Can't use functions from .c file even though it's declared in .h file. what to do?

Question

C++ / C#

Good day.
Yesterday I created a topic where I had problems compiling c++ source code using the libgd library.
As a result, not without help on this resource, it turned out to connect everything, but a new
problem arose - the function, the implementation of which is described in gd_filter.c, does not allow compiling an executable file in any way. There are 2 scenarios, not really sure about the second one, so I'll give the first one first:

/* Bring in gd library functions */
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include "src/gd.h"
//#include "src/gd_filter.c"



int main() {
  /* Declare the image */
  gdImagePtr im;
  /* Declare output files */
  FILE *pngout, *jpegout;
  /* Declare color indexes */
  int black;
  int white;

  /* Allocate the image: 64 pixels across by 64 pixels tall */
  im = gdImageCreate(64, 64);

  /* Do the same for a JPEG-format file. */
  jpegout = fopen("test.jpg", "wb");

    
  gdImageContrast(im,20);
  /* Output the same image in JPEG format, using the default
    JPEG quality setting. */
  gdImageJpeg(im, jpegout, -1);

  /* Close the files. */
  fclose(pngout);
  fclose(jpegout);

  /* Destroy the image in memory. */
  gdImageDestroy(im);
}

To which I get the following result when compiling:

C:\CPPImageCorrecing>gcc CPPImageCorrector.cpp -o CPPImageCorrector.exe -lgd.dll

C:\Users\0BD6~1\AppData\Local\Temp\cc3tfPz0.o:CPPImageCorrector.cpp:(.text+0x5a)
: undefined reference to `[email protected]'
collect2.exe: error: ld returned 1 exit status

If I include gd_filter.c :

/* Bring in gd library functions */
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include "src/gd.h"
#include "src/gd_filter.c"



int main() {
  /* Declare the image */
  gdImagePtr im;
  /* Declare output files */
  FILE *pngout, *jpegout;
  /* Declare color indexes */
  int black;
  int white;

  /* Allocate the image: 64 pixels across by 64 pixels tall */
  im = gdImageCreate(64, 64);

  /* Do the same for a JPEG-format file. */
  jpegout = fopen("test.jpg", "wb");

    
  gdImageContrast(im,20);
  /* Output the same image in JPEG format, using the default
    JPEG quality setting. */
  gdImageJpeg(im, jpegout, -1);

  /* Close the files. */
  fclose(pngout);
  fclose(jpegout);

