1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
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25 .TH GETPID 2 2017-11-26 "Linux" "Linux Programmer's Manual"
27 getpid, getppid \- get process identification
29 .B #include <sys/types.h>
31 .B #include <unistd.h>
33 .B pid_t getpid(void);
35 .B pid_t getppid(void);
38 returns the process ID (PID) of the calling process.
39 (This is often used by
40 routines that generate unique temporary filenames.)
43 returns the process ID of the parent of the calling process.
44 This will be either the ID of the process that created this process using
46 or, if that process has already terminated,
47 the ID of the process to which this process has been reparented (either
49 or a "subreaper" process defined via the
51 .BR PR_SET_CHILD_SUBREAPER
54 These functions are always successful.
56 POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
58 If the caller's parent is in a different PID namespace (see
59 .BR pid_namespaces (7)),
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
70 and the discussion of the
75 .SS C library/kernel differences
76 From glibc version 2.3.4 up to and including version 2.24,
77 the glibc wrapper function for
80 with the goal of avoiding additional system calls when a process calls
83 Normally this caching was invisible,
84 but its correct operation relied on support in the wrapper functions for
89 if an application bypassed the glibc wrappers for these system calls by using
93 in the child would return the wrong value
94 (to be precise: it would return the PID of the parent process).
95 .\" The following program demonstrates this "feature":
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>
105 .\" main(int argc, char *argv[])
107 .\" /* The following statement fills the getpid() cache */
109 .\" printf("parent PID = %ld\n", (long) getpid());
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);
120 In addition, there were cases where
122 could return the wrong value even when invoking
124 via the glibc wrapper function.
125 (For a discussion of one such case, see BUGS in
127 Furthermore, the complexity of the caching code had been
128 the source of a few bugs within glibc over the years.
130 Because of the aforementioned problems,
131 since glibc version 2.25, the PID cache is removed:
132 .\" commit c579f48edba88380635ab98cb612030e3ed8691e
133 .\" https://sourceware.org/glibc/wiki/Release/2.25#pid_cache_removal
136 always invoke the actual system call, rather than returning a cached value.
138 .\" Review progress of https://bugzilla.redhat.com/show_bug.cgi?id=1469757
140 On Alpha, instead of pair of
141 .BR getpid "() and " getppid ()
144 system call is provided, which returns a pair of PID and parent PID.
149 wrapper functions transparently deal with this.
152 for details regarding register mapping.
153 Other examples of such peculiarity are Alpha-specific
154 .BR getxuid "(2) and " getxgid (2),
157 on several architectures.
169 .BR pid_namespaces (7)