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fea681da | 1 | .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) |
2297bf0e | 2 | .\" |
93015253 | 3 | .\" %%%LICENSE_START(VERBATIM) |
fea681da MK |
4 | .\" Permission is granted to make and distribute verbatim copies of this |
5 | .\" manual provided the copyright notice and this permission notice are | |
6 | .\" preserved on all copies. | |
7 | .\" | |
8 | .\" Permission is granted to copy and distribute modified versions of this | |
9 | .\" manual under the conditions for verbatim copying, provided that the | |
10 | .\" entire resulting derived work is distributed under the terms of a | |
11 | .\" permission notice identical to this one. | |
c13182ef | 12 | .\" |
fea681da MK |
13 | .\" Since the Linux kernel and libraries are constantly changing, this |
14 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
15 | .\" responsibility for errors or omissions, or for damages resulting from | |
16 | .\" the use of the information contained herein. The author(s) may not | |
17 | .\" have taken the same level of care in the production of this manual, | |
18 | .\" which is licensed free of charge, as they might when working | |
19 | .\" professionally. | |
c13182ef | 20 | .\" |
fea681da MK |
21 | .\" Formatted or processed versions of this manual, if unaccompanied by |
22 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 23 | .\" %%%LICENSE_END |
4784c377 | 24 | .\" |
9ba01802 | 25 | .TH GETPID 2 2019-03-06 "Linux" "Linux Programmer's Manual" |
fea681da MK |
26 | .SH NAME |
27 | getpid, getppid \- get process identification | |
28 | .SH SYNOPSIS | |
29 | .B #include <sys/types.h> | |
30 | .br | |
31 | .B #include <unistd.h> | |
68e4db0a | 32 | .PP |
fea681da MK |
33 | .B pid_t getpid(void); |
34 | .br | |
35 | .B pid_t getppid(void); | |
36 | .SH DESCRIPTION | |
e511ffb6 | 37 | .BR getpid () |
80b435e1 | 38 | returns the process ID (PID) of the calling process. |
c13182ef | 39 | (This is often used by |
2c5f1089 | 40 | routines that generate unique temporary filenames.) |
cd065c7a | 41 | .PP |
e511ffb6 | 42 | .BR getppid () |
a1ffe9f5 | 43 | returns the process ID of the parent of the calling process. |
cbd17ac0 MK |
44 | This will be either the ID of the process that created this process using |
45 | .BR fork (), | |
46 | or, if that process has already terminated, | |
47 | the ID of the process to which this process has been reparented (either | |
48 | .BR init (1) | |
49 | or a "subreaper" process defined via the | |
50 | .BR prctl (2) | |
51 | .BR PR_SET_CHILD_SUBREAPER | |
52 | operation). | |
5f234475 MK |
53 | .SH ERRORS |
54 | These functions are always successful. | |
47297adb | 55 | .SH CONFORMING TO |
e4abf822 | 56 | POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4. |
67f0f50d | 57 | .SH NOTES |
da83e8a2 MK |
58 | If the caller's parent is in a different PID namespace (see |
59 | .BR pid_namespaces (7)), | |
60 | .BR getppid () | |
61 | returns 0. | |
57c80845 MK |
62 | .PP |
63 | From a kernel perspective, | |
64 | the PID (which is shared by all of the threads in a multithreaded process) | |
65 | is sometimes also known as the thread group ID (TGID). | |
66 | This contrasts with the kernel thread ID (TID), | |
67 | which is unique for each thread. | |
68 | For further details, see | |
69 | .BR gettid (2) | |
70 | and the discussion of the | |
71 | .BR CLONE_THREAD | |
72 | flag in | |
73 | .BR clone (2). | |
da83e8a2 | 74 | .\" |
0722a578 | 75 | .SS C library/kernel differences |
9338d37b | 76 | From glibc version 2.3.4 up to and including version 2.24, |
67f0f50d MK |
