Libjpeg write image to memory data - c++

Question

I would like to save image into memory (vector) using libjpeg library.
I found there funcitons:
init_destination
empty_output_buffer
term_destination
My question is how to do it safely and properly in parallel programs ? My function may be executed from different threads.
I want to do it in c++ and Visual Studio 2010.
Other libraries with callback functionality always have additional function parameter to store some additional data.
I don't see any way to add any additional parameters e.g. pointer to my local instance of vector.
Edit:
The nice solution of mmy question is here: http://www.christian-etter.de/?cat=48

Answer 1

The nice solution is described here: http://www.christian-etter.de/?cat=48
typedef struct _jpeg_destination_mem_mgr
{
jpeg_destination_mgr mgr;
std::vector<unsigned char> data;
} jpeg_destination_mem_mgr;
Initialization:
static void mem_init_destination( j_compress_ptr cinfo )
{
jpeg_destination_mem_mgr* dst = (jpeg_destination_mem_mgr*)cinfo->dest;
dst->data.resize( JPEG_MEM_DST_MGR_BUFFER_SIZE );
cinfo->dest->next_output_byte = dst->data.data();
cinfo->dest->free_in_buffer = dst->data.size();
}
When we finished then we need to resize buffer to actual size:
static void mem_term_destination( j_compress_ptr cinfo )
{
jpeg_destination_mem_mgr* dst = (jpeg_destination_mem_mgr*)cinfo->dest;
dst->data.resize( dst->data.size() - cinfo->dest->free_in_buffer );
}
When the buffer size is too small then we need to increase it:
static boolean mem_empty_output_buffer( j_compress_ptr cinfo )
{
jpeg_destination_mem_mgr* dst = (jpeg_destination_mem_mgr*)cinfo->dest;
size_t oldsize = dst->data.size();
dst->data.resize( oldsize + JPEG_MEM_DST_MGR_BUFFER_SIZE );
cinfo->dest->next_output_byte = dst->data.data() + oldsize;
cinfo->dest->free_in_buffer = JPEG_MEM_DST_MGR_BUFFER_SIZE;
return true;
}
Callbacks configuration:
static void jpeg_mem_dest( j_compress_ptr cinfo, jpeg_destination_mem_mgr * dst )
{
cinfo->dest = (jpeg_destination_mgr*)dst;
cinfo->dest->init_destination = mem_init_destination;
cinfo->dest->term_destination = mem_term_destination;
cinfo->dest->empty_output_buffer = mem_empty_output_buffer;
}
And sample usage:
jpeg_destination_mem_mgr dst_mem;
jpeg_compress_struct_wrapper cinfo;
j_compress_ptr pcinfo = cinfo;
jpeg_mem_dest( cinfo, &dst_mem);