Hello!
While moving a large-scaled software product to static linking due to many
problems with incompatible system libraries (like libc / glibc) among the
linux distributions, I discovered the following what seems to me like a BUG:
gethostbyname() does not seem to work when I use it in a shared object on
both of my redhat 6.1 and redhat 6.0 boxes with standard RPMs from CD.
It does work, however, when I do not compile my modules with static linking.
I have the three files (Makefile, shared.c, and loader.c) that repoduce this
bug appended to this email.
I tried to debug the libc resolving mechanism, but unfortunately gdb does not
know sharedFunc contained in the .so file even after dlopen() (using
GNU gdb 4.18 and supplied "-g" to gcc).
I greatly appreciate your hints, since this problem buggers me for weeks! :-)
If it is rather improblable that my question can be answered in this list,
feel free to forward it to forums that will do.
The 6.1 box's configuration:
sven@irving:c/shared.objects > uname -a
Linux irving.XXXXX.XX 2.2.12-20 #1 Mon Sep 27 10:40:35 EDT 1999 i686 unknown
sven@irving:c/shared.objects > cat /etc/redhat-release
Red Hat Linux release 6.1 (Cartman)
sven@irving:c/shared.objects > gcc -v
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/egcs-2.91.66/specs
gcc version egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)
The output of the appended programs looks like this:
sven@irving:c/shared.objects > make clean && make dynamic && ./loader
rm -f loader shared.so
gcc -g -shared -fPIC shared.c -o shared.so
gcc -g -Wall loader.c -o loader -ldl
Entered loader
gethostbyname() info for www.ansi.org:
hostname => www.ansi.org
ip => 165.254.114.5
Entered shared.so
gethostbyname() info for www.ansi.org:
hostname => www.ansi.org
ip => 165.254.114.5
[ So far it works ok, but now look at what follows! ]
sven@irving:c/shared.objects > make clean && make static && ./loader
rm -f loader shared.so
gcc -g -shared -Wall -fPIC shared.c -o shared.so -Wl,"-Bstatic"
gcc -g -Wall loader.c -o loader -ldl -static
Entered loader
gethostbyname() info for www.ansi.org:
hostname => www.ansi.org
ip => 165.254.114.5
Entered shared.so
gethostbyname() info for www.ansi.org:
www.ansi.org: Unknown host /* XXXXXXXXXXXX */
Regards,
Sven Koehler
### Makefile
static: shared.static loader.static
@true
dynamic: shared.dynamic loader.dynamic
@true
shared.static:
gcc -g -shared -Wall -fPIC shared.c -o shared.so -Wl,"-Bstatic"
shared.dynamic:
gcc -g -shared -fPIC shared.c -o shared.so
loader.static:
gcc -g -Wall loader.c -o loader -ldl -static
loader.dynamic:
gcc -g -Wall loader.c -o loader -ldl
clean:
rm -f loader shared.so
### shared.c
#include <stdio.h>
#include <netdb.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
extern int h_errno;
void test_gethostbyname() {
struct hostent *host;
char *hostQuery = "www.ansi.org";
printf(" gethostbyname() info for %s:\n", hostQuery);
host = gethostbyname(hostQuery);
if (host) {
struct in_addr ia = *(struct in_addr *) host->h_addr_list[0];
printf(" hostname => %s\n", host->h_name);
printf(" ip => %s\n", inet_ntoa(ia));
} else {
fprintf(stderr, " %s: %s\n", hostQuery, hstrerror(h_errno));
}
}
int sharedFunc () {
printf("Entered shared.so\n");
test_gethostbyname();
return 0;
}
### loader.c
#include <stdio.h>
#include <dlfcn.h>
#include <netdb.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
extern int h_errno;
void test_gethostbyname() {
struct hostent *host;
char *hostQuery = "www.ansi.org";
printf(" gethostbyname() info for %s:\n", hostQuery);
host = gethostbyname(hostQuery);
if (host) {
struct in_addr ia = *(struct in_addr *) host->h_addr_list[0];
printf(" hostname => %s\n", host->h_name);
printf(" ip => %s\n", inet_ntoa(ia));
} else {
fprintf(stderr, " %s: %s\n", hostQuery, hstrerror(h_errno));
}
}
int main(int argc, char **argv) {
const char *lib = "./shared.so";
void *handle = dlopen(lib, RTLD_LAZY);
printf("Entered loader\n");
test_gethostbyname();
if (handle) {
const char *error;
int (*sFunc)()=dlsym(handle, "sharedFunc");
error = dlerror();
if (error) {
fprintf(stderr, "sharedFunc() not found in shared object '%s':
%s\n",
lib, error);
exit(1);
} else {
sFunc();
}
} else {
fprintf(stderr, "cannot load shared object '%s': %s\n", lib,
dlerror());
exit(1);
}
exit(0);
}
EOE
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