Error handling in libjpeg

Question

I am using the C++ JPEG library (libjpeg) and I have realized that when some functions fail exit() is called and the application is closed. How can I override this behavior and prevent the application from closing on libjpeg errors?

Answer 1

This is the default behavior of libjpeg. In order to handle errors with libjpeg, you'll have to define an error handling routine like this:

struct jpegErrorManager {
    /* "public" fields */
    struct jpeg_error_mgr pub;
    /* for return to caller */
    jmp_buf setjmp_buffer;
};
char jpegLastErrorMsg[JMSG_LENGTH_MAX];
void jpegErrorExit (j_common_ptr cinfo)
{
    /* cinfo->err actually points to a jpegErrorManager struct */
    jpegErrorManager* myerr = (jpegErrorManager*) cinfo->err;
    /* note : *(cinfo->err) is now equivalent to myerr->pub */

    /* output_message is a method to print an error message */
    /*(* (cinfo->err->output_message) ) (cinfo);*/      

    /* Create the message */
    ( *(cinfo->err->format_message) ) (cinfo, jpegLastErrorMsg);

    /* Jump to the setjmp point */
    longjmp(myerr->setjmp_buffer, 1);

}

And then register it using jpeg_std_error.

FILE* fileHandler;
/* ... */
struct jpeg_decompress_struct cinfo;
jpegErrorManager jerr;
cinfo.err = jpeg_std_error(&jerr.pub);
jerr.pub.error_exit = jpegErrorExit;
/* Establish the setjmp return context for my_error_exit to use. */
if (setjmp(jerr.setjmp_buffer)) {
    /* If we get here, the JPEG code has signaled an error. */
    cerr << jpegLastErrorMsg << endl;
    jpeg_destroy_decompress(&cinfo);
    fclose(fileHandler);
    return 1;
}

You can find a complete example here.

Answer 2

As the question was targetting C++, an alternative approach with exceptions:

Error handler:

void jpegErrorExit ( j_common_ptr cinfo )
{
    char jpegLastErrorMsg[JMSG_LENGTH_MAX];
    /* Create the message */
    ( *( cinfo->err->format_message ) ) ( cinfo, jpegLastErrorMsg );

    /* Jump to the setjmp point */
    throw std::runtime_error( jpegLastErrorMsg ); // or your preffered exception ...
}

Use it:

FILE* fileHandler;
/* ... */
struct jpeg_decompress_struct cinfo;
struct jpeg_error_mgr jerr;
cinfo.err = jpeg_std_error( &jerr );
jerr.error_exit = jpegErrorExit;
try {
    jpeg_create_decompress( &cinfo );
    jpeg_stdio_src( &cinfo, fileHandler );
    /// ...
    jpeg_destroy_decompress( &cinfo );
    fclose( fileHandler );
}
catch ( std::runtime_exception & e ) {
    jpeg_destroy_decompress( &cinfo );
    fclose( fileHandler );
    throw; // or return an error code
}

Answer 3

Using c++11 I implemented this using a lambda (similar to Marco's answer):

[](j_common_ptr cinfo){throw cinfo->err;}

which works well. Only then catch on 'struct jpeg_error_mgr *err'

i.e.

struct jpeg_error_mgr jerr_mgr;
cinfo.err = jpeg_std_error(&jerr_mgr);
jerr_mgr.error_exit = [](j_common_ptr cinfo){throw cinfo->err;};

and

   try
   {
      jpeg_create_decompress(&cinfo);

      ...
   }
   catch (struct jpeg_error_mgr *err)
    {
        char pszErr[1024];
        (cinfo.err->format_message)((j_common_ptr)&cinfo, pszErr);

         ...
    }