MinGW / msys，链接器（ld.exe）找不到libpng（-lpng），即使它已经安装，运行xpdf configure脚本

Question

I installed versions 1.2, 1.4, and 1.6 of libpng from source from the makefiles they include for MinGW/msys. It seemed to work - I've got the header files in various places:

• /include
• /include/libpng
• /include/libpng12
• /include/libpng14
• /local/include
• /local/include/libpng
• /local/include/libpng16

As well as library files:

• /lib/libpng.a
• /lib/libpng.dll.a
• /lib/libpng12.a
• /lib/libpng12.dll.a
• /lib/libpng14.a
• /lib/libpng14.dll.a
• /local/lib/libpng.a
• /local/lib/libpng.dll
• /local/lib/libpng16.a
• /local/lib/libpng16.dll

I call the configure script with msys's bash, and it chokes in the same place no matter what combination of include directory and library I feed it. Config.log gives me the following:

configure:7732: checking png.h usability  
configure:7732: gcc -c -g -O2  -I/local/include  conftest.c >&5  
configure:7732: $? = 0  
configure:7732: result: yes  
configure:7732: checking png.h presence  
configure:7732: gcc -E  -I/local/include  conftest.c  
configure:7732: $? = 0  
configure:7732: result: yes  
configure:7732: checking for png.h  
configure:7732: result: yes  
configure:7761: checking for png_write_row in -lpng  
configure:7786: gcc -o conftest.exe -g -O2   conftest.c -lpng -I/local/include  /local/lib/libpng.a -lz  >&5  
c:/mingw/bin/../lib/gcc/mingw32/5.3.0/../../../../mingw32/bin/ld.exe: cannot find -lpng  
collect2.exe: error: ld returned 1 exit status

I've tried every combination of include directory and library listed above, and it's the same, every time. It will be

configure:7786: gcc -o conftest.exe -g -O2   conftest.c -lpng -I/include/libpng  /lib/libpng.dll.a -lz  >&5  
c:/mingw/bin/../lib/gcc/mingw32/5.3.0/../../../../mingw32/bin/ld.exe: cannot find -lpng

or

configure:7786: gcc -o conftest.exe -g -O2   conftest.c -lpng -I/local/include/libpng16  /local/lib/libpng16.dll -lz  >&5  
c:/mingw/bin/../lib/gcc/mingw32/5.3.0/../../../../mingw32/bin/ld.exe: cannot find -lpng

or

configure:7786: gcc -o conftest.exe -g -O2   conftest.c -lpng -I/include/libpng12  /lib/libpng12.a -lz  >&5  
c:/mingw/bin/../lib/gcc/mingw32/5.3.0/../../../../mingw32/bin/ld.exe: cannot find -lpng

You get the picture. I've successfully compiled several other xpdf pieces that don't require libpng. But here, I'm at a total loss.

Answer 1

Try this:

1. Add the MinGW redist to the path.
2. Ensure that the MinGW library and include paths precede those of Cygwin. (The png.h header for MinGW targets is incompatible with the png.h header for Cygwin targets.)
3. Dynamically link libpng. (The mingw64-*-libpng packages in Cygwin do not provide a static libpng.a library, which is unusual.)

For example, for a 64-bit build:

$ export CC=x86_64-w64-mingw32-gcc
$ export CXX=x86_64-w64-mingw32-g++
$ export PATH="/usr/x86_64-w64-mingw32/sys-root/mingw/bin:$PATH"
$ export CFLAGS="-L/usr/x86_64-w64-mingw32/sys-root/mingw/lib -I/usr/x86_64-w64-mingw32/sys-root/mingw/include -lpng -Wl,Bdynamic"
$ export CXXFLAGS="$CFLAGS"
$ which libpng-config
/usr/x86_64-w64-mingw32/sys-root/mingw/bin/libpng-config
$ ./configure

EDIT: Adding the redist folder to the PATH before compile time, which contains the libpng.dll file, is the important non-obvious step that applies to both the Cygwin and MSYS environments.