From: Greg Colvin (gcolvin_at_[hidden])
Date: 1999-07-28 21:02:11
From: Beman Dawes <beman_at_[hidden]>
> At 07:03 PM 7/28/99 -0600, Greg Colvin wrote:
> >> Hum... This might be a problem in theory, but in practice are there
> >> really any compilers out there that would prevent (2)? Anyone know
> >> of a real-world compiler with either of these limitations?
> >I don't know about the real world (:-) but the standard seems to
> >say you should use the "name" form for including files.
> I have read paragraphs 2 and 3 several times and can't see any
> difference, other that the ordering of sentances and "..." falling
> back to <...>. Oddly enough, the only problem I ever had with a real
> compiler was with "..." although that was clearly a compiler bug.
> What are you seeing that I am missing?
Not much, that's why I said "seems to". We took these paragraphs
verbatim from the C standard, and the rational says, in effect,
"we wanted to say what these mean but couldn't" so its all pretty
much implementation defined. I had thought that the intent was
<name> forms were intended for standard and system C++ headers;
<name.h> forms were intended for standard and system C headers;
"whatever" forms were intended for user source files;
where "headers" need not be files at all.
In the real world the main difference is how and where to search.
So for GCC we have:
Add the directory dir to the head of the list of directories to be
searched for header files. This can be used to override a system
header file, substituting your own version, since these
directories are searched before the system header file
directories. If you use more than one `-I' option, the directories
are scanned in left-to-right order; the standard system
directories come after.
Any directories you specify with `-I' options before the `-I-'
option are searched only for the case of `#include "file"'; they
are not searched for `#include <file>'. If additional directories
are specified with `-I' options after the `-I-', these directories
are searched for all `#include' directives. (Ordinarily all `-I'
directories are used this way.) In addition, the `-I-' option
inhibits the use of the current directory (where the current input
file came from) as the first search directory for `#include
"file"'. There is no way to override this effect of `-I-'. With
`-I.' you can specify searching the directory which was current when
the compiler was invoked. That is not exactly the same as what the
preprocessor does by default, but it is often satisfactory. `-I-'
does not inhibit the use of the standard system directories for
header files. Thus, `-I-' and `-nostdinc' are independent.
And for VC++ we have:
The path-spec is a filename optionally preceded by a directory
specification. The filename must name an existing file. The syntax of
the path-spec depends on the operating system on which the program is
Both syntax forms cause replacement of that directive by the entire
contents of the specified include file. The difference between the two
forms is the order in which the preprocessor searches for header files
when the path is incompletely specified.
This form instructs the preprocessor to look for include files in the
same directory of the file that contains the #include statement, and
then in the directories of whatever files that include (#include) that
file. The preprocessor then searches along the path specified by the /I
compiler option, then along paths specified by the INCLUDE environment
This form instructs the preprocessor to search for include files first
along the path specified by the /I compiler option, then along the path
specified by the INCLUDE environment variable.
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