Revision of HOWTO Specify the Location of External Libraries for use with MinGW from 2009, March 30 - 16:14

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FIXME: Page under construction; little of value here, yet.

I will add to this as time permits; please bear with me. My intent is to offer advice, complementing the information relating to include file search paths, on circumventing the limitations of MinGW's GCC when installing, and subsequently linking with, user added function libraries.


Introduction

This Mini-HOWTO further develops the theme introduced by the IncludePathHOWTO, progressing to a complementary consideration of where the MinGW implementation of GCC, (or more accurately, of the linker, ld, which is actually a component of the GNU binutils package), searches for object code libraries. In particular, it address the question: "How do I ensure that the MinGW linker will find my object libraries?"

In the case of header files, we observed that the GCC Manual makes a definitive statement, regarding the search path for header files, yet we also noted that this statement is not applicable to the MinGW implementation of GCC. In the case of libraries, the GCC Manual makes a much less definitive statement, which is paradoxically more applicable to the MinGW implementation, regarding the default search path:

-llibrary
-l library
    Search the library named library when linking.  (The second alternative
    with the library as a separate argument is only for POSIX compliance and
    is not recommended.)

    It makes a difference where in the command you write this option; the
    linker searches and processes libraries and object files in the order they
    are specified.  Thus, `foo.o -lz bar.o' searches library `z' after file foo.o
    but before bar.o.  If bar.o refers to functions in `z', those functions may
    not be loaded.

    The linker searches a standard list of directories for the library, which is
    actually a file named liblibrary.a.  The linker then uses this file as if it
    had been specified precisely by name.

    The directories searched include several standard system directories plus
    any that you specify with -L.

    :

Note: the second paragraph of the above extract is not strictly relevant to the subject of this Mini-HOWTO; I've chosen not to elide it, because it answers another FAQ: "Why do I get undefined reference errors, when the symbols in question are definitely present in the libraries I've specified with -l options?" -- the usual answer is that the -l options have been placed too early in the command line argument sequence. The remainder, and in particular the final paragraph, are pertinent, vaguely indicating that libraries will be searched for in several standard system directories; these will be identified more explicitly in this Mini-HOWTO.

Determining MinGW's Default Library Search Path

We have observed that the GCC Manual makes no definitive statement, regarding the default directories to be searched for libraries. Since the actual library search is performed by the linker, ld, which is a component of the GNU Binary File Utilities, (more commonly known as binutils), we might hope to find a more definitive statement in the applicable section of the Binutils Manual, but this appears to be equally noncommittal; the only references to library search paths, to be found in this manual are in the
Command Line Options section:

 :

-Lsearchdir
--library-path=searchdir
    Add path searchdir to the list of paths that ld will search for archive
    libraries and ld control scripts.  You may use this option any number of
    times.  The directories are searched in the order in which they are specified
    on the command line.  Directories specified on the command line are searched
    before the default directories.  All -L options apply to all -l options,
    regardless of the order in which the options appear.

    If searchdir begins with =, then the = will be replaced by the sysroot prefix,
    a path specified when the linker is configured.

    The default set of paths searched (without being specified with `-L') depends
    on which emulation mode ld is using, and in some cases also on how it was
    configured.  See Environment.

    The paths can also be specified in a link script with the SEARCH_DIR command.
    Directories specified this way are searched at the point in which the linker
    script appears in the command line.

 :

and in the Linker Scripts / File Commands section:

 :

SEARCH_DIR(path)
    The SEARCH_DIR command adds path to the list of paths where ld looks for
    archive libraries.  Using SEARCH_DIR(path) is exactly like using `-L path'
    on the command line (see Command Line Options).  If both are used, then the
    linker will search both paths.  Paths specified using the command line option
    are searched first.

 :

while the Environment reference, appearing within the Command Line Options reference, apparently has nothing to say, which is related to this subject.

Re: HOWTO Specify the Location of Libraries for use with MinGW

The LIBRARY_PATH environment variable is used by gcc. This is what the man page says about it:

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can’t find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first).

Re: HOWTO Specify the Location of Libraries for use with MinGW

keith's picture

Yes indeed. Notice that this is still a work in progress, and LIBRARY_PATH is one of the customisation options, which needs to be mentioned in the text yet to be written. Do note, however, that on MS-Windows it is not a colon-separated list; it needs to be semicolon-separated, with all of its members specified in native MS-Windows format; (however, MSYS users may specify it as a colon-separated list, if all of its members are specified in MSYS' emulated POSIX style, and it will be converted appropriately, but mixed POSIX and MS-Windows styles are unsupported).

A further point to note: the manpage snippet cited is somewhat ambiguous, but it suggests that LIBRARY_PATH is not used if mingw32-gcc is built as a cross-compiler. This feature may be of use only to those running MinGW as a native compiler on MS-Windows; it may have no value, for those of us who run it cross hosted.

Thanks for this comment. The focus of the article is more on setting the system up to avoid dependency on a proliferation of environment variables; however, the comment is pertinent, and a valuable source of additional information, until such time as I cover this aspect within the body text. When I get to that point, I will discard this attached comment thread.

Re: HOWTO Specify the Location of Libraries for use with MinGW

Thank you for explaining in more details that man page paragraph. When reading it, It's indeed easy to miss the fact it's not usable with a cross-compiler!
I was also wondering what the man page refers to with "searching for special linker files". Are these the specs files described in the SpecsFileHOWTO page?

Re: HOWTO Specify the Location of Libraries for use with MinGW

keith's picture

No, the specs files are specific to configuration of the GCC front-end drivers. As I understand it, special linker files would be the linker scripts, referred to in the ld section of the binutils manual.

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