CHAPTER 3
178
Syntax
Errors in Type 4 Functions
The code that reads a type 4 function (analogous to the PostScript
scanner)
must
detect and report syntax errors. It may also be able to detect some errors that will
occur when the function is used, although this is not always possible. Any errors
detected by the scanner are considered to be errors in the PDF file and are
handled like other errors in the file.
The code that executes a type 4 function (analogous to the PostScript
interpreter)
must detect and report errors. PDF does not define a representation for the
errors; those details are provided by the application that processes the PDF file.
The following types of errors can occur (among others):
•
Stack overflow
•
Stack underflow
•
A type error (for example, applying
not
to a real number)
•
A range error (for example, applying
sqrt
to a negative number)
•
An undefined result (for example, dividing by 0)
3.10 File Specifications
A PDF file can refer to the contents of another file by using a
file specification
(PDF 1.1),
which can take either of two forms:
•
A
simple
file specification gives just the name of the target file in a standard for-
mat, independent of the naming conventions of any particular file system. It
can take the form of either a string or a dictionary
•
A
full
file specification includes information related to one or more specific file
systems. It can only be represented as a dictionary.
Although the file designated by a file specification is normally external to the
PDF file referring to it, PDF 1.3 permits a copy of the external file to be
embedded within the referring PDF file, allowing its contents to be stored or
transmitted along with the PDF file. However, embedding a file does not change
the presumption that it is external to the PDF file. Consequently, to ensure that
the PDF file can be processed correctly, it may be necessary to copy its embedded
files back into a local file system.