man2/getpid.2

   1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
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  25 .TH GETPID 2 2017-05-03 "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 .sp
  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 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 .\"
  63 .SS C library/kernel differences
  64 From glibc version 2.3.4 up to and including version 2.24,
  65 the glibc wrapper function for
  66 .BR getpid ()
  67 cached PIDs,
  68 with the goal of avoiding additional system calls when a process calls
  69 .BR getpid ()
  70 repeatedly.
  71 Normally this caching was invisible,
  72 but its correct operation relied on support in the wrapper functions for
  73 .BR fork (2),
  74 .BR vfork (2),
  75 and
  76 .BR clone (2):
  77 if an application bypassed the glibc wrappers for these system calls by using
  78 .BR syscall (2),
  79 then a call to
  80 .BR getpid ()
  81 in the child would return the wrong value
  82 (to be precise: it would return the PID of the parent process).
  83 .\" The following program demonstrates this "feature":
  84 .\"
  85 .\" #define _GNU_SOURCE
  86 .\" #include <sys/syscall.h>
  87 .\" #include <sys/wait.h>
  88 .\" #include <stdio.h>
  89 .\" #include <stdlib.h>
  90 .\" #include <unistd.h>
  91 .\"
  92 .\" int
  93 .\" main(int argc, char *argv[])
  94 .\" {
  95 .\"    /* The following statement fills the getpid() cache */
  96 .\"
  97 .\"    printf("parent PID = %ld\n", (long) getpid());
  98 .\"
  99 .\"    if (syscall(SYS_fork) == 0) {
 100 .\"        if (getpid() != syscall(SYS_getpid))
 101 .\"            printf("child getpid() mismatch: getpid()=%ld; "
 102 .\"                    "syscall(SYS_getpid)=%ld\n",
 103 .\"                    (long) getpid(), (long) syscall(SYS_getpid));
 104 .\"        exit(EXIT_SUCCESS);
 105 .\"    }
 106 .\"    wait(NULL);
 107 .\"}
 108 In addition, there were cases where
 109 .BR getpid ()
 110 could return the wrong value even when invoking
 111 .BR clone (2)
 112 via the glibc wrapper function.
 113 (For a discussion of one such case, see BUGS in
 114 .BR clone (2).)
 115 Furthermore, the complexity of the caching code had been
 116 the source of a few bugs within glibc over the years.
 117 .PP
 118 Because of the aforementioned problems,
 119 since glibc version 2.25, the PID cache is removed:
 120 .\" commit c579f48edba88380635ab98cb612030e3ed8691e
 121 .\" https://sourceware.org/glibc/wiki/Release/2.25#pid_cache_removal
 122 calls to
 123 .BR getpid ()
 124 always invoke the actual system call, rather than returning a cached value.
 125 .SH SEE ALSO
 126 .BR clone (2),
 127 .BR fork (2),
 128 .BR kill (2),
 129 .BR exec (3),
 130 .BR mkstemp (3),
 131 .BR tempnam (3),
 132 .BR tmpfile (3),
 133 .BR tmpnam (3),
 134 .BR credentials (7),
 135 .BR pid_namespaces (7)