Wsrtjtyk

Reading the fine print of the -I switch in GCC, I'm rather shocked to find that using it on the command line overrides system includes. From the preprocessor docs

"You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories."

They don't seem to be lying. On two different Ubuntu systems with GCC 7, if I create a file endian.h:

#error "This endian.h shouldn't be included"

...and then in the same directory create a main.cpp (or main.c, same difference):

#include <stdlib.h>

int main() {}

Then compiling with g++ main.cpp -I. -o main (or clang, same difference) gives me:

In file included from /usr/include/x86_64-linux-gnu/sys/types.h:194:0,

                 from /usr/include/stdlib.h:394,

                 from /usr/include/c++/7/cstdlib:75,

                 from /usr/include/c++/7/stdlib.h:36,

                 from main.cpp:1:

./endian.h:1:2: error: #error "This endian.h shouldn't be included"

So stdlib.h includes this types.h file, which on line 194 just says #include <endian.h>. My apparent misconception (and perhaps that of others) was that the angle brackets would have prevented this, but -I is stronger than I'd thought.

Though not strong enough, because you can't even fix it by sticking /usr/include in on the command line first, because:

"If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain."

Indeed, the verbose output for g++ -v main.cpp -I/usr/include -I. -o main leaves /usr/include at the bottom of the list:

#include "..." search starts here:

#include <...> search starts here:

 .

 /usr/include/c++/7

 /usr/include/x86_64-linux-gnu/c++/7

 /usr/include/c++/7/backward

 /usr/lib/gcc/x86_64-linux-gnu/7/include

 /usr/local/include

 /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed

 /usr/include/x86_64-linux-gnu

 /usr/include

Color me surprised. I guess to make this a question:

What legitimate reason is there for most projects to use -I considering this extremely serious issue? You can override arbitrary headers on systems based on incidental name collisions. Shouldn't pretty much everyone be using -iquote instead?

What legitimate reasons are there for -I over -iquote? -I is standardized (at least by POSIX) while -iquote isn't. (Practically, I'm using -I because tinycc (one of the compilers I want my project to compile with) doesn't support -iquote.)

How do projects manage with -I given the dangers? You'd have the includes wrapped in a directory and use -I to add the directory containing that directory.

• filesystem: includes/mylib/endian.h
  


• command line: -Iincludes
  


• C/C++ file: #include "mylib/endian.h" //or <mylib/endian.h>
  

With that, as long as you don't clash on the mylib name, you don't clash (at least as far header names are concerned).

Looking back at the GCC manuals it looks like -iquote and other options were only added in GCC 4: https://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Directory-Options.html#Directory%20Options

So the use of "-I" is probably some combination of: habit, lazyness, backwards compatibility, ignorance of the new options, compatibility with other compilers.

The solution is to "namespace" your header files by putting them in sub directories. For example put your endian header in "include/mylib/endian.h" then add "-Iinclude" to the command line and you can #include "mylib/endian.h" which shouldn't conflict with other libraries or system libraries.

I this your premise that it's -I that's dangerous is false. The language leaves the search for header files with either form of #include sufficiently implementation-defined that it's unsafe to use header files that conflict with the names of the standard header files at all. Simply refrain from doing this.

An obvious case is cross-compilation. GCC suffers a bit from a historical UNIX assumption that you're always compiling for your local system, or at least something that's very close. That's why the compiler's header files are in the system root. The clean interface is missing.

In comparison, Windows assumes no compiler, and Windows compilers do not assume you're targeting the local system. That's why you can have a set of compilers and a set of SDK's installed.

Now in cross-compilation, GCC behaves much more like a compiler for Windows. It no longer assumes that you intend to use the local system headers, but lets you specify exactly which headers you want. And obviously, the same then goes for the libraries you link in.

Now note that when you do this, the set of replacement headers is designed to go on top of the base system. You can leave out headers in the replacement set if their implementation would be identical. E.g. chances are that <complex.h> is the same. There's not that much variation in complex number implementations. However, you can't randomly replace internal implementation bits like <endian.h>.

TL,DR : this option if for people who know what they're doing. "Being unsafe" is not an argument for the target audience.