How to write a binary file with 4 bytes per entry using R?

Hi, y'all

I'm trying to write a data frame into a binary file. I know how to do it using writeBin, but I don't know how to specify the format that the data are to be stored.

This output binary file is to be read by a FORTRAN application, and the data it expects should be read as a sequence of 320 number per row, each number a 4-byte integer.

I happen to know that in FORTRAN, for instance, you could specify such format as: 320(I4), where I4 stands for 4-byte integer, but I am uncertain about doing it in R. I tried the following but I received an error message back.

# Fake data
df = matrix(data = rnorm(320*100, mean = 100), ncol = 320, nrow = 100)
# Set up a temppfile
xfil = tempfile("testbin")
x = file(xfil, "wb")
# Write your data to a binary file
writeBin(object = df,
         con = x,
         size = 320*4)
#> Error in writeBin(object = df, con = x, size = 320 * 4): can only write vector objects

Could anyone help me out?
Thank you.