Can GDB reload executable with a statically linked library? - c

Question

Normally when using gdb I can stop execution and rebuild the executable and restart without loosing my breakpoints. When I try this with an executable that has a statically linked library I get an error stating that I cannot open the executable file during the build.
A concrete example:
The library files:
libtest.h:
int square(int a);
libtest.c:
int square(int a) {
return a * a;
}
The library is compiled with:
gcc -g -c libfile.c
ar rcs libtest.a libfile.o
Main file a.c contains:
#include <stdio.h>
#include <stdlib.h>
#include "libfile.h"
int main() {
printf( "2 squared is %d\n", square(2) );
return 0;
}
The project is compiled and linked like this:
gcc -g -c a.c
gcc a.o -g --static -L. -ltest -o gdb_test
If I load the resulting file gdb_test into gdb it doesn't matter if it is running it not. As long is gdb is open a subsequent build will fail during the link step:
/usr/bin/ld: cannot open output file gdb_test: Permission denied
Is there a way around this? I would like to be able to work with gdb without having to restart it and loose my breakpoints.

Answer 1

It is probably a matter of versions of GDB or GCC; it works for me:
% gcc -g -c libtest.c
gcc -g -c libtest.c
% ar rcs libtest.a libtest.o
ar rcs libtest.a libtest.o
% gcc -g -c -Wall a.c
gcc -g -c -Wall a.c
% gcc -Wall -g a.o -L. -ltest -o gdb_test
gcc -Wall -g a.o -L. -ltest -o gdb_test
% ./gdb_test
2 squared is 4
% gdb ./gdb_test
GNU gdb (GDB) 7.3.50.20111117-cvs-debian
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/basile/tmp/gdb_test...done.
(gdb) r
r
Starting program: /home/basile/tmp/gdb_test
2 squared is 4
[Inferior 1 (process 12271) exited normally]
(gdb) quit
quit
% gcc -v
gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc-4.6.real
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.6.2-9'
--with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr
--program-suffix=-4.6 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc
--with-arch-32=i586
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.2 (Debian 4.6.2-9)
My system is Debian/Sid/amd64. GCC is (Debian 4.6.2-9); ld = binutils = ar is GNU gold (GNU Binutils for Debian 2.22); GDB is GNU gdb (GDB) 7.3.50.20111117-cvs-debian; Gnu Libc is (Debian EGLIBC 2.13-24). Kernel is Linux version 3.1.0-1-amd64 (Debian 3.1.5-1)
And I am able to recompile the program from inside gdb and to run it:
% gdb ./gdb_test
gdb ./gdb_test
GNU gdb (GDB) 7.3.50.20111117-cvs-debian
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/basile/tmp/gdb_test...done.
(gdb) shell gcc -g -c -Wall a.c
shell gcc -g -c -Wall a.c
(gdb) r
r
Starting program: /home/basile/tmp/gdb_test
2 squared is 4
[Inferior 1 (process 12335) exited normally]
(gdb) shell gcc -Wall -g a.o -L. -ltest -o gdb_test
shell gcc -Wall -g a.o -L. -ltest -o gdb_test
(gdb)
(gdb) r
r
`/home/basile/tmp/gdb_test' has changed; re-reading symbols.
Starting program: /home/basile/tmp/gdb_test
2 squared is 4
[Inferior 1 (process 12346) exited normally]
(gdb) quit
quit
%

Answer 2

/usr/bin/ld: cannot open output file gdb_test: Permission denied
This is unlikely to be related to either GDB or ld (or their versions), and is most certainly not related to your use of archive library.
Much more likely this is happening because you are using some "strange" filesystem. Perhaps you are using NTFS or CIFS mount? What does df . say?