This Mini‑HOWTO answers the question: “Why can’t the MinGW compilers find my project’s header files?”
Prior to the publication of the GCC‑7 documentation, (when the wording changed, to provide only a brief outline of the identification method described below), the CPP Section of the GCC Manual indicated that header files may be located in the following directories:–
On a normal Unix system, if you do not instruct it otherwise,
[GCC] will look for headers requested with
#include
<file>
in:
/usr/local/include libdir/gcc/target/version/include /usr/target/include /usr/include
For C++
programs,
it will also look in /usr/include/g++-v3
, first.
Of course, the MinGW ports of GCC target Microsoft’s Windows system, which is not a “normal UNIX system”. Nevertheless, many users, particularly those coming from a UNIX or GNU/Linux background, and especially when they use MSYS to emulate a UNIX file system model on their MS‑Windows hosts, are surprised to find that MinGW, using its default configuration, does not automatically search for header files in these directories.
So, if MinGW compilers don’t search in these “normal” directories, where do they search?
Before answering this, it may be useful to understand why the MinGW compilers do not search in these directories. Firstly, on a native MS‑Windows host, these directories do not typically exist; the nature of the file system, with multiple distinct roots distributed over multiple distinctly named devices, means that such path names would be ambiguous. Secondly, since MinGW itself is not a suite of MSYS applications, even if such directories are created in the MSYS environment, being native MS‑Windows applications, the MinGW compilers would have no means to locate them.
Thus,
to avoid imposing draconian restrictions on how users install MinGW,
the maintainers have chosen to adopt a restricted search paradigm,
which is determined on the basis of where MinGW is itself installed.
The actual search path,
for any specific installation of MinGW may be determined,
by running the compiler itself,
with the ‘-v
’ option;
typically, for a MinGW‑GCC installation in C:\MinGW
,
we might see something similar to the following,
from (the now dated, but nonetheless still typical)
MinGW‑GCC‑3.4.5:–
$ touch foo.c bar.cpp $ gcc -v -c foo.c : #include "..." search starts here: #include <...> search starts here: c:/mingw/bin/../lib/gcc/mingw32/3.4.5/../../../../include c:/mingw/bin/../lib/gcc/mingw32/3.4.5/include End of search list. : $ g++ -v -c bar.cpp : #include "..." search starts here: #include <...> search starts here: c:/mingw/bin/../lib/gcc/mingw32/3.4.5/../../../../include/c++/3.4.5 c:/mingw/bin/../lib/gcc/mingw32/3.4.5/../../../../include/c++/3.4.5/mingw32 c:/mingw/bin/../lib/gcc/mingw32/3.4.5/../../../../include/c++/3.4.5/backward c:/mingw/bin/../lib/gcc/mingw32/3.4.5/../../../../include c:/mingw/bin/../lib/gcc/mingw32/3.4.5/include End of search list. :
This raw‑format output, as displayed by the compilers, may be difficult for human readers to interpret. Thus, it may be helpful to reduce it to canonical path forms:–
$ gcc -v -c foo.c 2>&1 | sed ':1;s,/[^/.][^/]*/\.\./,/,;t 1' : #include "..." search starts here: #include <...> search starts here: c:/mingw/include c:/mingw/lib/gcc/mingw32/3.4.5/include End of search list. : $ g++ -v -c bar.cpp 2>&1 | sed ':1;s,/[^/.][^/]*/\.\./,/,;t 1' : #include "..." search starts here: #include <...> search starts here: c:/mingw/include/c++/3.4.5 c:/mingw/include/c++/3.4.5/mingw32 c:/mingw/include/c++/3.4.5/backward c:/mingw/include c:/mingw/lib/gcc/mingw32/3.4.5/include End of search list. :
The minimal list of directories,
identified as described above,
specifies the only locations
which will be searched by default,
for system header files,
or for headers associated with user installed libraries;
however, there is one exception.
Astute readers may have noticed that
the include file search path is itemised in a pair of sequential lists,
with the second being concatenated to the first; however, the first,
identified as the
#include
"..."
search
list appears to be empty.
In reality, this apparent emptiness may be misleading;
unless the user specifies the ‘-I-
’ option
when invoking GCC, this list contains exactly one directory:
the directory in which the source file containing the
#include
"file"
directive resides.
Notice that this one additional directory
is not searched for headers specified
with the #include
<file>
form of
the #include
directive;
it applies only for those headers specified using
the #include
"file"
form of the directive.
Also note that the location of this additional directory is not fixed;
it is dependent on the location of the source file being compiled,
and, while unique for each individual source file,
it may vary even within the compilation scope for any project,
and even within the scope of a single translation unit,
if multiple source files are distributed among different directories
in the project’s source hierarchy.
