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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>Magick++ API: Exceptions </title>
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<link rel="stylesheet" href="magick.css" type="text/css" />
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</head>
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<body>
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Magick::Exception Classes
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Exception represents the base class of objects thrown when
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Magick++reports an error. Magick++ throws C++ exceptions synchronous
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with the operation where the error occurred. This allows errors to be
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trapped within the enclosing code (perhaps the code to process a
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single image) while allowing the code to be written with a simple
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coding style.
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A try/catch block should be placed around any sequence of
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operations which can be considered an important body of work. For
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example, if your program processes lists of images and some of these
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images may be defective, by placing the try/catch block around the
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entire sequence of code that processes one image (including
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instantiating the image object), you can minimize the overhead of
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error checking while ensuring that all objects created to deal with
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that object are safely destroyed (C++ exceptions unroll the stack
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until the enclosing try block, destroying any created objects).
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The pseudo code for the main loop of your program may look like:
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using namespace std;
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for infile in list
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{
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try {
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// Construct an image instance first so that we don't have to worry
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// about object construction failure due to a minor warning exception
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// being thrown.
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Magick::Image image;
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try {
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// Try reading image file
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image.read(infile);
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}
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catch( Magick::WarningCoder &warning )
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{
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// Process coder warning while loading file (e.g. TIFF warning)
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// Maybe the user will be interested in these warnings (or not).
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// If a warning is produced while loading an image, the image
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// can normally still be used (but not if the warning was about
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// something important!)
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cerr << "Coder Warning: " << warning.what() << endl;
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}
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catch( Magick::Warning &warning )
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{
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// Handle any other Magick++ warning.
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cerr << "Warning: " << warning.what() << endl;
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}
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catch( Magick::ErrorFileOpen &error )
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{
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// Process Magick++ file open error
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cerr << "Error: " << error.what() << endl;
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continue; // Try next image.
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}
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try {
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image.rotate(90);
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image.write("outfile");
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}
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catch ( MagickExeption & error)
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{
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// Handle problem while rotating or writing outfile.
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cerr << "Caught Magick++ exception: " << error.what() << endl;
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}
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}
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catch( std::exception & error )
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{
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// Process any other exceptions derived from standard C++ exception
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cerr << "Caught C++ STD exception: " << error.what() << endl;
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}
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catch( ... )
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{
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// Process *any* exception (last-ditch effort). There is not a lot
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// you can do here other to retry the operation that failed, or exit
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}
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}
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The desired location and number of try/catch blocks in your program
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depends how sophisticated its error handling must be. Very simple
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programs may use just one try/catch block.
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The Exception class is derived from the C++ standard
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exception class. This means that it contains a C++ string containing
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additional information about the error (e.g to display to the user).
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Obtain access to this string via the what() method. For
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example:
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catch( Exception & error_ )
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{
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cout << "Caught exception: " << error_.what() << endl;
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}
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The classes Warning and Error derive from the
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Exception class. Exceptions derived from Warning are
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thrown to represent non-fatal errors which may effect the
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completeness or quality of the result (e.g. one image provided as an
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argument to montage is defective). In most cases, a Warning
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exception may be ignored by catching it immediately, processing it
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(e.g. printing a diagnostic) and continuing on. Exceptions derived
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from Error are thrown to represent fatal errors that can not
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produce a valid result (e.g. attempting to read a file which does not
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exist).
