assign to extern in shared library

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I am building a user defined shell where the user can dynamically load new pluggins into the shell.

In some of my shared libraries I reference global variables using the keyword extern, such as the pluggin below

C++

extern char *prompt;

int setprompt(char *argv[]) {
	prompt = argv[1];
	return 0;
}

only, when I execute my shell program (which links this library) I get the error

./setprompt.so: undefined symbol: prompt

Why can't I assign something to the global variable?

Here's the code from the shell code which includes the definition of prompt

C++

char *builtin_functions[64] =  {
	"comment", "setprompt", "cd", "bgjobs", "fg", "loadpluggin", "culater"
};
char *prompt = "upsh";
void *pointers_to_functions[64];
char *background_jobs[64];
int ptr_to_func_index = 7;
int bg_index = 0;


int main()
{

Posted 16-Nov-15 18:11pm

Member 12144933

Updated 16-Nov-15 18:51pm

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Jochen Arndt · Answer 1 · 2015-11-16T21:10:00

So you have an executable and a shared library. The library exports symbols to be used by the executable.

But you also want the library to access a symbol from the executable. This is called 'reverse dependency' and your executable must also export the symbol. This can be done by using the --export-dynamic linker option. See also section 3.4. Creating a Shared Library of the Program Library HOWTO[^].

Sergey Alexandrovich Kryukov · Answer 2 · 2015-11-16T18:19:00

Solution 1

It has nothing to do with "global". This is a linker error, not compilation. You simply did not define prompt anywhere. The linker looks up for all object files/libraries and cannot find where it is defined. Apparently, you did not bother to defined it anywhere. I hope you understand what is definition and declaration, the basics of C and C conception of separate compilation model.

—SA

Posted 16-Nov-15 18:19pm

Sergey Alexandrovich Kryukov

Comments

Member 12144933 17-Nov-15 0:33am

Oh sorry, prompt was also defined in the main program -- the one that does the linking. It's defined as `char *prompt = "upsh";`

Sergey Alexandrovich Kryukov 17-Nov-15 0:40am

If it really was defined, you would not have this error. It could be not a part of the solution, not defined in proper context, something like that.
For example, if it is defined in the body of the function main it only exists on the stack frame of this function.
You need to use static linkage, say, define it outside of any functions and types, in the top-level context of a compilation unit.
—SA

Member 12144933 17-Nov-15 0:42am

...it is defined outside lol

Sergey Alexandrovich Kryukov 17-Nov-15 0:44am

If it was properly defined, you would not get this error message.
—SA

Member 12144933 17-Nov-15 0:51am

please see above edit

Sergey Alexandrovich Kryukov 17-Nov-15 10:02am

Normally, 3 or more files are involved. You define the object in one C file, declare in a header file, include this header in another C file which gives you access to this object in another compilation unit. Where is all that?

You don't have to have all three, but this is a usual reasonable practice. You don't know what is in what file (instead, you mention shell and other insignificant notions), what goes to compilation, in what order, what goes to the linker, etc.

—SA

Sergey Alexandrovich Kryukov · Answer 3 · 2015-11-16T18:43:00

Most likely, your problem is that you don't understand how C linkage works. You really need to learn it, because this is one of the basics; without it, any non-nonsense C or C++ programming is simply impossible. Please see my comment to Solution 1.

You can start here:
https://bytes.com/topic/c/answers/168304-static-linkage-extern-c[^],
http://en.cppreference.com/w/cpp/language/storage_duration[^].

—SA

assign to extern in shared library

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