Do you need to use the `-I` flag in `gcc` if the program has the `#include` statements?

Question

In my C my program, I included the header files I need with a #include statement.

When compiling [on a Linux server] do I need to an -I/ flag in the gcc command as well?

I noticed in some places it's included, but in some others its not. I'd really appreciate a definitive answer.

Thanks a lot!

score 2 · Answer 1 · answered Nov 27 '13 at 13:08

2

There is a standard set of include directories. For gcc, you check the preprocessor:

 cpp -v < /dev/null

Which outputs:

#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/gcc/x86_64-redhat-linux/4.4.7/include
 /usr/include
End of search list.

These generally cover the standard headers and such. For your own headers, especially in a folder hierarchy, you may need additional -I flags.

answered Nov 27 '13 at 13:08

Joe

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I don't understand... what does this tell me? – CodyBugstein Nov 27 '13 at 13:22
1

It says: if you `#include` something, it will look there without having to specify `-I`. If your include files are elsewhere you may need to use `-I` for those paths. – Joe Nov 27 '13 at 13:35
So including something in quotations does nothing basically? – CodyBugstein Nov 27 '13 at 14:08
http://stackoverflow.com/questions/21593/what-is-the-difference-between-include-filename-and-include-filename – Joe Nov 27 '13 at 14:42

score 0 · Answer 2 · answered Nov 27 '13 at 13:55

In short, -I tells the compiler where to look for the headers.

Some paths are already searched by the compiler, such as /usr/include. For those headers, you don't -I (e.g. the standard headers or installed libraries). For headers in a location unknown to the compiler, such as your own program or a library whose source you have downloaded, you need to specify the include path (via -I) otherwise the compiler has no idea where to find the include file.

Note that when you include with "file" instead of <file>, the compiler first searches the directory where the including file itself is. For example, let's say you have a directory structure like this:

project
|
+- include
|  |
|  +- header1.h
|  |
|  `- header2.h
|
`- src
   |
   +- funcs1.c
   |
   +- funcs2.c
   |
   `- internal.h

Now let's see some examples:

funcs1.c:

#include "internal.h"

Compiling this doesn't need -I because internal.h is in the same location as funcs1.c.

funcs1.c:

#include <internal.h>

This needs -I/path/to/project/src (or -I. if you are calling the compiler in the same directory as funcs1.c) because #include <...> doesn't search in the directory of the source file.

funcs2.c:

#include <header1.h>
/* or */
#include "header1.h"

This needs -I/path/to/project/include (or -I../include if you are calling the compiler in the same direcotry as funcs2.c) because the location of header1.h is unknown to the compiler in either case. Note that this is regardless of where you call the compiler since the compiler doesn't look at the directory it is being called from, but in the directory the source file is.

funcs1.c:

#include "internal.h"

internal.h:

#include "header2.h"

This still needs -I/path/to/project/include because even though internal.h is found to be included in funcs1.c, it tries to include header2.h whose path needs to be told to the compiler.

Do you need to use the `-I` flag in `gcc` if the program has the `#include` statements?

2 Answers2