Page 128 - C-Language

However, very few programs actually use the facilities of <locale.h> or <tgmath.h>.

• See also How to link multiple implementation files in C?

Include What You Use (IWYU)

Google's Include What You Use project, or IWYU, ensures source files include all headers used in
the code.

Suppose a source file source.c includes a header arbitrary.h which in turn coincidentally includes
freeloader.h, but the source file also explicitly and independently uses the facilities from
freeloader.h. All is well to start with. Then one day arbitrary.h is changed so its clients no longer
need the facilities of freeloader.h. Suddenly, source.c stops compiling — because it didn't meet the
IWYU criteria. Because the code in source.c explicitly used the facilities of freeloader.h, it should
have included what it uses — there should have been an explicit #include "freeloader.h" in the
source too. (Idempotency would have ensured there wasn't a problem.)

The IWYU philosophy maximizes the probability that code continues to compile even with
reasonable changes made to interfaces. Clearly, if your code calls a function that is subsequently
removed from the published interface, no amount of preparation can prevent changes becoming
necessary. This is why changes to APIs are avoided when possible, and why there are
deprecation cycles over multiple releases, etc.

This is a particular problem in C++ because standard headers are allowed to include each other.
Source file file.cpp could include one header header1.h that on one platform includes another
header header2.h. file.cpp might turn out to use the facilities of header2.h as well. This wouldn't be
a problem initially - the code would compile because header1.h includes header2.h. On another
platform, or an upgrade of the current platform, header1.h could be revised so it no longer includes
header2.h, and thenfile.cpp would stop compiling as a result.

IWYU would spot the problem and recommend that header2.h be included directly in file.cpp. This
would ensure it continues to compile. Analogous considerations apply to C code too.

Notation and Miscellany

The C standard says that there is very little difference between the #include <header.h> and
#include "header.h" notations.

[#include <header.h>] searches a sequence of implementation-defined places for a
header identified uniquely by the specified sequence between the < and > delimiters,
and causes the replacement of that directive by the entire contents of the header. How
the places are specified or the header identified is implementation-defined.

[#include "header.h"] causes the replacement of that directive by the entire contents of
the source file identified by the specified sequence between the "…" delimiters. The
named source file is searched for in an implementation-defined manner. If this search

is not supported, or if the search fails, the directive is reprocessed as if it read [#include
<header.h>] …

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