To clarify this point, let us consider a trivial example. Suppose that we have a small project, which provides all of it’s sources in a single directory; among them we might have:–
~/src : :--- bar.h : :--- foo.c :--- foo.h :
Further suppose that the file, foo.c
,
includes the project‑local header, foo.h
:–
/* foo.c */ : #include "foo.h" :
and that foo.h
, in turn,
includes bar.h
:–
/* foo.h */ : #include "bar.h" :
With this arrangement,
we may create a build directory which is a sibling of src
,
and with that as current directory,
we may compile foo.c
:–
$ mkdir ~/build $ cd ~/build $ gcc -c ../src/foo.c $
and we might have every expectation that it will compile correctly;
it certainly should have no problem finding the pair of header files,
foo.h
and bar.h
.
Now, suppose that the project grows
to such a size that we decide to separate our headers
into a subdirectory of src
:–
~/src : :--- foo.c : :--- hdr : : : :--- bar.h : :--- foo.h : :
After making this rearrangement,
if we again try to compile foo.c
,
we will likely see errors resulting from the failure
to locate the header file foo.h
:–
$ gcc -c ../src/foo.c ../src/foo.c:3:17: foo.h: No such file or directory : $
To correct this, we must also modify foo.c
,
to reflect the relocation of the header files:–
/* foo.c */ : #include "hdr/foo.h" :
and this should be sufficient to again let us compile
foo.c
successfully:–
$ gcc -c ../src/foo.c $
Note here,
that we did not make any corresponding change
in bar.h
; that would have been wrong!
The rule is that, in any #include
directive of the form:–
#include "path/file"
path
must be a relative path,
and it must be relative to the directory in which the file containing
the pertinent #include
directive is itself located.
In our example,
since foo.h
and bar.h
are still
in the same directory,
searches for files included by either will commence in that same directory.
If we had changed foo.h
,
in a manner similar to the change we made in foo.c
,
then foo.h
would have attempted
to #include
the file ~/src/hdr/hdr/bar.h
,
instead of the intended ~/src/hdr/bar.h
.
While the default header search paths may suffice for many simple projects, as project complexity grows, it will often become more convenient to collect header files into a central directory which is not in the default search path; this is particularly true of projects which provide, and possibly install, one or more function libraries, either exclusively for their own use, or maybe even for general use by other projects. In such cases, GCC must be explicitly notified, by specifying appropriate command line options, to identify the additional directories in which to search.
The relevant sections of
the GCC Manual,
and its associated C preprocessor manual,
document several options,
which may be used to specify directories to search for header files;
of these,
the most widely used is the
-I
dir
option.
As an example of its use, let us reconsider the previous example,
with the project source files organised within the hierarchy:–
~/src : :--- foo.c : :--- hdr : : : :--- bar.h : :--- foo.h : :
but this time,
we will not adapt foo.c
to locate foo.h
using the default include file search path:–
/* foo.c */ : #include "foo.h" :
As we saw previously,
if we try to compile this unmodified foo.c
,
without explicitly specifying any header search path,
the compilation will fail:–
$ gcc -c ../src/foo.c ../src/foo.c:3:17: foo.h: No such file or directory : $
Previously,
we corrected this failure by modifying foo.c
,
to bring foo.h
into the
default #include
"..."
search path.
This time, we will correct it, not by modifying foo.c
,
but by explicitly adding our project’s
local hdr
directory to the search path,
when we invoke GCC to compile foo.c
:–
$ gcc -I ../src/hdr -c ../src/foo.c $
which, we may observe, again results in successful compilation.
Many projects have dependencies on external libraries, which are not provided by a standard MinGW installation. Users who wish to build the applications provided by such projects must first install the appropriate prerequisite libraries. Such libraries are referred to as “locally installed libraries”, and they must be installed such that MinGW’s GCC can readily locate both the libraries themselves, and any header files installed as prerequisites for specifying the interface to the library functions.
The installation of
such external libraries may be viewed as a challenge,
for some users.
Typically,
and particularly in the case of library packages
originally developed for Unix‑like platforms,
such libraries would be installed into
the /usr/local/...
directory hierarchy.
However, as we have seen above,
MinGW’s GCC
does not consider any such hierarchy in its default search paths,
either for header files, or for libraries.
Thus, the challenge is to choose an installation strategy
which achieves a satisfactory compromise between:–
<mingw-root>/include
and <mingw-root>/lib
directory hierarchies;
this offers the advantage that the libraries,
and their associated header files,
are placed directly into MinGW‑GCC’s default search paths,
so there is no additional burden placed on the user,
to add additional -I
dir
options,
throughout the build systems of any project using an external library.
However, while perhaps being the simplest option,
this strategy is not without its disadvantages:–
-I
dir
,
(and associated -L
dir
),
options to GCC, through his own projects’ build systems.
(This may be mitigated, in turn, by customising the
GCC Specs File,
to supply the necessary options automatically,
for all invocations of GCC,
or by appropriately defining GCC’s
CPATH and related environment variables).
-I
dir
and -L
dir
options
must be added to each GCC invocation; however,
whereas option (2) requires only one additional path of each type,
this option requires an additional one of each type,
for each and every external library to be deployed.
(While this may still be achieved by appropriate customisation of the
GCC Specs File,
or by appropriately specifying the
CPATH and related environment variables,
the additional complexity, and maintenance overhead,
may be considered unacceptable to many users).