undefined reference to `fwrite'

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originally posted by kwokwong@Dec 29 2004, 10:51 AM

hi all,

we got this error if we use the (small) newlib c library:

-- start --

error pure.o * in function `__cxa_pure_virtual': undefined reference to `fwrite' pact_sb line 0

-- end

fyi: we do remove/comment all printf statements

how can we get rid of this error?

any one helps?

regards,

kwok wong

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The reference is made in the file pure.cc:

--start --# ifdef _GLIBCPP_HAVE_UNISTD_H# include <unistd.h># define writestr(str) write(2, str, sizeof(str) - 1)# ifdef __GNU_LIBRARY__

/* Avoid forcing the library&#39;s meaning of `write&#39; on the user program

by using the "internal" name (for use within the library). */

/*# define write(fd, buf, n) __write((fd), (buf), (n))*/# endif# else# include <cstdio># define writestr(str) std::fputs(str, stderr)# endif

extern "C" void

__cxa_pure_virtual (void)

{

writestr ("pure virtual method called\n");

std::terminate ();

}

-- end

Who can tell what this funtions does and/or which ohter (libray) funtion requires this functions?

The function __cxa_pure_virtual() is a placeholder function inserted automatically by the compiler. It is used whenever you declare a pure virtual function in a base class (that is, a function that won&#39;t have an implementation provided:

class an_abstract_base_class
{
    virtual void do_something() = 0;
}

Since you can&#39;t create instances of an_abstract_base_class, you shouldn&#39;t be able to call the non-existant function an_abstract_base_class::do_something(). The compiler still needs to create a vtable (virtual function dispatch table) for abstract classes (those with one or more pure virutal methods like an_abstract_base_class), so it has to put something in the vtable entries corresponding to pure virtual functions: it uses __cxa_pure_virtual(). In properly written code __cxa_pure_virtual() should never get called. If it does somehow get called it&#39;s a signal that something has gone seriously wrong with your code.

So, to eliminate the undefined reference error you have two choices: Either remove all pure virtual functions from you project, or provide your own version of __cxa_pure_virtual() that doesn&#39;t use facilities which aren&#39;t available on your hardware platform (e.g. just go into an infinite loop flashing an LED).

Thanks for the reply.

I&#39;ll look for all unimplemented pure virtual methods.