Q: Why do I get so many undefined references when linking my
programs?
Q: Why do I get "Undefined reference to yywrap" when linking
programs produced by Flex?
Q: GCC complains that it cannot find -liostream. Where can I find
this library?
A: By default, GCC instructs the linker to only look in two
libraries: libgcc.a and libc.a. Some functions aren't
included there, so the linker can't find them. If you need to link
against some optional library, say libxy.a, put the library into
the DJGPP lib/ subdirectory and append a -lxy to the link
command line. The Standard C++ Template classes are in
libstdcxx.a (it's called libstdc++.a on Unix); append
-lstdcxx. To use the additional GNU C++ classes in the
libgpp.a library (it's called libg++.a on Unix systems),
append -lgpp. Flex-generated lexical analyzers call functions in
the libfl.a library; you need to append -lfl when linking
them. Append -lgrx if you are using GRX library, and
-lalleg for linking with Allegro.
When linking C++ programs, you should use either the gpp or
gxx commands instead of gcc; they will then instruct the
linker to also scan the C++ libraries automatically, so you don't
have to remember doing that yourself.
Another reason for undefined references when linking C++ programs is
that you mix GCC and libstdcxx.a from different releases: they
are usually incompatible. In particular, sometimes people install an
additional compiler based on (old releases of) GCC, such as GNU Pascal
or GNAT, the GNU Ada development environment, and these additional
compilers overwrite some of the libraries, like libgcc.a, with
older and incompatible versions. You should always make sure you don't
mix different releases of the compiler and libraries; if you must
install different releases, install them in separate directories and
prepare some batch files or shortcuts to set up the environments for
each of the compilers by pointing the DJGPP variable to different
directories and changing the order of directories in PATH.
If your program uses a lot of floating-point math, or needs math
functions beyond those specified in the ANSI/ISO standard, consider
appending -lm to your link command line. The basic math functions
required by ANSI/ISO standard are included in the libc.a library,
but libm.a includes different versions of these functions which
sometimes are more accurate or more compatible with widely-accepted
standards for numeric computations; libm.a also includes some
functions not included in the default library, like Gamma function and
Bessel functions, support for different standards of behavior in case of
errors, a matherr facility, etc.
Old C++ programs used to be built with the GNU iostream library,
libiostream.a. The iostream classes are now part of the standard
C++ library, libstdcxx.a, so if you come across a Makefile
that passes the -liostream option to the compiler, change that
to -lstdcxx instead.
Further problems which cause the linker to fail with C++ programs are discussed in listing libraries in the correct order, and in exceptions and inline functions.