Shriramana Sharma wrote: > main.c calls function firstcall() > a.c defines function firstcall() which calls secondcall() > b.c defines function secondcall() > > To build the executable main, I need to do: > > gcc -fPIC -c a.c > gcc -fPIC -shared -o liba.so a.o > gcc -fPIC -c b.c > gcc -fPIC -shared -o libb.so b.o > gcc -o main main.c -L. -la -lb > > My question is, why does not the creation of liba.so require the > presence of and linking against libb.so, seeing as a.o contains > firstcall() which calls secondcall() present only in libb.so? > > The gcc command building liba.so does not use -c which would prevent the > linker from being called. So if the linker is called then it should > require the presence of and linking to libb.so to resolve the reference > to the external function secondcall(). But this does not happen. Libraries can have undefined references. Sometimes this is necessary, e.g. if the library depends upon a function which is supposed to be provided by the executable. > What does happen is that I am supposed to specify at the point of > building the *executable* not only the libraries that the executable > depends on, but also the libraries that *those* libraries depend on, as > well as, presumably, any third-level, fourth-level and upto n-th level > dependencies. Not if the libraries themselves have dependency information. E.g.: gcc -fPIC -c b.c gcc -fPIC -shared -o libb.so b.o gcc -fPIC -c a.c gcc -fPIC -shared -o liba.so a.o -L. -lb gcc -o main main.c -L. -Wl,-rpath-link,. -la > This seems highly counter-intuitive. Why would the linker require *only* > at *executable* build-time the libraries, You're looking it at the wrong way. Why would the linker require all external symbols to be resolved at the time the library is built? > and the whole dependency *tree* at that, to be specified? Ultimately, any symbols which are required must be provided, so this is checked when the executable is built. There isn't any need to check it before then. > Why does it not need at each level of > build of an executable or library its and only its own dependencies? Because there is no reason for such a requirement. -- Glynn Clements <glynn@xxxxxxxxxxxxxxxxxx> - To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
