Where is the function definition of system headers?

[buddha87]

When we write a C or C++ code, it is very common we include system headers at the beginning, such as;

#include <stdio.h>
#include <stdlib.h>
#include <string>

In Linux, these headers are usually in /usr/include, which belongs to search path. However, these are just header files. Where are the source codes or library of these header files?

Thank you.

[Zhuge]

They are defined in system libraries, such as libc or libstdc++. They are linked in by the linker after your sources are compiled. I would expect them to be found in the /usr/lib directory.

[buddha87]

Thank you. I can see a lot of .so and .a files are in /usr/lib

So when g++ or gcc link them, they will automatically search these paths to find the library files ?
Any idea how to check what paths g++ or gcc will go thru to search the library files?

Thank you a lot.

[JLBorges]

ld -verbose | grep SEARCH

gcc -print-search-dirs

http://coliru.stacked-crooked.com/a/fcb66a6f01026dda

[buddha87]

Thank you very much!

I have another questions.

When I write the makefile to compile the codes, if in my codes I have used the library in /usr/lib, for example : libpcap.a or libpcap.so. Do I have to add flag : -lpcap in my makefile ?

Generally speaking is, if I use a library file (either .a or .so), I need add the corresponding flags to makefile as gcc or g++ option, correct?

Thank you.

[JLBorges]

> if in my codes I have used the library in /usr/lib, for example : libpcap.a or libpcap.so.
> Do I have to add flag : -lpcap in my makefile ?
> if I use a library file (either .a or .so), I need add the corresponding flags to makefile as gcc or g++ option, correct?

Yes.

If the library is not in one of the directories listed with ld -verbose | grep SEARCH, you would also need to add the directory containing the library with the -L option.
https://gcc.gnu.org/onlinedocs/gcc-6.1.0/gcc/Directory-Options.html#Directory-Options

[buddha87]

Thank you!