Hi: given a C source file that beginsWhere does the compiler look for the source where the objects declared in stdio.h are defined?

that depends on the compiler itself, usually in /usr/include, but it is only the header
the source code of those objects are already compiled and will be linked together with your code.

OK. Let's subtitute usb.h for stdio.h in a sentence like
In a certain library the functions declared by usb.h are already compiled. Specifically, there is the function usb_control_msg in usb.h. So the sources may come from many places?! As I have in vain looked for complete specifications for the use of that function, I wanted to go to the source. Now I see I was being naive.

in general you do not need the source, just the documentation.
http://libusb.sourceforge.net/doc/fu...ontrolmsg.html
but sometimes it is not sufficient. In that case you can try to browse the source here:
http://lxr.free-electrons.com/source.../message.c#L81
(but I'm not really sure if it was the same version)

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stdio is part of glibc, so the sources for the functions are in the glibc source code.

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should work on Debian-based distros. On redhat you need to look at rpm info to find the source rpm.

Yes, the http://libusb.sourceforge.net/doc/fu...ontrolmsg.html is insufficient. And the second link, well I have /usb/core/message.c in my disk already, and the order and quantity of the arguments are different to those declared in /usr/include/usb.h. Doesn't matter, for it would be hard for me to draw safe conclusions form the sources, now I see.

What I need, is something like The GNU C Library.htm, which explains everything in such detail and so much intelligibility. Look at the sourceforge.net link and tell me if you can understand what it is about! And all the pages from Google refer to this kind of documentation.

usb.h comes from libusb package, if you look at the package sources, there is libusb-compat-x.y.z.tar.xz containing usb.h and core.c which contains the implementations from usb.h

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