What is an EMF spool file?
When an application sends pages to a printer they are held in an intermediary file which the application can write to and the printer driver can read from simultaneously. This speeds up the print process as control can return to the application before the printer has finished printing the document.
Usually this spool file is held in the raw printer page definition language (which could be PCL, PostScript or one of many other options) but in Windows NT, 2000, XP and 2003, it is possible to make the spooler use a different, more device independent format known as an EMF spool file.
The file layout of an EMF spool file is not officially documented by Microsoft, but this article reveals that it is a series of enhanced metafile records (one for each page) and a number of other record types which are specific to the business of printing these pages.
The EMF spool file record types
The spool file is composed of the following record types:
Private Enum SpoolerRecordTypes
SRT_EOF = &H0 SRT_RESERVED_1 = &H1 SRT_FONTDATA = &H2 SRT_DEVMODE = &H3 SRT_FONT2 = &H4 SRT_RESERVED_5 = &H5 SRT_FONT_MM = &H6 SRT_FONT_SUB1 = &H7 SRT_FONT_SUB2 = &H8 SRT_RESERVED_9 = &H9
SRT_UNKNOWN = &H10 SRT_RESERVED_A = &HA
SRT_RESERVED_B = &HB
SRT_PAGE = &HC SRT_EOPAGE1 = &HD SRT_EOPAGE2 = &HE SRT_EXT_FONT = &HF SRT_EXT_FONT2 = &H10 SRT_EXT_FONT_MM = &H11 SRT_EXT_FONT_SUB1 = &H12 SRT_EXT_FONT_SUB2 = &H13 SRT_EXT_PAGE = &H14 End Enum
Of these the most important record types are:
This record contains a Windows standard enhanced metafile, consisting of an EMF header and one or more EMF graphics record structures. The EMF header record gives you the dimensions of the boundary rectangle (the area inside the print margins), the number of graphics records that make up the page, and the file size of this single EMF page.
This record contains the device settings that apply to the rest of the pages in the document (or until another
DEVMODE record is encountered that overrides it). The
DEVMODE structure holds details such as the number of copies, the page orientation (landscape versus portrait), the paper size and paper source, and so on.
This signifies the end of the spool file records.
Using the EMF spoolfile viewer
To view an EMF spool file, select the menu File -> Open. A dialog box will appear to allow you to browse to the spool file (extension .spl). These are typically held in your $winnt$\System32\spool\ directory. (I have included some samples with this application code as well.)
The viewer allows you to go to a particular page, zoom in and out, and even print the spool file using the following menu options:
- Open - Brings up a dialog box to find and open EMF spool files.
- Print - Allows you to reprint the currently open EMF spool file.
- Properties - Brings up a dialog box listing the print file properties.
- Exit - Quits the application.
- First Page - Goes to the beginning of the document.
- Previous Page - Goes back one page.
- Next Page - Goes to the next page.
- Last Page - Goes to the end of the document.
- Goto - Goes directly to the selected page.
- Properties = Brings up a dialog with the properties of this page.
- Zoom in enlarges the preview.
- Zoom out reduces the size of the preview.
Uses for EMF spoolfiles
Since pretty much every application has a print function, the EMF spool file can serve as a quick and dirty portable document format (though much less featured than a PDF). Thus if you have an application and you want to send a file to someone who doesn't have that application to view, you simply use the application's print function and send them the spool file.
In addition, knowing the structure of the spool file means that it is possible to parse the file to extract information from it. You might want to extract just the text from a printed document for archival purposes, and you can do this by parsing the file and extracting the
The source code in this application draws heavily on the work of Feng Yuan and Christophe Lindemann as well as numerous others on the various forums and news groups.
Portions of this article based on www.undocprint.org/formats/winspool/spl. These portions are subject to the GNU Free Documentation License and are copyright © 2003-2005 Free Software Foundation, Inc.