diff --git a/checkin_notes b/checkin_notes
index c346a8fa9e..a5a5696ea9 100755
--- a/checkin_notes
+++ b/checkin_notes
@@ -2625,3 +2625,15 @@ David  Dec 11 2002
         tools_work.html
     test/
         *wu
+
+David  Dec 13 2002
+    - use lock file mechanism to prevent multiple instances of
+       core client from running in same directory
+    
+    configure
+    configure.in
+    client/
+        file_names.h
+        main.C
+    lib/
+        util.C,y
diff --git a/client/file_names.h b/client/file_names.h
index 789fe069ea..ae0280a782 100644
--- a/client/file_names.h
+++ b/client/file_names.h
@@ -48,3 +48,4 @@ extern bool is_account_file(char*);
 #define STDERR_FILE_NAME        "stderr.txt"
 #define STDOUT_FILE_NAME        "stdout.txt"
 #define TIME_TESTS_FILE_NAME	"time_tests.xml"
+#define LOCK_FILE_NAME          "lockfile"
diff --git a/client/main.C b/client/main.C
index 6777866d1b..29a2c62f7a 100644
--- a/client/main.C
+++ b/client/main.C
@@ -75,6 +75,10 @@ int main(int argc, char** argv) {
     int retval;
 
     setbuf(stdout, 0);
+    if (lock_file(LOCK_FILE_NAME)) {
+        fprintf(stderr, "Another copy of BOINC is already running\n");
+        exit(1);
+    }
     read_log_flags();
     gstate.parse_cmdline(argc, argv);
     retval = gstate.init();
diff --git a/configure b/configure
index 982a13e44f..ff81b5d1c0 100755
--- a/configure
+++ b/configure
@@ -1888,8 +1888,6 @@ main() {
   r.ru_majflt = r.ru_minflt = 0;
   switch (fork()) {
   case 0: /* Child.  */
-    /* Unless we actually _do_ something, the kernel sometimes doesn't chalk up any system time to this process. */
-    if(fork()) { i = 123; wait(NULL); } else { i = 234; exit(0); }
     sleep(1); /* Give up the CPU.  */
     _exit(0);
   case -1: _exit(0); /* What can we do?  */
@@ -1901,7 +1899,7 @@ main() {
   }
 }
 EOF
-if { (eval echo configure:1905: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
+if { (eval echo configure:1903: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
 then
   ac_cv_func_wait3_rusage=yes
 else
@@ -1923,15 +1921,15 @@ EOF
 
 fi
 
-for ac_func in gethostname gettimeofday mkdir select socket strstr uname
+for ac_func in gethostname gettimeofday mkdir select socket strstr uname lockf flock
 do
 echo $ac_n "checking for $ac_func""... $ac_c" 1>&6
-echo "configure:1930: checking for $ac_func" >&5
+echo "configure:1928: checking for $ac_func" >&5
 if eval "test \"`echo '$''{'ac_cv_func_$ac_func'+set}'`\" = set"; then
   echo $ac_n "(cached) $ac_c" 1>&6
 else
   cat > conftest.$ac_ext <<EOF
-#line 1935 "configure"
+#line 1933 "configure"
 #include "confdefs.h"
 /* System header to define __stub macros and hopefully few prototypes,
     which can conflict with char $ac_func(); below.  */
@@ -1954,7 +1952,7 @@ $ac_func();
 
 ; return 0; }
 EOF
-if { (eval echo configure:1958: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext}; then
+if { (eval echo configure:1956: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext}; then
   rm -rf conftest*
   eval "ac_cv_func_$ac_func=yes"
 else
diff --git a/configure.in b/configure.in
index 10d16f19ab..738237ca1b 100644
--- a/configure.in
+++ b/configure.in
@@ -40,6 +40,6 @@ dnl Checks for library functions.
 AC_PROG_GCC_TRADITIONAL
 AC_FUNC_VPRINTF
 AC_FUNC_WAIT3
-AC_CHECK_FUNCS(gethostname gettimeofday mkdir select socket strstr uname)
+AC_CHECK_FUNCS(gethostname gettimeofday mkdir select socket strstr uname lockf flock)
 
 AC_OUTPUT(RSAEuro/source/Makefile apps/Makefile db/Makefile sched/Makefile lib/Makefile tools/Makefile Makefile api/Makefile)
diff --git a/doc/boinc.gif b/doc/boinc.gif
index 50cde0af9c..bbaa5c2d07 100644
Binary files a/doc/boinc.gif and b/doc/boinc.gif differ
diff --git a/lib/util.C b/lib/util.C
index 2968346366..7909c70682 100755
--- a/lib/util.C
+++ b/lib/util.C
@@ -20,6 +20,9 @@
 #include <string.h>
 #include <math.h>
 #include <ctype.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
 
 #ifdef _WIN32
 #include <time.h>
@@ -147,3 +150,29 @@ int parse_command_line(char* p, char** argv) {
     return argc;
 }
 
+int lock_file(char* filename) {
+    int retval;
+        
+    // some systems have both!
+#ifdef HAVE_LOCKF
+    int lock = open(filename, O_WRONLY|O_CREAT, 0644);
+    retval = lockf(lock, F_TLOCK, 1);
+    // must leave fd open
+#else
+#ifdef HAVE_FLOCK
+    int lock = open(filename, O_WRONLY|O_CREAT, 0644);
+    retval = flock(lock, LOCK_EX|LOCK_NB);
+#endif
+#endif
+                        
+#ifdef _WIN32
+    HANDLE hfile = CreateFile(
+        filename, GENERIC_WRITE,
+        0, 0, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0
+    );
+    if (hfile == INVALID_HANDLE_VALUE) retval = 1;
+    else retval = 0;
+#endif
+    return retval;
+}
+
diff --git a/lib/util.h b/lib/util.h
index 2997e8ae53..1bb264bc7c 100755
--- a/lib/util.h
+++ b/lib/util.h
@@ -23,6 +23,7 @@ extern int double_to_ydhms (double x, int smallest_timescale, char *buf);
 extern double dtime();
 extern void boinc_sleep( int seconds );
 extern int parse_command_line( char *, char ** );
+extern int lock_file(char*);
 
 #ifndef max
 #define max(a,b)            (((a) > (b)) ? (a) : (b))
diff --git a/todo b/todo
index 9fb5581684..90d632551f 100755
--- a/todo
+++ b/todo
@@ -1,3 +1,7 @@
+write garbage collector
+
+prevent 2 core clients from running at once
+
 support for HTTP and SOCKS proxies
 make get_local_ip_addr() work in all cases
 est_time_to_completion doesn't work for non-running tasks