How to convert image matrix to char stream?

I want to POST an image to the server. I have a matrix read from file:

I = imread('file.jpg');%This line can't be changed

And what I do now is saving it to a file then read it back:

imwrite(I, 'temp.jpg');
fid = fopen('temp.jpg', 'r');
imgData = char(fread(fid)');
fclose(fid);

But when call this function frequently, there will be an error:

Unable to open file "temp.jpg" for writing.  You might not have write permission.

Is there any solution to convert the matrix I to char stream imgData directly?

Answer 1

[Image Analyst]

What is your current folder? You're evidently not allowed to save the (badly-named) I there.

Or else you already have a file called that and some app has that file open and it's locked for modifying. If so, close it in the other app that has it open. If no other app has it open, then try

fclose('all');

before you call imwrite().

[Yuheng Huang]

Is there any way to convert these two formats directly in the memory instead of saving on the disk and read back? I think this is a bit not elegant :)