Depending on the tools available there are various ways:
Your debugger may have options to select whether to follow the parent or
the child process (or both) after a fork(), which may be
sufficient for some purposes.
Alternatively, your debugger may have an option which allows you to
attach to a running process. This can be used to attach to the child
process after it has been started. If you don't need to examine the very
start of the child process, this is usually sufficient. Otherwise, you
may wish to insert a sleep() call after the fork() in the
child process, or a loop such as the following:
{
volatile int f = 1;
while(f);
}
which will hang the child process until you explicitly set f to 0
using the debugger.
Remember, too, that actively using a debugger isn't the only way to find errors in your program; utilities are available to trace system calls and signals on many unix flavours, and verbose logging is also often useful.
Assuming we're talking about an archive (static) library, the easiest way is to explode all the constituent libraries into their original objects using `ar x' in an empty directory, and combine them all back together. Of course, there is the potential for collision of filenames, but if the libraries are large, you probably don't want to be combining them in the first place....
The precise method for creating shared libraries varies between different systems. There are two main parts to the process; firstly the objects to be included in the shared library must be compiled, usually with options to indicate that the code is to be position-independent; secondly, these objects are linked together to form the library.
Here's a trivial example that should illustrate the idea:
/* file shrobj.c */
const char *myfunc()
{
return "Hello World";
}
/* end shrobj.c */
/* file hello.c */
#include <stdio.h>
extern const char *myfunc();
main()
{
printf("%s\n", myfunc());
return 0;
}
/* end hello.c */
$ gcc -fpic -c shrobj.c
$ gcc -shared -o libshared.so shrobj.o
$ gcc hello.c libshared.so
$ ./a.out
Hello World
By far the best method if you want the library and build procedure to be anything approaching portable is to use GNU Libtool. This is a small suite of utilities which know about the platform-dependent aspects of building shared libraries; you can distribute the necessary bits with your program, so that when the installer configures the package, he or she can decide what libraries to build. Libtool works fine on systems which do not support shared libraries. It also knows how to hook into GNU Autoconf and GNU Automake (if you use those tools to manage your program's build procedure).
If you don't want to use Libtool, then for compilers other than gcc, you should change the compiler options as follows:
(Submission of additional entries for the above table is encouraged.)
Other issues to watch out for:
LD_LIBRARY_PATH environment
variable, but there is usually an additional option to specify this at
link time.
LD_RUN_PATH environment variable).
Generally, no.
On most systems (except AIX), when you link objects to form a shared library, it's rather like linking an executable; the objects don't retain their individual identity. As a result, it's generally not possible to extract or replace individual objects from a shared library.
Some systems provide library functions for unwinding the stack, so that you can (for example) generate a stack dump in an error-handling function. However, these are highly system-specific, and only a minority of systems have them.
A possible workaround is to get your program to invoke a debugger on itself -- the details still vary slightly between systems, but the general idea is to do this:
void dump_stack(void)
{
char s[160];
sprintf(s, "/bin/echo 'where\ndetach' | dbx -a %d", getpid());
system(s);
return;
}
You will need to tweak the commands and parameters to dbx according to
your system, or even substitute another debugger such as gdb,
but this is still the most general solution to this particular problem
that I've ever seen. Kudos to Ralph Corderoy for this one :-)
Here's a list of the command lines required for some systems:
"/bin/echo 'where\ndetach' | dbx /path/to/program %d"
"/bin/echo 'where\ndetach' | dbx -a %d"
"/bin/echo 'where\ndetach' | dbx -p %d"
Go to the first, previous, next, last section, table of contents.