man2/getpid.2

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  25 .TH GETPID 2 2019-03-06 "Linux" "Linux Programmer's Manual"
  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>
  32 .PP
  33 .B pid_t getpid(void);
  34 .br
  35 .B pid_t getppid(void);
  36 .SH DESCRIPTION
  37 .BR getpid ()
  38 returns the process ID (PID) of the calling process.
  39 (This is often used by
  40 routines that generate unique temporary filenames.)
  41 .PP
  42 .BR getppid ()
  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
  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).
  53 .SH ERRORS
  54 These functions are always successful.
  55 .SH CONFORMING TO
  56 POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
  57 .SH NOTES
  58 If the caller's parent is in a different PID namespace (see
  59 .BR pid_namespaces (7)),
  60 .BR getppid ()
  61 returns 0.
  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).
  74 .\"
  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
  78 .BR getpid ()
  79 cached PIDs,
  80 with the goal of avoiding additional system calls when a process calls
  81 .BR getpid ()
  82 repeatedly.
  83 Normally this caching was invisible,
  84 but its correct operation relied on support in the wrapper functions for
  85 .BR fork (2),
  86 .BR vfork (2),
  87 and
  88 .BR clone (2):
  89 if an application bypassed the glibc wrappers for these system calls by using
  90 .BR syscall (2),
  91 then a call to
  92 .BR getpid ()
  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":
  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 .\"}
 120 In addition, there were cases where
 121 .BR getpid ()
 122 could return the wrong value even when invoking
 123 .BR clone (2)
 124 via the glibc wrapper function.
 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.
 129 .PP
 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
 134 calls to
 135 .BR getpid ()
 136 always invoke the actual system call, rather than returning a cached value.
 137 .\" FIXME .
 138 .\" Review progress of https://bugzilla.redhat.com/show_bug.cgi?id=1469757
 139 .PP
 140 On Alpha, instead of a pair of
 141 .BR getpid ()
 142 and
 143 .BR getppid ()
 144 system calls, a single
 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.
 155 .SH SEE ALSO
 156 .BR clone (2),
 157 .BR fork (2),
 158 .BR gettid (2),
 159 .BR kill (2),
 160 .BR exec (3),
 161 .BR mkstemp (3),
 162 .BR tempnam (3),
 163 .BR tmpfile (3),
 164 .BR tmpnam (3),
 165 .BR credentials (7),
 166 .BR pid_namespaces (7)