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The specific derived exception classes
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are shown in the following tables:
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Warning Sub-Classes
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<font size="2">Warning</font>
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<font size="2">Warning Description</font>
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<font size="2">WarningUndefined</font>
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<font size="2">Unspecified warning type.</font>
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<font size="2">WarningBlob</font>
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<font size="2">NOT
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CURRENTLY USED</font>
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<font size="2">WarningCache</font>
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<font size="2">NOT
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CURRENTLY USED</font>
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<font size="2">WarningCoder</font>
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<font size="2">Warnings issued by some coders.</font>
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<font size="2">WarningConfigure</font>
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<font size="2">NOT
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CURRENTLY USED</font>
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<font size="2">WarningCorruptImage</font>
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<font size="2">Warning issued when an image is determined to be
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corrupt.</font>
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<font size="2">WarningDelegate</font>
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<font size="2">Warnings reported by the delegate (interface to
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external programs) subsystem.</font>
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<font size="2">WarningDraw</font>
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<font size="2">Warnings reported by the rendering subsystem.</font>
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<font size="2">WarningFileOpen</font>
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<font size="2">Warning reported when The image file could not be
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opened (permission problem, wrong file type, or does not exist).</font>
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<font size="2">WarningImage</font>
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<font size="2">NOT CURRENTLY USED</font>
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<font size="2">WarningMissingDelegate</font>
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<font size="2">NOT CURRENTLY USED</font>
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<font size="2">WarningModule</font>
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<font size="2">NOT CURRENTLY USED</font>
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<font size="2">WarningMonitor</font>
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<font size="2">NOT CURRENTLY USED</font>
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<font size="2">WarningOption</font>
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<font size="2">Warning reported when an option is malformed or
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out of range.</font>
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<font size="2">WarningRegistry</font>
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<font size="2">NOT CURRENTLY USED</font>
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<font size="2">WarningResourceLimit</font>
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<font size="2">Warning reported when a program resource is
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exhausted (e.g. not enough memory).</font>
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<font size="2">WarningStream</font>
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<font size="2">NOT CURRENTLY USED</font>
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<font size="2">WarningType</font>
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<font size="2">NOT CURRENTLY USED</font>
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<font size="2">WarningXServer</font>
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<font size="2">Warnings reported by the X11 subsystem.</font>
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Error Sub-Classes
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<font size="2">Error</font>
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<font size="2">Error Description</font>
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<font size="2">ErrorUndefined</font>
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<font size="2">Unspecified error type.</font>
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<font size="2">ErrorBlob</font>
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<font size="2">Error reported by BLOB I/O subsystem.</font>
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<font size="2">ErrorCache</font>
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<font size="2">Error reported by the pixel cache subsystem.</font>
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<font size="2">ErrorCoder</font>
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<font size="2">Error reported by coders (image format support).</font>
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<font size="2">ErrorConfigure</font>
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<font size="2">Errors reported while loading configuration files.</font>
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<font size="2">ErrorCorruptImage</font>
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<font size="2">Error reported when the image file is corrupt.</font>
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<font size="2">ErrorDelegate</font>
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<font size="2">Errors reported by the delegate (interface to
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external programs) subsystem.</font>
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<font size="2">ErrorDraw</font>
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<font size="2">Error reported while drawing on image.</font>
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<font size="2">ErrorFileOpen</font>
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<font size="2">Error reported when the image file can not be
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opened.</font>
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<font size="2">ErrorImage</font>
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<font size="2">Errors reported while drawing.</font>
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<font size="2">ErrorMissingDelegate</font>
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<font size="2">Error reported when an add-on library or program
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is necessary in order to support the requested operation.</font>
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<font size="2">ErrorModule</font>
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<font size="2">Errors reported by the module loader subsystem.</font>
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<font size="2">ErrorMonitor</font>
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<font size="2">NOT CURRENTLY USED</font>
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<font size="2">ErrorOption</font>
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<font size="2">Error reported when an option is malformed or out
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of range.</font>
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<font size="2">ErrorRegistry</font>
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<font size="2">Errors reported by the image/BLOB registry
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subsystem.</font>
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<font size="2">ErrorResourceLimit</font>
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<font size="2">Error reported when a program resource is
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exhausted (e.g. not enough memory).</font>
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<font size="2">ErrorStream</font>
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<font size="2">Errors reported by the pixel stream subsystem.</font>
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<font size="2">ErrorType</font>
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<font size="2">Errors reported by the type (font) rendering
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subsystem.</font>
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<font size="2">ErrorXServer</font>
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<font size="2">Errors reported by the X11 subsystem.</font>
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</body>
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</html>
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