77 | the glibc wrapper function for |
78 | .BR getpid () | |
9338d37b MK |
79 | cached PIDs, |
80 | with the goal of avoiding additional system calls when a process calls | |
67f0f50d MK |
81 | .BR getpid () |
82 | repeatedly. | |
9338d37b MK |
83 | Normally this caching was invisible, |
84 | but its correct operation relied on support in the wrapper functions for | |
67f0f50d MK |
85 | .BR fork (2), |
86 | .BR vfork (2), | |
87 | and | |
88 | .BR clone (2): | |
9338d37b | 89 | if an application bypassed the glibc wrappers for these system calls by using |
67f0f50d MK |
90 | .BR syscall (2), |
91 | then a call to | |
92 | .BR getpid () | |
9338d37b MK |
93 | in the child would return the wrong value |
94 | (to be precise: it would return the PID of the parent process). | |
67f0f50d MK |
95 | .\" The following program demonstrates this "feature": |
96 | .\" | |
97 | .\" #define _GNU_SOURCE | |
98 | .\" #include <sys/syscall.h> | |
99 | .\" #include <sys/wait.h> | |
100 | .\" #include <stdio.h> | |
101 | .\" #include <stdlib.h> | |
102 | .\" #include <unistd.h> | |
103 | .\" | |
104 | .\" int | |
105 | .\" main(int argc, char *argv[]) | |
106 | .\" { | |
107 | .\" /* The following statement fills the getpid() cache */ | |
108 | .\" | |
109 | .\" printf("parent PID = %ld\n", (long) getpid()); | |
110 | .\" | |
111 | .\" if (syscall(SYS_fork) == 0) { | |
112 | .\" if (getpid() != syscall(SYS_getpid)) | |
113 | .\" printf("child getpid() mismatch: getpid()=%ld; " | |
114 | .\" "syscall(SYS_getpid)=%ld\n", | |
115 | .\" (long) getpid(), (long) syscall(SYS_getpid)); | |
116 | .\" exit(EXIT_SUCCESS); | |
117 | .\" } | |
118 | .\" wait(NULL); | |
119 | .\"} | |
9338d37b | 120 | In addition, there were cases where |
67f0f50d | 121 | .BR getpid () |
9338d37b | 122 | could return the wrong value even when invoking |
67f0f50d MK |
123 | .BR clone (2) |
124 | via the glibc wrapper function. | |
9338d37b MK |
125 | (For a discussion of one such case, see BUGS in |
126 | .BR clone (2).) | |
127 | Furthermore, the complexity of the caching code had been | |
128 | the source of a few bugs within glibc over the years. | |
cd065c7a | 129 | .PP |
9338d37b MK |
130 | Because of the aforementioned problems, |
131 | since glibc version 2.25, the PID cache is removed: | |
995865aa | 132 | .\" commit c579f48edba88380635ab98cb612030e3ed8691e |
9338d37b | 133 | .\" https://sourceware.org/glibc/wiki/Release/2.25#pid_cache_removal |
995865aa | 134 | calls to |
7693d1e5 | 135 | .BR getpid () |
995865aa | 136 | always invoke the actual system call, rather than returning a cached value. |
e0ccee7f MK |
137 | .\" FIXME . |
138 | .\" Review progress of https://bugzilla.redhat.com/show_bug.cgi?id=1469757 | |
70ea1968 | 139 | .PP |
03936fb4 | 140 | On Alpha, instead of a pair of |
b30b425b MK |
141 | .BR getpid () |
142 | and | |
143 | .BR getppid () | |
03936fb4 | 144 | system calls, a single |
70ea1968 ES |
145 | .BR getxpid () |
146 | system call is provided, which returns a pair of PID and parent PID. | |
147 | The glibc | |
148 | .BR getpid () | |
149 | and | |
150 | .BR getppid () | |
151 | wrapper functions transparently deal with this. | |
152 | See | |
153 | .BR syscall (2) | |
154 | for details regarding register mapping. | |
47297adb | 155 | .SH SEE ALSO |
67f0f50d | 156 | .BR clone (2), |
fea681da | 157 | .BR fork (2), |
12fe9b17 | 158 | .BR gettid (2), |
fea681da MK |
159 | .BR kill (2), |
160 | .BR exec (3), | |
161 | .BR mkstemp (3), | |
162 | .BR tempnam (3), | |
163 | .BR tmpfile (3), | |
53a1443c | 164 | .BR tmpnam (3), |
4effb5be | 165 | .BR credentials (7), |
7e0e902b | 166 | .BR pid_namespaces (7) |