Calling a function from another C file

I am writing a graphics program in C/C++ using OpenGL. I have a bunch of .c files and right now, I am trying to call a function from main.cpp in assignment1gui.c. The teacher has given me a file with the compile code. It looks like this:

OBJS   = assignment1gui.o shadertools.o main.o
TARGET = ass1

CXX = gcc

DBFLAGS  = -O0 -g3 -ggdb3 -fno-inline

WFLAGS   = -Wall -ansi
GLFLAGS  = `pkg-config --cflags gtk+-2.0`
LGLFLAGS = `pkg-config --libs gtk+-2.0` -lGL -lGLEW -lGLU -lglut

CXXFLAGS = $(WFLAGS) $(DFLAGS) $(GLFLAGS)
LDFLAGS  =  -export-dynamic -lXext -lX11 $(LGLFLAGS)


all: $(TARGET)
clean:
    rm -f $(OBJS) $(TARGET)

.SUFFIXES: .cc
.cc.o:
    $(CXX) -c $(CXXFLAGS) $<
.c.o:
    $(CXX) -c $(CXXFLAGS) $<
$(TARGET): $(OBJS)
    $(CXX) -o $(TARGET) $(OBJS) $(LDFLAGS)

The function I am trying to call is "drawPloygonLine()". I have declared this in "assignment1gui.h", and the definition, as I mentioned earlier, is in main.cpp.

From all the help on the Internet, I realized that there must some problem with the compile code. But I am not sure. Could someone please help me out?

Sorry I left out this part earlier. The errors I am getting are:

assignment1gui.o: In function `on_btn_color_color_set':
assignment1gui.c:(.text+0x1c): undefined reference to `drawPolygonLine'
assignment1gui.o: In function `on_rb_tool_line_toggled':
assignment1gui.c:(.text+0x8b): undefined reference to `drawPolygonLine'
assignment1gui.o: In function `on_rb_tool_circle_toggled':
assignment1gui.c:(.text+0xfc): undefined reference to `drawPolygonLine'
assignment1gui.o: In function `on_rb_tool_rect_toggled':
assignment1gui.c:(.text+0x16d): undefined reference to `drawPolygonLine'
assignment1gui.o: In function `on_rb_tool_edit_toggled':
assignment1gui.c:(.text+0x24a): undefined reference to `drawPolygonLine'
collect2: ld returned 1 exit status
make: *** [ass1] Error 1

The C/C++ files I have in the folder are main.cpp, assignment1gui.c and shadertools.c. I also have two header files- assignment1gui.h and shadertools.h. As I have mentioned earlier, I used the assignment1gui.h file to declare the new function "drawPolygonLine();"

Addition: The declaration of the "drawPolygonLine();" is now in "main.h" and it looks like this:

#ifndef MAIN_H
#define MAIN_H

extern void drawPolygonLine();

#endif  /* MAIN_H */

Both main.cpp and assignment1gui.c have "#include "main.h"" at their beginning. The definition of drawPolygonLine() is:

void drawPolygonLine()
{
    if(option[opCount-1]==4 && drawn==false)
    {           
        vCount++;
        vertex[vCount].x=A;
        vertex[vCount].y=B;
        lines=lines+2;
        vCount++;
        vertex[vCount].x=polyX;
        vertex[vCount].y=polyY;   
        clicks=0;     
        drawn=true;
    }

}

All the variables used in the function are global in main.cpp.

An example of how I am calling drawPolygonLine(); in assignment1gui.c is in this function:

void on_btn_color_color_set(GtkColorButton *colorbutton, gpointer user_data)
{
    drawPolygonLine();

    GdkColor color;
    gtk_color_button_get_color(colorbutton, &color);
    printf("Color set rgb=(%i,%i,%i)\n",color.red,color.green,color.blue);
}

I hope this makes it clear.

Solution

The problem is that main.cpp isn't understood by this make file, since it only has rules for .cc or .c files. I suspect that main.o isn't being built - or, more likely, since you're getting as far as a failed compile, there is another file called main.c or main.cc which IS included in your program, but doesn't have drawPolygonLine() defined in it.

To solve this:

Remove or rename any existing main.c or main.cc files.
If your program is actually a C program, then change the filename to main.c
If your program is a C++ program, you could try changing the extension to .cc, but I suspect you'll then run in to problems since you're using gcc rather than g++. In this case, I'd also change:
```
CXX = gcc
```
to
```
CXX = g++
```

Remember that although a lot of C code is also valid C++ code, there are many subtle differences - and the best practices are very different in each language. For this reason, it's best to treat C and C++ as different languages.

If this doesn't fix it:

Check that the function signature of drawPolygonLine() declared in your header file is the same as the way it's defined in your main file. Check that these declarations match with the way you've called it.
Check that the files that call drawPolygonLine() have #include "assignment1gui.h" at the top
Check that drawPolygonLine() isn't declared as static (static functions aren't visible outside their translation unit - so the linker won't see them when compiling two output files together)

Also, it's good practice to have the function declaration in the header file corresponding to the source file it's defined in - so if you want to call a function in main.c from assignment.c, you would usually put the declaration in main.h, and #include it at the top of assignment.c. This won't be the cause of the problem that you're having, but it's good practice to get used to.