93

So, when we do the following:

#include <stdio.h>

versus 

#include "myFile.h"

the compiler, GCC in my case, knows where that stdio.h (and even the object file) are located on my hard drive. It just utilizes the files with no interaction from me.

I think that on my Ubuntu Linux machine the files are stored at /usr/include/. How does the compiler know where to look for these files? Is this configurable or is this just the expected default? Where would I look for this configuration?

Since I'm asking a question on these include files, what are the source of the files? I know this might be fuzzy in the Linux community but who manages these? Who would provide and manage the same files for a Windows compiler.

I was always under the impression that they come with the compiler but that was an assumption...

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5 Answers 5

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63

See here: Search Path

Summary:

#include <stdio.h>

When the include file is in brackets the preprocessor first searches in paths specified via the -I flag. Then it searches the standard include paths (see the above link, and use the -v flag to test on your system).

#include "myFile.h"

When the include file is in quotes the preprocessor first searches in the current directory, then paths specified by -iquote, then -I paths, then the standard paths.

-nostdinc can be used to prevent the preprocessor from searching the standard paths at all.

Environment variables can also be used to add search paths.

When compiling if you use the -v flag you can see the search paths used.

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54

gcc is a rich and complex "orchestrating" program that calls many other programs to perform its duties. For the specific purpose of seeing where #include "goo" and #include <zap> will search on your system, I recommend:

$ touch a.c
$ gcc -v -E a.c
 ...
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/gcc/i686-apple-darwin9/4.0.1/include
 /usr/include
 /System/Library/Frameworks (framework directory)
 /Library/Frameworks (framework directory)
End of search list.
# 1 "a.c"

This is one way to see the search lists for included files, including (if any) directories into which #include "..." will look but #include <...> won't. This specific list I'm showing is actually on Mac OS X (aka Darwin) but the commands I recommend will show you the search lists (as well as interesting configuration details that I've replaced with ... here;-) on any system on which gcc runs properly.

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Can you explain where from GCC knows to search in /Library/Frameworks in the example you provided? Say, it's hardcoded to search in /usr/include and /usr/local/include but where from additional directories come? From environment?
10

Karl answered your search-path question, but as far as the "source of the files" goes, one thing to be aware of is that if you install the libfoo package and want to do some development with it (i.e., use its headers), you will also need to install libfoo-dev. The standard library header files are already in /usr/include, as you saw.

Note that some libraries with a lot of headers will install them to a subdirectory, e.g., /usr/include/openssl. To include one of those, just provide the path without the /usr/include part, for example:

#include <openssl/aes.h>
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The link doesn't work anymore. Is there any alternative that you can provide?
6

The #include files of gcc are stored in /usr/include . The standard include files of g++ are stored in /usr/include/c++.

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0

As the GNU docs suggest, run cpp -v /dev/null -o /dev/null (for the GNU C PreProcessor, not C plus plus).

Or just cpp -v which will hang waiting for input.

As a quick reference, here is the result on my Ubuntu 22.04:

ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/11/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/11/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.

Installing packages from apt such as libfoo-dev will install to /usr/include. Stuff manually installed not by apt, such as a header-only library, should be put in /usr/local/include. It's a safe bet Linux systems will have these.

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