Extension to previous posts: Problems with ld, libc, and "struct stat"

[Doug Hardie]

> On 16 October 2019, at 16:09, MJ <mafsys1234 at gmail.com> wrote:
> 
> 
> On 17/10/2019 8:02 am, Doug Hardie wrote:
>> Here is an issue that has plagued me for some time:
>> 
>> testlib.c:
>> #include <sys/stat.h>
>> #include <stdio.h>
>> #include <string.h>
>> #include <strings.h>
>> 
>> char id[4];
>> int sock;
>> 
>> void testfunc() {
>>   struct stat sb;
>>   stat("testlib.c", &sb);
>>   strcpy (id, "aa");
>>   sock = 5;
>>   printf("Size of testlib.c is %i bytes.\n", (int)sb.st_size);
>> }
>> 
>> 
>> testprog.c:
>> #include <stdio.h>
>> 
>> extern char id[4];
>> extern int sock;
>> 
>> void testfunc(void);
>> int main(int argc, char **argv) {
>>   testfunc();
>>   printf ("id = %s\n", id);
>>   printf ("sock = %d\n", sock);
>>   return 0;
>> }
>> 
>> 
>> Makefile:
>> all:    clean testprog run
>> 
>> testprog:
>>         cc -Wall -g -c -fPIC -o testlib.o testlib.c
>>         cc  -shared -Wl,-export-dynamic -o testlib.so testlib.o
>>         cc -Wall -g -o testprog ./testlib.so testprog.c
>> 
>> clean:
>>         rm -f testlib.o testlib.so testprog
>> 
>> run:
>>         ./testprog
>> 
>> 
>> Using make:
>> rm -f testlib.o testlib.so testprog
>> cc -Wall -g -c -fPIC -o testlib.o testlib.c
>> cc  -shared -Wl,-export-dynamic -o testlib.so testlib.o
>> cc -Wall -g -o testprog ./testlib.so testprog.c
>> ./testprog
>> Size of testlib.c is 268 bytes.
>> id = aa
>> sock = 5
>> 
>> 
>> Running lldb:
>> master# lldb testprog
>> (lldb) target create "testprog"
>> Current executable set to 'testprog' (x86_64).
>> (lldb) b main
>> Breakpoint 1: where = testprog`main + 22 at testprog.c:8, address = 0x0000000000201366
>> (lldb) r
>> Process 34787 launching
>> Process 34787 launched: '/home/doug/zzz/testprog' (x86_64)
>> Process 34787 stopped
>> * thread #1, name = 'testprog', stop reason = breakpoint 1.1
>>     frame #0: 0x0000000000201366 testprog`main(argc=1, argv=0x00007fffffffeb38) at testprog.c:8
>>    5   	
>>    6   	void testfunc(void);
>>    7   	int main(int argc, char **argv) {
>> -> 8   	  testfunc();
>>    9   	  printf ("id = %s\n", id);
>>    10  	  printf ("sock = %d\n", sock);
>>    11  	  return 0;
>> (lldb) n
>> Size of testlib.c is 268 bytes.
>> Process 34787 stopped
>> * thread #1, name = 'testprog', stop reason = step over
>>     frame #0: 0x000000000020137f testprog`main(argc=1, argv=0x00007fffffffeb38) at testprog.c:9
>>    6   	void testfunc(void);
>>    7   	int main(int argc, char **argv) {
>>    8   	  testfunc();
>> -> 9   	  printf ("id = %s\n", id);
>>    10  	  printf ("sock = %d\n", sock);
>>    11  	  return 0;
>>    12  	}
>> (lldb) p id
>> error: use of undeclared identifier 'id'
>> (lldb) p sock
>> error: Couldn't materialize: couldn't get the value of variable sock: testlib.so[0x4004] can't be resolved, testlib.so is not currently loaded
>> error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression
>> (lldb) c
>> id = aa
>> sock = 5
>> Process 34787 resuming
>> 
>> 
>> You notice that lldb cannot display values for id or sock.  It even gives quite different messages about them.  However the program can access the values and it prints them out properly.  Why can't lldb see them?  How can that be corrected?
>> 
>> What is even more interesting is that in the real application there are quite a few of these global variables and lldb can display some of them, just not all.  Possibly it has to do with the specific names as DATE generally works.  sock and id never seem to work.
>> 
>> -- Doug
> 
> Well it's obviously wrong. It's a bug in lldb. Unless you have to specifically load the shared library in? (process load testlib?)
> 
> I tested this with gdb, it works as expected. That's probably why I still use gdb...
> 

I tried gdb on 12.0 and got the following:

master# /usr/libexec/gdb testprog
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "amd64-marcel-freebsd"...
(gdb) b main
Breakpoint 1 at 0x201366: file testprog.c, line 8.
(gdb) r
Starting program: /usr/home/doug/zzz/testprog 

Breakpoint 1, main (argc=1, argv=0x7fffffffeb30) at testprog.c:8
8	  testfunc();
Current language:  auto; currently minimal
(gdb) n
Size of testlib.c is 268 bytes.
9	  printf ("id = %s\n", id);
(gdb) p id
$1 = 0x80024b000 ""
(gdb) p sock
$2 = 0
(gdb) n
id = aa
10	  printf ("sock = %d\n", sock);
(gdb) 
sock = 5
11	  return 0;
(gdb) ^DThe program is running.  Exit anyway? (y or n) y


It doesn't throw an error, but it shows incorrect values.

-- Doug