Why do projects use the -I include switch given the dangers?

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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?


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.


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