cached PIDs, +with the goal of avoiding additional system calls when a process calls +.BR getpid () +repeatedly. +Normally this caching was invisible, +but its correct operation relied on support in the wrapper functions for +.BR fork (2), +.BR vfork (2), +and +.BR clone (2): +if an application bypassed the glibc wrappers for these system calls by using +.BR syscall (2), +then a call to +.BR getpid () +in the child would return the wrong value +(to be precise: it would return the PID of the parent process). +.\" The following program demonstrates this "feature": +.\" +.\" #define _GNU_SOURCE +.\" #include <sys/syscall.h> +.\" #include <sys/wait.h> +.\" #include <stdint.h> +.\" #include <stdio.h> +.\" #include <stdlib.h> +.\" #include <unistd.h> +.\" +.\" int +.\" main(int argc, char *argv[]) +.\" { +.\" /* The following statement fills the getpid() cache */ +.\" +.\" printf("parent PID = %ld\n", (intmax_t) getpid()); +.\" +.\" if (syscall(SYS_fork) == 0) { +.\" if (getpid() != syscall(SYS_getpid)) +.\" printf("child getpid() mismatch: getpid()=%jd; " +.\" "syscall(SYS_getpid)=%ld\n", +.\" (intmax_t) getpid(), (long) syscall(SYS_getpid)); +.\" exit(EXIT_SUCCESS); +.\" } +.\" wait(NULL); +.\"} +In addition, there were cases where +.BR getpid () +could return the wrong value even when invoking +.BR clone (2) +via the glibc wrapper function. +(For a discussion of one such case, see BUGS in +.BR clone (2).) +Furthermore, the complexity of the caching code had been +the source of a few bugs within glibc over the years. +.P +Because of the aforementioned problems, +since glibc 2.25, the PID cache is removed: +.\" commit c579f48edba88380635ab98cb612030e3ed8691e +.\" https://sourceware.org/glibc/wiki/Release/2.25#pid_cache_removal +calls to +.BR getpid () +always invoke the actual system call, rather than returning a cached value. +.\" FIXME . +.\" Review progress of https://bugzilla.redhat.com/show_bug.cgi?id=1469757 +.SH NOTES +If the caller's parent is in a different PID namespace (see +.BR pid_namespaces (7)), +.BR getppid () +returns 0. +.P +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 +.BR gettid (2) +and the discussion of the +.B CLONE_THREAD +flag in +.BR clone (2). +.SH SEE ALSO +.BR clone (2), +.BR fork (2), +.BR gettid (2), +.BR kill (2), +.BR exec (3), +.BR mkstemp (3), +.BR tempnam (3), +.BR tmpfile (3), +.BR tmpnam (3), +.BR credentials (7), +.BR pid_namespaces (7) |