man2/getpid.2

   1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
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  23 .TH GETPID 2 2008-09-23 "Linux" "Linux Programmer's Manual"
  24 .SH NAME
  25 getpid, getppid \- get process identification
  26 .SH SYNOPSIS
  27 .B #include <sys/types.h>
  28 .br
  29 .B #include <unistd.h>
  30 .sp
  31 .B pid_t getpid(void);
  32 .br
  33 .B pid_t getppid(void);
  34 .SH DESCRIPTION
  35 .BR getpid ()
  36 returns the process ID of the calling process.
  37 (This is often used by
  38 routines that generate unique temporary filenames.)
  39
  40 .BR getppid ()
  41 returns the process ID of the parent of the calling process.
  42 .SH ERRORS
  43 These functions are always successful.
  44 .SH CONFORMING TO
  45 POSIX.1-2001, 4.3BSD, SVr4.
  46 .SH NOTES
  47 Since glibc version 2.3.4,
  48 the glibc wrapper function for
  49 .BR getpid ()
  50 caches PIDs,
  51 so as to avoid additional system calls when a process calls
  52 .BR getpid ()
  53 repeatedly.
  54 Normally this caching is invisible,
  55 but its correct operation relies on support in the wrapper functions for
  56 .BR fork (2),
  57 .BR vfork (2),
  58 and
  59 .BR clone (2):
  60 if an application bypasses the glibc wrappers for these system calls by using
  61 .BR syscall (2),
  62 then a call to
  63 .BR getpid ()
  64 in the child will return the wrong value
  65 (to be precise: it will return the PID of the parent process).
  66 .\" The following program demonstrates this "feature":
  67 .\"
  68 .\" #define _GNU_SOURCE
  69 .\" #include <sys/syscall.h>
  70 .\" #include <sys/wait.h>
  71 .\" #include <stdio.h>
  72 .\" #include <stdlib.h>
  73 .\" #include <unistd.h>
  74 .\"
  75 .\" int
  76 .\" main(int argc, char *argv[])
  77 .\" {
  78 .\"    /* The following statement fills the getpid() cache */
  79 .\"
  80 .\"    printf("parent PID = %ld\n", (long) getpid());
  81 .\"
  82 .\"    if (syscall(SYS_fork) == 0) {
  83 .\"        if (getpid() != syscall(SYS_getpid))
  84 .\"            printf("child getpid() mismatch: getpid()=%ld; "
  85 .\"                    "syscall(SYS_getpid)=%ld\n",
  86 .\"                    (long) getpid(), (long) syscall(SYS_getpid));
  87 .\"        exit(EXIT_SUCCESS);
  88 .\"    }
  89 .\"    wait(NULL);
  90 .\"}
  91 See also
  92 .BR clone (2)
  93 for discussion of a case where
  94 .BR getpid ()
  95 may return the wrong value even when invoking
  96 .BR clone (2)
  97 via the glibc wrapper function.
  98 .SH SEE ALSO
  99 .BR clone (2),
 100 .BR fork (2),
 101 .BR kill (2),
 102 .BR exec (3),
 103 .BR mkstemp (3),
 104 .BR tempnam (3),
 105 .BR tmpfile (3),
 106 .BR tmpnam (3),
 107 .BR credentials (7)