I have found this function which uses libjpeg to write to a file:

int write_jpeg_file( char *filename )
{
    struct jpeg_compress_struct cinfo;
    struct jpeg_error_mgr jerr;

    /* this is a pointer to one row of image data */
    JSAMPROW row_pointer[1];
    FILE *outfile = fopen( filename, "wb" );

    if ( !outfile )
    {
        printf("Error opening output jpeg file %s\n!", filename );
        return -1;
    }
    cinfo.err = jpeg_std_error( &jerr );
    jpeg_create_compress(&cinfo);
    jpeg_stdio_dest(&cinfo, outfile);

    /* Setting the parameters of the output file here */
    cinfo.image_width = width;  
    cinfo.image_height = height;
    cinfo.input_components = bytes_per_pixel;
    cinfo.in_color_space = color_space;
    /* default compression parameters, we shouldn't be worried about these */
    jpeg_set_defaults( &cinfo );
    /* Now do the compression .. */
    jpeg_start_compress( &cinfo, TRUE );
    /* like reading a file, this time write one row at a time */
    while( cinfo.next_scanline < cinfo.image_height )
    {
        row_pointer[0] = &raw_image[ cinfo.next_scanline * cinfo.image_width *  cinfo.input_components];
        jpeg_write_scanlines( &cinfo, row_pointer, 1 );
    }
    /* similar to read file, clean up after we're done compressing */
    jpeg_finish_compress( &cinfo );
    jpeg_destroy_compress( &cinfo );
    fclose( outfile );
    /* success code is 1! */
    return 1;
}

I would actually need to write the jpeg compressed image just to memory buffer, without saving it to a file, to save time. Could somebody give me an example how to do it?

I have been searching the web for a while but the documentation is very rare if any and examples are also difficult to come by.