Daniil Mashkin Daniil Mashkin - 1 month ago 15
Python Question

struct.error: unpack requires a string argument of length 16

While processing a PDF file (2.pdf) with pdfminer (pdf2txt.py) I received the following error:

pdf2txt.py 2.pdf

Traceback (most recent call last):
File "/usr/local/bin/pdf2txt.py", line 115, in <module>
if __name__ == '__main__': sys.exit(main(sys.argv))
File "/usr/local/bin/pdf2txt.py", line 109, in main
interpreter.process_page(page)
File "/usr/local/lib/python2.7/dist-packages/pdfminer/pdfinterp.py", line 832, in process_page
self.render_contents(page.resources, page.contents, ctm=ctm)
File "/usr/local/lib/python2.7/dist-packages/pdfminer/pdfinterp.py", line 843, in render_contents
self.init_resources(resources)
File "/usr/local/lib/python2.7/dist-packages/pdfminer/pdfinterp.py", line 347, in init_resources
self.fontmap[fontid] = self.rsrcmgr.get_font(objid, spec)
File "/usr/local/lib/python2.7/dist-packages/pdfminer/pdfinterp.py", line 195, in get_font
font = self.get_font(None, subspec)
File "/usr/local/lib/python2.7/dist-packages/pdfminer/pdfinterp.py", line 186, in get_font
font = PDFCIDFont(self, spec)
File "/usr/local/lib/python2.7/dist-packages/pdfminer/pdffont.py", line 654, in __init__
StringIO(self.fontfile.get_data()))
File "/usr/local/lib/python2.7/dist-packages/pdfminer/pdffont.py", line 375, in __init__
(name, tsum, offset, length) = struct.unpack('>4sLLL', fp.read(16))
struct.error: unpack requires a string argument of length 16


While the similar file (1.pdf) doesn't cause a problem.

I can't find any information about the error. I added an issue on the pdfminer GitHub repository, but it remained unanswered. Can someone explain to me why this is happening? What can I do to parse 2.pdf?




Update: I get a similar error with
BytesIO
instead of
StringIO
after installing pdfminer directly from the GitHub repository.

$ pdf2txt.py 2.pdf
Traceback (most recent call last):
File "/home/danil/projects/python/pdfminer-source/env/bin/pdf2txt.py", line 116, in <module>
if __name__ == '__main__': sys.exit(main(sys.argv))
File "/home/danil/projects/python/pdfminer-source/env/bin/pdf2txt.py", line 110, in main
interpreter.process_page(page)
File "/home/danil/projects/python/pdfminer-source/env/local/lib/python2.7/site-packages/pdfminer/pdfinterp.py", line 839, in process_page
self.render_contents(page.resources, page.contents, ctm=ctm)
File "/home/danil/projects/python/pdfminer-source/env/local/lib/python2.7/site-packages/pdfminer/pdfinterp.py", line 850, in render_contents
self.init_resources(resources)
File "/home/danil/projects/python/pdfminer-source/env/local/lib/python2.7/site-packages/pdfminer/pdfinterp.py", line 356, in init_resources
self.fontmap[fontid] = self.rsrcmgr.get_font(objid, spec)
File "/home/danil/projects/python/pdfminer-source/env/local/lib/python2.7/site-packages/pdfminer/pdfinterp.py", line 204, in get_font
font = self.get_font(None, subspec)
File "/home/danil/projects/python/pdfminer-source/env/local/lib/python2.7/site-packages/pdfminer/pdfinterp.py", line 195, in get_font
font = PDFCIDFont(self, spec)
File "/home/danil/projects/python/pdfminer-source/env/local/lib/python2.7/site-packages/pdfminer/pdffont.py", line 665, in __init__
BytesIO(self.fontfile.get_data()))
File "/home/danil/projects/python/pdfminer-source/env/local/lib/python2.7/site-packages/pdfminer/pdffont.py", line 386, in __init__
(name, tsum, offset, length) = struct.unpack('>4sLLL', fp.read(16))
struct.error: unpack requires a string argument of length 16

Answer

TL; DR

Thanks to @mkl and @hynecker for the extra info... With that I can confirm this is a bug in pdfminer and your PDF. Whenever pdfminer tries to get embedded file streams (e.g. font definitions), it is picking up the last one in the file before an endobj. Sadly, not all PDFs rigorously add the end tag and so pdfminer should be resilient to this.

Quick fix for this issue

I've created a patch - which has been submitted as a pull request on github. See https://github.com/euske/pdfminer/pull/159.

Detailed diagnosis

As mentioned in the other answers, the reason you're seeing this is that you're not getting the expected number of bytes from the stream as pdfminer is unpacking the data. But why?

As you can see in your stack trace, pdfminer (rightly) spots that it has a CID font to process. It then goes on to process the embedded font file as a TrueType font (in pdffont.py). It tries to parse the associated stream (stream ID 18) by reading out a set of binary tables.

This doesn't work for 2.pdf because it has a text stream. You can see this by running dumppdf -b -i 18 2.pdf. I've put the start here:

/CIDInit /ProcSet findresource begin
12 dict begin
begincmap
/CIDSystemInfo << /Registry (Adobe) /Ordering (UCS) /Supplement 0
>> def /CMapName /Adobe-Identity-UCS def
...

So, garbage in, garbage out... Is this a bug in your file or pdfminer? Well, the fact that other readers can handle it made me suspicious.

Digging around a little more, I see that this stream is identical to stream ID 17, which is the cmap for the ToUnicode field. A quick look at the PDF spec shows that these cannot be the same.

Digging in to the code further, I see that all streams are getting the same data. Oops! This is the bug. The cause appears to be related to the fact that this PDF is missing some end tags - as noted by @hynecker.

The fix is to return the right data for each stream. Any other fix to just swallow the error will result in bad data being used for all streams and so, for example, incorrect font definitions.

I believe the attached patch will fix your problem and should be safe to use in general.