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What's the difference between a header file and a library?

Can anyone tell me what’s the actual meaning of a header file and a library file and their difference?

For example we include header file with .h extension in our program and its just the definition but the actual implementation is defined in library files and this is done at linking stage this is what people say but sometimes we include the library files directory too for the programs to generate the exec file for example in posix threads people say to include the -lpthread in the command line but why when we included the header file #include<> why we still need to include the library files too may i know the reason please??

Answer 1

Generally, a header file notifies the compiler of certain things (mostly their existence or declarations) so that the compiler can correctly build a single translation unit (such as a single C file).

A library file is the actual executable code that does the work as specified in that header file. This is linked in by the linker to provide the actual functionality (the _definitions rather than just the declarations).

So, in your example, you may have the line:

#include <pthread.h>

which tells the compiler all about the existence of the pthread_mutex_this, pthread_condvar_that and pthread_thread_the_other stuff but doesn’t actually provide the implementations of those things.

The -lpthread option tells the linker that it should locate a library based on the pthread name from which it can pull in the actual implementations, in order to forn the final executable.

Similarly, while stdio.h holds information about the I/O stuff, the actual code for it will be in the runtime library (though you rarely have to link that library specifically since the compiler will try to take care of it for you). Because you usually link with the compiler (i.e., the compiler invokes the linker for you), it knows that you’re probably going to need the C run time library. If you were to use the linker directly (such as by using the ld command), that probably wouldn’t happen, and you’d have to be explicit.