A compilation/linking problem

[aardvarc]

Please someone help with this compilation/linking problem that I am having.

I am writing some code that uses the libpng library version 1.4.1 to work with PNG files. The library is available as source code and compiles fine using NetBeans and g++ on Debian. My code uses functions declared in the header png.h that is part of libpng. The following code results in the exact same compilation/linking error that I get for almost, if not all functions that I try to use from png.h:

#include <stdlib.h>
#include <png.h>
int main(int argc, char** argv) {
	png_structp png_ptr = png_create_write_struct (
		PNG_LIBPNG_VER_STRING, 0, 0, 0);
    	return (EXIT_SUCCESS);
}

It seems that though the linker can find the declaration, it cannot find the code for png_create_write_struct().

newmain.o: In function `main':
/home/user01/NetBeansProjects/ErrTest/newmain.cc:6: undefined reference to `png_create_write_struct'

In png.h (a very large file), png_create_write_struct is defined as:

extern PNG_EXPORT(png_structp,png_create_write_struct)
   PNGARG((png_const_charp user_png_ver, png_voidp error_ptr,
   png_error_ptr error_fn, png_error_ptr warn_fn)) PNG_ALLOCATED;

And resolving the typedefs for PNG_EXPORT, PNG_ARG, and PNG_ALLOCATED yields something like:

extern  png_structp  png_create_write_struct (
      png_const_charp  user_png_ver, 
      png_voidp             error_ptr, 
      png_error_ptr       error_fn, 
      png_error_ptr       warn_fn));

This declaration is found amongst those for many many other functions, but the code for these functions is not all contained in a file png.c. Instead, that code is contained in many smaller files. The code for the function png_create_write_struct() is in one called pngwrite.c.

I've put the actual code for png_create_write_struct() at the end of this post, but, thankfully, I don't think that it is relevant to the compilation problem I am having. The code is very hard to read! Instead, it seems that locating that code is the problem.

In trying, I thought it best to explicitly include both the png.o file (actually named libpng14_la-png.o) as well as the pngwrite.o file (actually named libpng14_la-pngwrite.o), as below. However, this results in exactly the same error -- no difference at all.

Running "/usr/bin/make  -f Makefile CONF=Debug" in /home/user01/NetBeansProjects/ErrTest

/usr/bin/make -f nbproject/Makefile-Debug.mk SUBPROJECTS= .build-conf
make[1]: Entering directory `/home/user01/NetBeansProjects/ErrTest'
mkdir -p build/Debug/GNU-Linux-x86/_ext/home/user01/NetBeansProjects/ErrTest
g++    -c -g -I/home/user01/NetBeansProjects/libpng-1.4.1 -o build/Debug/GNU-Linux-x86/_ext/home/user01/NetBeansProjects/ErrTest/newmain.o /home/user01/NetBeansProjects/ErrTest/newmain.cc
mkdir -p dist/Debug/GNU-Linux-x86
g++     -o /home/user01/NetBeansProjects/libpng-1.4.1/libpng14_la-pngwrite.o -o /home/user01/NetBeansProjects/libpng-1.4.1/libpng14_la-png.o -o dist/Debug/GNU-Linux-x86/errtest build/Debug/GNU-Linux-x86/_ext/home/user01/NetBeansProjects/ErrTest/newmain.o  
build/Debug/GNU-Linux-x86/_ext/home/user01/NetBeansProjects/ErrTest/newmain.o: In function `main':
/home/user01/NetBeansProjects/ErrTest/newmain.cc:6: undefined reference to `png_create_write_struct'
collect2: ld returned 1 exit status
make[1]: *** [dist/Debug/GNU-Linux-x86/errtest] Error 1
make[1]: Leaving directory `/home/user01/NetBeansProjects/ErrTest'
make: *** [.build-impl] Error 2

Build failed. Exit value 2.

Does anyone have any ideas as what might be the problem? Any help would be much appreciated.

Below is the png_create_write_struct code. As far as I can tell, all of the preprocessor variables are defined except USE_FAR_KEYWORD and PNG_ERROR_NUMBERS_SUPPORTED. Oddly, because PNG_USER_MEM_SUPPORTED is defined, it seems the function is actually only the first 4-5 lines, and only calls the next function, png_create_write_struct_2()!

png_structp PNGAPI
png_create_write_struct(png_const_charp user_png_ver, png_voidp error_ptr,
   png_error_ptr error_fn, png_error_ptr warn_fn)
{
#ifdef PNG_USER_MEM_SUPPORTED
   return (png_create_write_struct_2(user_png_ver, error_ptr, error_fn,
      warn_fn, NULL, NULL, NULL));
}

/* Alternate initialize png_ptr structure, and allocate any memory needed */
png_structp PNGAPI
png_create_write_struct_2(png_const_charp user_png_ver, png_voidp error_ptr,
   png_error_ptr error_fn, png_error_ptr warn_fn, png_voidp mem_ptr,
   png_malloc_ptr malloc_fn, png_free_ptr free_fn)
{
#endif /* PNG_USER_MEM_SUPPORTED */
   volatile int png_cleanup_needed = 0;
#ifdef PNG_SETJMP_SUPPORTED
   volatile
#endif
   png_structp png_ptr;
#ifdef PNG_SETJMP_SUPPORTED
#ifdef USE_FAR_KEYWORD
   jmp_buf jmpbuf;
#endif
#endif
   int i;

   png_debug(1, "in png_create_write_struct");

