libjpeg ver. 6b jpeg_stdio_src vs jpeg_mem_src

I am using Libjpeg version 6b. In version 8 they have a nice function to read data out of the memory called jpeg_mem_src(...), unfortunately ver. 6b does not have this function.

What can I use to read compressed data directly from memory? All I see is jpeg_stdio_src which reads f开发者_StackOverflow中文版rom the hard drive.

Write your own...

/* Read JPEG image from a memory segment */
static void init_source (j_decompress_ptr cinfo) {}
static boolean fill_input_buffer (j_decompress_ptr cinfo)
{
    ERREXIT(cinfo, JERR_INPUT_EMPTY);
return TRUE;
}
static void skip_input_data (j_decompress_ptr cinfo, long num_bytes)
{
    struct jpeg_source_mgr* src = (struct jpeg_source_mgr*) cinfo->src;

    if (num_bytes > 0) {
        src->next_input_byte += (size_t) num_bytes;
        src->bytes_in_buffer -= (size_t) num_bytes;
    }
}
static void term_source (j_decompress_ptr cinfo) {}
static void jpeg_mem_src (j_decompress_ptr cinfo, void* buffer, long nbytes)
{
    struct jpeg_source_mgr* src;

    if (cinfo->src == NULL) {   /* first time for this JPEG object? */
        cinfo->src = (struct jpeg_source_mgr *)
            (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
            SIZEOF(struct jpeg_source_mgr));
    }

    src = (struct jpeg_source_mgr*) cinfo->src;
    src->init_source = init_source;
    src->fill_input_buffer = fill_input_buffer;
    src->skip_input_data = skip_input_data;
    src->resync_to_restart = jpeg_resync_to_restart; /* use default method */
    src->term_source = term_source;
    src->bytes_in_buffer = nbytes;
    src->next_input_byte = (JOCTET*)buffer;
}

and then to use it:

...
    /* Step 2: specify data source (eg, a file) */
    jpeg_mem_src(&dinfo, buffer, nbytes);
...

where buffer is a pointer to the memory chunk containing the compressed jpeg image, and nbytes is the length of that buffer.

Answering to poor s093294 who has been waiting for an answer for more than a year. I can't comment, so creating a new answer is the only way.

ERREXIT is a macro in libjpeg. Include jerror.h and you're all set.

Or you can also try to use GNU's fmemopen() function which should be declared in stdio.h header file.

FILE * source = fmemopen(inbuffer, inlength, "rb");
if (source == NULL)
{
    fprintf(stderr, "Calling fmemopen() has failed.\n");
    exit(1);
}

// ...

jpeg_stdio_src(&cinfo, source);

// ...

fclose(source);