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

[Doug Hardie]

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