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                                                190
       CHAPTER 3                                                                       Syntax



3.10.6 Maintenance of File Specifications

       The techniques described in this section can be used to maintain the integrity of
       the file specifications within a PDF file during the following types of operations:

       • Updating the relevant file specification when a referenced file is renamed
       • Determining the complete collection of files that must be copied to a mirror
         site
       • When creating new links to external files, discovering existing file specifica-
         tions that refer to the same files and sharing them
       • Finding the file specifications associated with embedded files to be packed or
         unpacked

       It is not possible, in general, to find all file specification strings in a PDF file
       because there is no way to determine whether a given string is a file specification
       string. It is possible, however, to find all file specification dictionaries, provided
       that they meet the following conditions:

       • They are indirect objects.
       • They contain a Type entry whose value is the name Filespec.
       An application can locate all of the file specification dictionaries by traversing the
       PDF file’s cross-reference table (see Section 3.4.3, “Cross-Reference Table”) and
       finding all dictionaries with Type keys whose value is Filespec. For this reason, it
       is highly recommended that all file specifications be expressed in dictionary form
       and meet the conditions stated above. Note that any file specification dictionary
       specifying embedded files (that is, one that contains an EF entry) must satisfy
       these conditions (see Table 3.41 on page 182).

       Note: It may not be possible to locate file specification dictionaries that are direct
       objects, since they are neither self-typed nor necessarily reachable by any standard
       path of object references.

       Files may be embedded in a PDF file either directly, using the EF entry in a file
       specification dictionary, or indirectly, using related files arrays specified in the RF
       entry. If a file is embedded indirectly, its name is given by the string that precedes
       the embedded file stream in the related files array. If it is embedded directly, its
       name is obtained from the value of the corresponding entry in the file

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