help with error when compiling a C program with g++

Hello,

I have a C program (using OpenGL) that compiles and runs quite well with gcc. Now, to expand it I need to use a library that's written in C++, of which I know nothing.
I tried simply compiling my code with g++, to see if I could "fake" the C++, i.e. not change my existing code and simply use it when I need to call this new library.

I got only one error. If I comment out the function where the error is, everything seems to compile and run quite well. The thing is I do not understand the error, so I hope someone could give me a hint.

First and foremost, do you guys see any risk in doing what I intend to do?

Second, I put g++ instead of gcc in my Makefile, and changed my code to *.cpp. When I make it, I get this error:

visuals.c:465: error: invalid conversion from 'void*' to 'unsigned char*'

The code with the error is below. The line 465 of the error is

Code:

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pixels = malloc(3*width*height);

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Function with the error

Code:

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int capture_to_ppm(void)
{
        int width, height, colorDepth, maxColorValue,y;
        unsigned char        *pixels;
        int fd;
        char sbuf[256]; /* for sprintf() */

        /* open output file: you can name it as you like */
        fd = open(picfile,O_CREAT|O_TRUNC|O_WRONLY,
                                          S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH);
        if(fd == -1) return -1;

        /* width & height of the window */
        width = glutGet(GLUT_WINDOW_WIDTH);
        height = glutGet(GLUT_WINDOW_HEIGHT);

        /* maxColorValue is 255 in most cases... */
        colorDepth = glutGet(GLUT_WINDOW_RED_SIZE);
        maxColorValue = (1 << colorDepth) - 1;

        /* allocate pixels[]: 3 is for RGB */
        pixels = malloc(3*width*height);
        if( !pixels ) return -2;

        /* get RGB values from the frame buffer into pixels[] */
        glReadBuffer(GL_FRONT); /* if you are using "double buffer" */
        glReadPixels(0,0,width,height,GL_RGB,GL_UNSIGNED_BYTE,pixels);

        /* write ppm file header */
        sprintf(sbuf,"P6 %d %d %d\n",width,height,maxColorValue);
        write(fd,sbuf,strlen(sbuf));
       
        /* write ppm RGB data: we must invert upside down */
        for(y = height-1; y >= 0; --y) {
                write(fd, pixels+3*width*y, 3*width);
        }       

        close(fd);
        free(pixels);
        return 0;
}

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Thank you very much for any help,

mc61

Unlike C, C++ does not allow for implicit conversion from void* to other pointer types. You have to explicitly cast void* to unsigned char*.

You mean like this?

Code:

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pixels = (unsigned char*)malloc(3*width*height);

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Sorry if I abuse your kindness with one more thing, but then I take it that you do not see any danger with my strategy?

Thanks for your help,

mc61

Could you write a small wrapper for the lib you need - wrapper that will translate all C++ calls to C
Compile only this wrapper library by g++ to build some lib file - and link this file to your project that will be still compiled as C by gcc?

Quote:

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You mean like this?

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Yes.

Quote:

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Sorry if I abuse your kindness with one more thing, but then I take it that you do not see any danger with my strategy?

---

It is probably okay, but you might want to read How to mix C and C++.

laserlight: thanks so much for your help and for pointing to the article on mixing C and C++. I'll definitely read it.

vart: thanks for the suggestion, but I am afraid I have no idea how to do that... If you know of something I could read and learn, I am all ears!

What vart means is -- there are two ways to solve this problem. Either compile it as C++, with casts everywhere or something of the like, or use a wrapper of some sort.

As an example wrapper, this lets you call a C++ function from C. (Note: with this method, you could continue compiling your program as C.)

Code:

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// this is compiled as C++

void cpp_function() {
    // ...
}

extern "C" {

void c_function() {
    cpp_function();
}

}

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Basically, C++ functions can be wrapped with extern "C", and then they're treated as C ones. If only one function is extern "C", you can leave out the curly braces, I think.

Code:

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extern "C"
void c_function() {
    // ...
}

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dwks, vart, laserlight,

Thank you very much for all your help. I think I understand now what a wrapper is. Incidentally, after I replied to vart's post I read the article that laserlight suggested, and thus I discovered all this extern "C" stuff. I just have to try it now :-)

Thank again. This forum is amazing!

mc61