[R-sig-Geo] Error when combining raster and foreach packages

[Lukas Lehnert]

Dear list members,

I tried to process a large GeoTIFF file in a blockwise manner (following the 
vignette "Writing functions for large raster files").

I then combined the approach with a "foreeach"-loop to parallelize the 
calculation on the raster data:

library(raster)
library(doMC)
library(foreach)
registerDoMC(3)

ra <- raster("dem.tif")
tr <- blockSize(ra)
outfile <- "test_2.tif" 
res <- writeStart(raster(ra), outfile, overwrite = TRUE)
for (i in 1:tr$n) 
{
  v <- getValuesBlock(ra, row=tr$row[i], nrows=tr$nrows[i])

  v2 <- foreach(i=1:length(v), .combine = 'c') %dopar% {
     i*(-1)
  }
  res <- writeValues(res, v2, tr$row[i])
  cat(round(i/tr$n*100, 2), "%\n")
}
res <- writeStop(res)

However, I get errors and warnings like the following ones:
ERROR 1: /tmp/Rtmpz6qEOE/xortest_2.tif:Failed to allocate memory for to read 
TIFF directory (0 elements of 12 bytes each)            
ERROR 1: TIFFReadDirectory:Failed to read directory at offset 4639232 
Warning 1: TIFFFetchNormalTag:ASCII value for tag "GeoASCIIParams" contains 
null byte in value; value incorrectly truncated during reading due to 
implementation limitations 

The errors are different each time I restart the process. If I do not use 
foreach but a common for-loop, the errors do not occur. I tried several raster 
files (large ones and small ones) and two different computers, but the problem 
remains.

Does somebody has any idea, how to solve this problem?

Thank you in advance

Lukas