  /* Destroy the image in memory. */
  gdImageDestroy(im);
}

C:\CPPImageCorrecing>gcc CPPImageCorrector.cpp -o CPPImageCorrector.exe -lgd.dll

In file included from src/gd_intern.h:29:0,
                 from src/gd_filter.c:7,
                 from CPPImageCorrector.cpp:6:
src/msinttypes/inttypes.h:33:2: error: #error "Use this header only with Microso
ft Visual C++ compilers!"
 #error "Use this header only with Microsoft Visual C++ compilers!"
  ^
In file included from src/msinttypes/inttypes.h:43:0,
                 from src/gd_intern.h:29,
                 from src/gd_filter.c:7,
                 from CPPImageCorrector.cpp:6:
src/msinttypes/stdint.h:33:2: error: #error "Use this header only with Microsoft
 Visual C++ compilers!"
 #error "Use this header only with Microsoft Visual C++ compilers!"
  ^
In file included from src/gd_intern.h:29:0,
                 from src/gd_filter.c:7,
                 from CPPImageCorrector.cpp:6:
src/msinttypes/inttypes.h:278:1: error: '_inline' does not name a type
 _inline
 ^
In file included from CPPImageCorrector.cpp:6:0:
src/gd_filter.c:40:18: error: function 'int gdImageScatter(gdImagePtr, int, int)
' definition is marked dllimport
 BGD_DECLARE(int) gdImageScatter(gdImagePtr im, int sub, int plus)
                  ^
src/gd_filter.c: In function 'int gdImageScatter(gdImagePtr, int, int)':
src/gd_filter.c:40:18: warning: 'int gdImageScatter(gdImagePtr, int, int)' redec
lared without dllimport attribute: previous dllimport ignored [-Wattributes]
src/gd_filter.c: At global scope:
src/gd_filter.c:54:18: error: function 'int gdImageScatterColor(gdImagePtr, int,
 int, int*, unsigned int)' definition is marked dllimport
 BGD_DECLARE(int) gdImageScatterColor(gdImagePtr im, int sub, int plus, int colo
rs[], unsigned int num_colors)
                  ^
src/gd_filter.c: In function 'int gdImageScatterColor(gdImagePtr, int, int, int*
, unsigned int)':
src/gd_filter.c:54:18: warning: 'int gdImageScatterColor(gdImagePtr, int, int, i
nt*, unsigned int)' redeclared without dllimport attribute: previous dllimport i
gnored [-Wattributes]
src/gd_filter.c: At global scope:
src/gd_filter.c:69:18: error: function 'int gdImageScatterEx(gdImagePtr, gdScatt
erPtr)' definition is marked dllimport
 BGD_DECLARE(int) gdImageScatterEx(gdImagePtr im, gdScatterPtr scatter)
                  ^
src/gd_filter.c: In function 'int gdImageScatterEx(gdImagePtr, gdScatterPtr)':
src/gd_filter.c:69:18: warning: 'int gdImageScatterEx(gdImagePtr, gdScatterPtr)'
 redeclared without dllimport attribute after being referenced with dll linkage
src/gd_filter.c: At global scope:
src/gd_filter.c:131:18: error: function 'int gdImagePixelate(gdImagePtr, int, un
signed int)' definition is marked dllimport
 BGD_DECLARE(int) gdImagePixelate(gdImagePtr im, int block_size, const unsigned
int mode)
                  ^
src/gd_filter.c: In function 'int gdImagePixelate(gdImagePtr, int, unsigned int)
':
src/gd_filter.c:131:18: warning: 'int gdImagePixelate(gdImagePtr, int, unsigned
int)' redeclared without dllimport attribute: previous dllimport ignored [-Wattr
ibutes]
src/gd_filter.c: At global scope:
src/gd_filter.c:192:18: error: function 'int gdImageNegate(gdImagePtr)' definiti
on is marked dllimport
 BGD_DECLARE(int) gdImageNegate(gdImagePtr src)
                  ^
src/gd_filter.c: In function 'int gdImageNegate(gdImagePtr)':
src/gd_filter.c:192:18: warning: 'int gdImageNegate(gdImagePtr)' redeclared with
out dllimport attribute: previous dllimport ignored [-Wattributes]
src/gd_filter.c: At global scope:
src/gd_filter.c:228:18: error: function 'int gdImageGrayScale(gdImagePtr)' defin
ition is marked dllimport
 BGD_DECLARE(int) gdImageGrayScale(gdImagePtr src)
                  ^
src/gd_filter.c: In function 'int gdImageGrayScale(gdImagePtr)':
src/gd_filter.c:228:18: warning: 'int gdImageGrayScale(gdImagePtr)' redeclared w
ithout dllimport attribute: previous dllimport ignored [-Wattributes]
src/gd_filter.c: At global scope:
src/gd_filter.c:265:18: error: function 'int gdImageBrightness(gdImagePtr, int)'
 definition is marked dllimport
 BGD_DECLARE(int) gdImageBrightness(gdImagePtr src, int brightness)
                  ^
src/gd_filter.c: In function 'int gdImageBrightness(gdImagePtr, int)':
src/gd_filter.c:265:18: warning: 'int gdImageBrightness(gdImagePtr, int)' redecl
ared without dllimport attribute: previous dllimport ignored [-Wattributes]
src/gd_filter.c: At global scope:
src/gd_filter.c:313:18: error: function 'int gdImageContrast(gdImagePtr, double)
' definition is marked dllimport
 BGD_DECLARE(int) gdImageContrast(gdImagePtr src, double contrast)
                  ^
src/gd_filter.c: In function 'int gdImageContrast(gdImagePtr, double)':
src/gd_filter.c:313:18: warning: 'int gdImageContrast(gdImagePtr, double)' redec
lared without dllimport attribute: previous dllimport ignored [-Wattributes]
src/gd_filter.c: At global scope:
src/gd_filter.c:376:18: error: function 'int gdImageColor(gdImagePtr, int, int,
int, int)' definition is marked dllimport
 BGD_DECLARE(int) gdImageColor(gdImagePtr src, const int red, const int green, c
onst int blue, const int alpha)
                  ^
src/gd_filter.c: In function 'int gdImageColor(gdImagePtr, int, int, int, int)':

src/gd_filter.c:376:18: warning: 'int gdImageColor(gdImagePtr, int, int, int, in
t)' redeclared without dllimport attribute: previous dllimport ignored [-Wattrib
utes]
src/gd_filter.c: At global scope:
src/gd_filter.c:421:18: error: function 'int gdImageConvolution(gdImagePtr, floa
t (*)[3], float, float)' definition is marked dllimport
 BGD_DECLARE(int) gdImageConvolution(gdImagePtr src, float filter[3][3], float f
ilter_div, float offset)
                  ^
src/gd_filter.c: In function 'int gdImageConvolution(gdImagePtr, float (*)[3], f
loat, float)':
src/gd_filter.c:421:18: warning: 'int gdImageConvolution(gdImagePtr, float (*)[3
], float, float)' redeclared without dllimport attribute: previous dllimport ign
ored [-Wattributes]
src/gd_filter.c: At global scope:
src/gd_filter.c:484:18: error: function 'int gdImageSelectiveBlur(gdImagePtr)' d
efinition is marked dllimport
 BGD_DECLARE(int) gdImageSelectiveBlur( gdImagePtr src)
  
  ...

And so on (posting limit is 10,000 characters).
gd.h line 1135:

BGD_DECLARE(int) gdImageContrast(gdImagePtr src, double contrast);

Once again I ask everyone who knows to help and explain, What am I doing wrong?
I use for compilation only MinGW from CMD from under the Administrator.
Some points about how the library was built can be found in my yesterday's topic .

Answer 1

No need to include gd_filter.c!
Just add it to the compilation:
gcc CPPImageCorrector.cpp gd_filter.c -o CPPImageCorrector.exe -lgd.dll