To build a program in ELF, use gcc as always. To build
in a.out, use gcc -b i486-linuxaout .
$ cat >hello.c
main() { printf("hello, world\n"); }
^D
$ gcc -o hello hello.c
$ file hello
hello: ELF 32-bit LSB executable i386 (386 and up) Version 1
$ ./hello
hello, world
This is perhaps an appropriate time to answer the question ``if a.out
compilers default to producing a program called a.out, what
name does an ELF compiler give its output?''. Still a.out,
is the answer. Boring boring boring ... :-)
To build libfoo.so as a shared library, the basic steps look like this:
$ gcc -fPIC -c *.c
$ gcc -shared -Wl,-soname,libfoo.so.1 -o libfoo.so.1.0 *.o
$ ln -s libfoo.so.1.0 libfoo.so.1
$ ln -s libfoo.so.1 libfoo.so
$ export LD_LIBRARY_PATH=`pwd`:$LD_LIBRARY_PATH
This will generate a shared library called libfoo.so.1.0, and
the appropriate links for ld (libfoo.so) and the dynamic
linker (libfoo.so.1) to find it. To test, we add the current
directory to LD_LIBRARY_PATH.
When you're happpy that the library works, you'll have to move it to,
say, /usr/local/lib, and recreate the appropriate links.
Note that the libfoo.so link should point to
libfoo.so.1, so it doesn't need updating on every minor
version number change. The link from libfoo.so.1 to
libfoo.so.1.0 is kept up to date by ldconfig, which on
most systems is run as part of the boot process.
$ su
# cp libfoo.so.1.0 /usr/local/lib
# /sbin/ldconfig
# ( cd /usr/local/lib ; ln -s libfoo.so.1 libfoo.so )
These are covered extensively in H J Lu's ELF programming
document, and the dlopen(3) manual page, which can be found
in the ld.so package. Here's a nice simple example though: link it
with -ldl
#include <dlfcn.h>
#include <stdio.h>
main()
{
void *libc;
void (*printf_call)();
if(libc=dlopen("/lib/libc.so.5",RTLD_LAZY))
{
printf_call=dlsym(libc,"printf");
(*printf_call)("hello, world\n");
}
}
Your existing copy of gdb will most likely work unchanged
with ELF programs. The new version in the GCC directory on
tsx-11 is reported to be better at debugging programs that use shared
libraries and dynamic loading, and to understand ELF core dumps.
Note that 1.2 series kernels cannot generate core dumps from ELF programs anyway. 1.3 can.