Read binary files in R from a zipped file and a known starting position (byte offset)

Question

I have a zipped binary file under the Windows operating system that I am trying to read with R. So far it works using the unz() function in combination with the readBin() function.

> bin.con <- unz(zip_path, file_in_zip, open = 'rb')
> readBin(bin.con,
          "double", 
          n = byte_chunk, 
          size = 8L, 
          endian = "little")
> close(bin.con)

Where zip_path is the path to the zip file, file_in_zip is the filename within the zip file that is to be read and byte_chunk the number of bytes that I want to read.

In my use case, the readBin operation is part of a loop and gradually reads the whole binary file. However, I rarely want to read everything and often I know precisely which parts I want to read. Unfortunately, readBin doesn't have a start/skip argument to skip the first n bytes. Therefore I tried to conditionally replace readBin() with seek() in order to skip the actual reading of the unwanted parts.

When I try this, I get an error:

> bin.con <- unz(zip_path, file_in_zip, open = 'rb')
> seek(bin.con, where = bytes_to_skip, origin = 'current')
Error in seek.connection(bin.con, where = bytes_to_skip, origin = "current") : 
  seek not enabled for this connection
> close(bin.con)

So far, I didn't find a way to solve this error. Similar questions can be found here (unfortunately without a satisfactory answer):

• https://stat.ethz.ch/pipermail/r-help/2007-December/148847.html (no answer)
• http://r.789695.n4.nabble.com/reading-file-in-zip-archive-td4631853.html (no answer but reproducible example)

Tips all over the internet suggest adding the open = 'r' argument to unz() or dropping the open argument altogether but that only works for non-binary files (since the default is 'r'). People also suggest to unzip the files first, but since the files are quite big, this is practically impossible.

Is there any work-around to seek in a binary zipped file or read with a byte offset (potentially using C++ via the Rcpp package)?

Update:

Further research seems to indicate that seek() in zip files is not an easy problem. This question suggests a c++ library that can at best use a coarse seek. This Python question indicates that an exact seek is completely impossible because of the way how zip is implemented (although it doesn't contradict the coarse seek method).

Answer 1

Here's a bit of a hack that might work for you. Here's a fake binary file:

writeBin(as.raw(1:255), "file.bin")
readBin("file.bin", raw(1), n = 16)
#  [1] 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10

And here's the produced zip file:

zip("file.zip", "file.bin")
#   adding: file.bin (stored 0%)
readBin("file.zip", raw(1), n = 16)
#  [1] 50 4b 03 04 0a 00 02 00 00 00 7b ab 45 4a 87 1f

This uses a temporary intermediate binary file.

system('sh -c "unzip -p file.zip file.bin | dd of=tempfile.bin bs=1c skip=5c count=4c"')
# 4+0 records in
# 4+0 records out
# 4 bytes copied, 0.00044964 s, 8.9 kB/s
file.info("tempfile.bin")$size
# [1] 4
readBin("tempfile.bin", raw(1), n = 16)
# [1] 06 07 08 09

This method offsets the "expense" of dealing with the size of the stored binary data to the shell/pipe, out of R.

This worked on win10, R-3.3.2. I'm using dd from Git for Windows (version 2.11.0.3, though 2.11.1 is available), and unzip and sh from RTools.

Sys.which(c("dd", "unzip", "sh"))
#                                    dd 
# "C:\\PROGRA~1\\Git\\usr\\bin\\dd.exe" 
#                                 unzip 
#          "c:\\Rtools\\bin\\unzip.exe" 
#                                    sh 
#             "c:\\Rtools\\bin\\sh.exe"