#ifdef PNG_USER_MEM_SUPPORTED
   png_ptr = (png_structp)png_create_struct_2(PNG_STRUCT_PNG,
      (png_malloc_ptr)malloc_fn, (png_voidp)mem_ptr);
#else
   png_ptr = (png_structp)png_create_struct(PNG_STRUCT_PNG);
#endif /* PNG_USER_MEM_SUPPORTED */
   if (png_ptr == NULL)
      return (NULL);

   /* Added at libpng-1.2.6 */
#ifdef PNG_SET_USER_LIMITS_SUPPORTED
   png_ptr->user_width_max = PNG_USER_WIDTH_MAX;
   png_ptr->user_height_max = PNG_USER_HEIGHT_MAX;
#endif

#ifdef PNG_SETJMP_SUPPORTED
/* Applications that neglect to set up their own setjmp() and then
   encounter a png_error() will longjmp here.  Since the jmpbuf is
   then meaningless we abort instead of returning. */
#ifdef USE_FAR_KEYWORD
   if (setjmp(jmpbuf))
#else
   if (setjmp(png_jmpbuf(png_ptr))) /* sets longjmp to match setjmp */
#endif
#ifdef USE_FAR_KEYWORD
   png_memcpy(png_jmpbuf(png_ptr), jmpbuf, png_sizeof(jmp_buf));
#endif
      PNG_ABORT();
#endif

#ifdef PNG_USER_MEM_SUPPORTED
   png_set_mem_fn(png_ptr, mem_ptr, malloc_fn, free_fn);
#endif /* PNG_USER_MEM_SUPPORTED */
   png_set_error_fn(png_ptr, error_ptr, error_fn, warn_fn);

   if (user_png_ver)
   {
      i = 0;
      do
      {
         if (user_png_ver[i] != png_libpng_ver[i])
            png_ptr->flags |= PNG_FLAG_LIBRARY_MISMATCH;
      } while (png_libpng_ver[i++]);
   }

   if (png_ptr->flags & PNG_FLAG_LIBRARY_MISMATCH)
   {
     /* Libpng 0.90 and later are binary incompatible with libpng 0.89, so
      * we must recompile any applications that use any older library version.
      * For versions after libpng 1.0, we will be compatible, so we need
      * only check the first digit.
      */
     if (user_png_ver == NULL || user_png_ver[0] != png_libpng_ver[0] ||
         (user_png_ver[0] == '1' && user_png_ver[2] != png_libpng_ver[2]) ||
         (user_png_ver[0] == '0' && user_png_ver[2] < '9'))
     {
#ifdef PNG_STDIO_SUPPORTED
        char msg[80];
        if (user_png_ver)
        {
           png_snprintf(msg, 80,
              "Application was compiled with png.h from libpng-&#37;.20s",
              user_png_ver);
           png_warning(png_ptr, msg);
        }
        png_snprintf(msg, 80,
           "Application  is  running with png.c from libpng-%.20s",
           png_libpng_ver);
        png_warning(png_ptr, msg);
#endif
#ifdef PNG_ERROR_NUMBERS_SUPPORTED
        png_ptr->flags = 0;
#endif
        png_warning(png_ptr,
           "Incompatible libpng version in application and library");
        png_cleanup_needed = 1;
     }
   }

   /* Initialize zbuf - compression buffer */
   png_ptr->zbuf_size = PNG_ZBUF_SIZE;
   if (!png_cleanup_needed)
   {
      png_ptr->zbuf = (png_bytep)png_malloc_warn(png_ptr,
         png_ptr->zbuf_size);
      if (png_ptr->zbuf == NULL)
         png_cleanup_needed = 1;
   }
   if (png_cleanup_needed)
   {
       /* Clean up PNG structure and deallocate any memory. */
       png_free(png_ptr, png_ptr->zbuf);
       png_ptr->zbuf = NULL;
#ifdef PNG_USER_MEM_SUPPORTED
       png_destroy_struct_2((png_voidp)png_ptr,
          (png_free_ptr)free_fn, (png_voidp)mem_ptr);
#else
       png_destroy_struct((png_voidp)png_ptr);
#endif
       return (NULL);
   }

   png_set_write_fn(png_ptr, NULL, NULL, NULL);

#ifdef PNG_WRITE_WEIGHTED_FILTER_SUPPORTED
   png_set_filter_heuristics(png_ptr, PNG_FILTER_HEURISTIC_DEFAULT,
      1, NULL, NULL);
#endif

   return (png_ptr);
}

[Lindley]

Are you able to build the libpng distributable? If so, it should generate libpng.a, which you can link against just by adding "-lpng" to the command line.

Make sure that is *after* your main source file on the command line----order matters for gcc. In fact, that's probably your immediate problem.

[aardvarc]

It took me a long time to put the fix in, but this was indeed the answer I needed.

Thanks again, Lindley.