What is the correct way to return an image from a C function?

Question

I'm new to C trying to understand the basics.

I am wanting to call a function that creates and returns a jpeg image.

In some other language like Python you would just put the return value from the function into a variable. C obviously does not work that way.

What is the correct way then for one function to ask another for a jpeg image and to receive back that image, which is of an unknown size?

Is the correct way to create a struct that defines a pointer to a buffer and a length, and then to set a variable outside the scope of both functions, so that both functions can access the data?

Presumably I also need to properly free the memory used inside the makeimg function via tjFree(&_compressedImage);

I copied the code for this function from somewhere on the net. It creates a jpeg function. I want to get the resulting jpeg back.

static void makeimg() {
        const int JPEG_QUALITY = 75;
        const int COLOR_COMPONENTS = 3;
        int _width = 1920;
        int _height = 1080;
        long unsigned int _jpegSize = 0;
        unsigned char* _compressedImage = NULL; //!< Memory is allocated by tjCompress2 if _jpegSize == 0
        unsigned char buffer[_width*_height*COLOR_COMPONENTS]; //!< Contains the uncompressed image
        tjhandle _jpegCompressor = tjInitCompress();
        tjCompress2(_jpegCompressor, buffer, _width, 0, _height, TJPF_RGB,
                  &_compressedImage, &_jpegSize, TJSAMP_444, JPEG_QUALITY,
                  TJFLAG_FASTDCT);
        tjDestroy(_jpegCompressor);
        //to free the memory allocated by TurboJPEG (either by tjAlloc(),
        //or by the Compress/Decompress) after you are done working on it:
        tjFree(&_compressedImage);
}

Any guidance on a good approach to take appreciated.

I am a beginner with C (experienced with Python), so if you could please explain as much as possible about your response it would be much appreciated.

Answer 1

There are multiple ways of doing this. I prefer the following:

/*
 * Makes an image and returns the length
 * returns < 0 on error
 *
 * Call as follows:
 *  char *image ;
 *  int len = make_image( &image );
 *  if (len < 0) { /* Process error code */ }
*/
int makeImage(void **image) {
    unsigned char *_image ;
    int length ;
    /* Create image and set the length of buffer in length variable */
    /* Return the image */
    *image = _image ;
    return length;
}

If you do not need multiple error codes, setting image parameter to null and checking for it may be enough.