Date: Wed, 6 Nov 2019 21:48:32 -0800 From: Doug Hardie <bc979@lafn.org> To: FreeBSD Questions <freebsd-questions@freebsd.org> Subject: Re: Extension to previous posts: Problems with ld, libc, and "struct stat" Message-ID: <5FBE3E7E-D9B2-486B-98F0-DBCF3E72B6E7@mail.sermon-archive.info> In-Reply-To: <E92C7F26-912F-4443-A37B-E7AF9E025CD8@mail.sermon-archive.info> References: <E92C7F26-912F-4443-A37B-E7AF9E025CD8@mail.sermon-archive.info>
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We now have 12.1 with an updated compiler and linker. Things have =
changed. I added a bit to testprog.c:
master# more testprog.c
#include <stdio.h>
extern char id[4];
extern int sock;
int unknown;
void testfunc(void);
int main(int argc, char **argv) {
testfunc();
unknown =3D 9;
printf ("id (%x) =3D %s\n", (int)&id, id);
printf ("sock(%x) =3D %d\n", (int)&sock, sock);
printf ("unknown (%x) =3D %d\n", (int)&unknown, unknown);
return 0;
}
running it yields:
master# ./testprog
Size of testlib.c is 268 bytes.
id (20400c) =3D aa
sock(204010) =3D 5
unknown (204008) =3D 9
That appears to be correct. Everything worked. However, with lldb, =
instead of error messages I get wrong values:
master# lldb testprog
(lldb) target create "testprog"
Current executable set to 'testprog' (x86_64).
(lldb) b main
Breakpoint 1: where =3D testprog`main + 22 at testprog.c:9:3, address =3D =
0x00000000002012f6
(lldb) r
Process 6039 launching
Process 6039 launched: '/home/doug/zzz/testprog' (x86_64)
Process 6039 stopped
* thread #1, name =3D 'testprog', stop reason =3D breakpoint 1.1
frame #0: 0x00000000002012f6 testprog`main(argc=3D1, =
argv=3D0x00007fffffffeb38) at testprog.c:9:3
6 =09
7 void testfunc(void);
8 int main(int argc, char **argv) {
-> 9 testfunc();
10 unknown =3D 9;
11 printf ("id (%x) =3D %s\n", (int)&id, id);
12 printf ("sock(%x) =3D %d\n", (int)&sock, sock);
(lldb) n
Size of testlib.c is 268 bytes.
Process 6039 stopped
* thread #1, name =3D 'testprog', stop reason =3D step over
frame #0: 0x0000000000201307 testprog`main(argc=3D1, =
argv=3D0x00007fffffffeb38) at testprog.c:10:11
7 void testfunc(void);
8 int main(int argc, char **argv) {
9 testfunc();
-> 10 unknown =3D 9;
11 printf ("id (%x) =3D %s\n", (int)&id, id);
12 printf ("sock(%x) =3D %d\n", (int)&sock, sock);
13 printf ("unknown (%x) =3D %d\n", (int)&unknown, unknown);
(lldb)=20
Process 6039 stopped
* thread #1, name =3D 'testprog', stop reason =3D step over
frame #0: 0x0000000000201312 testprog`main(argc=3D1, =
argv=3D0x00007fffffffeb38) at testprog.c:11:3
8 int main(int argc, char **argv) {
9 testfunc();
10 unknown =3D 9;
-> 11 printf ("id (%x) =3D %s\n", (int)&id, id);
12 printf ("sock(%x) =3D %d\n", (int)&sock, sock);
13 printf ("unknown (%x) =3D %d\n", (int)&unknown, unknown);
14 return 0;
(lldb) p id
(char [4]) $0 =3D ""
(lldb) p &id
(char (*)[4]) $1 =3D 0x000000080024d000
(lldb) p unknown
(int) $2 =3D 9
(lldb) p &unknown
(int *) $3 =3D 0x0000000000204008
(lldb)=20
For the variable unknown lldb displays the correct address and value. =
For id and sock, it displays the wrong address and the value at the =
wrong address. The correct values are in the proper addresses:
x 0x204008
0x00204008: 09 00 00 00 61 61 00 00 05 00 00 00 00 00 00 00 =
....aa..........
-- Doug
> On 16 October 2019, at 14:02, Doug Hardie <bc979@lafn.org> wrote:
>=20
> Here is an issue that has plagued me for some time:
>=20
> testlib.c:
> #include <sys/stat.h>
> #include <stdio.h>
> #include <string.h>
> #include <strings.h>
>=20
> char id[4];
> int sock;
>=20
> void testfunc() {
> struct stat sb;
> stat("testlib.c", &sb);
> strcpy (id, "aa");
> sock =3D 5;
> printf("Size of testlib.c is %i bytes.\n", (int)sb.st_size);
> }
>=20
>=20
> testprog.c:
> #include <stdio.h>
>=20
> extern char id[4];
> extern int sock;
>=20
> void testfunc(void);
> int main(int argc, char **argv) {
> testfunc();
> printf ("id =3D %s\n", id);
> printf ("sock =3D %d\n", sock);
> return 0;
> }
>=20
>=20
> Makefile:
> all: clean testprog run
>=20
> 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
>=20
> clean:
> rm -f testlib.o testlib.so testprog
>=20
> run:
> ./testprog
>=20
>=20
> 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 =3D aa
> sock =3D 5
>=20
>=20
> Running lldb:
> master# lldb testprog
> (lldb) target create "testprog"
> Current executable set to 'testprog' (x86_64).
> (lldb) b main
> Breakpoint 1: where =3D testprog`main + 22 at testprog.c:8, address =3D =
0x0000000000201366
> (lldb) r
> Process 34787 launching
> Process 34787 launched: '/home/doug/zzz/testprog' (x86_64)
> Process 34787 stopped
> * thread #1, name =3D 'testprog', stop reason =3D breakpoint 1.1
> frame #0: 0x0000000000201366 testprog`main(argc=3D1, =
argv=3D0x00007fffffffeb38) at testprog.c:8
> 5 =09
> 6 void testfunc(void);
> 7 int main(int argc, char **argv) {
> -> 8 testfunc();
> 9 printf ("id =3D %s\n", id);
> 10 printf ("sock =3D %d\n", sock);
> 11 return 0;
> (lldb) n
> Size of testlib.c is 268 bytes.
> Process 34787 stopped
> * thread #1, name =3D 'testprog', stop reason =3D step over
> frame #0: 0x000000000020137f testprog`main(argc=3D1, =
argv=3D0x00007fffffffeb38) at testprog.c:9
> 6 void testfunc(void);
> 7 int main(int argc, char **argv) {
> 8 testfunc();
> -> 9 printf ("id =3D %s\n", id);
> 10 printf ("sock =3D %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 =3D aa
> sock =3D 5
> Process 34787 resuming
>=20
>=20
> 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?
>=20
> 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.
>=20
> -- Doug
>=20
> _______________________________________________
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