/usr/share/zsh/functions/Zftp/zfcput is in zsh-common 5.3.1-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 | # function zfcput {
# Continuation put of files from remote server.
# For each file, if it's shorter over there, put the remainder from
# over here.  This uses append, which is standard, so unlike zfcget it's
# expected to work on any reasonable server... err, as long as it
# supports SIZE and MDTM.  (It could be enhanced so you can enter the
# size so far by hand.)  You should probably be in binary transfer
# mode, thought it's not enforced.
#
# To read from midway through a local file, `tail +<n>c' is used.
# It would be nice to find a way of doing this which works on all OS's.
emulate -L zsh
[[ $curcontext = :zf* ]] || local curcontext=:zfcput
local loc rem stat=0 locst remst offs tailtype
# find how tail works.  this is intensely annoying, since it's completely
# standard in C.  od's no use, since we can only skip whole blocks.
if [[ $(echo abcd | tail +2c) = bcd ]]; then
  tailtype=c
elif [[ $(echo abcd | tail --bytes=+2) = bcd ]]; then
  tailtype=b
else
  print "I can't get your \`tail' to start from arbitrary characters.\n" \
  "If you know how to do this, let me know." 2>&1
  return 1
fi
for loc in $*; do
  # zfcd directory hack to put the front back to ~
  rem=$loc
  if [[ $rem = $HOME || $rem = $HOME/* ]]; then
    rem="~${rem#$HOME}"
  fi
  if [[ ! -r $loc ]]; then
    print "Can't read file $loc"
    stat=1
  else
    # Compare the sizes.
    locst=($(zftp local $loc))
    () {
      zftp remote $rem >|$1
      rstat=$?
      remst=($(<$1))
    } =(<<<'temporary file')
    if [[ $rstat = 2 ]]; then
      print "Server does not support remote status commands.\n" \
      "You will have to find out the size by hand and use zftp append." 2>&1
      stat=1
      continue
    elif [[ $rstat = 1 ]]; then
      # Not found, so just do a standard put.
      zftp put $rem <$loc
    elif [[ $remst[1] -gt $locst[1] ]]; then
      print "Remote file is larger!" 2>&1
      continue;
    elif [[ $locst[1] == $remst[1] ]]; then
      print "Files are already the same size." 2>&1
      continue
    else
      # tail +<N>c takes the count of the character
      # to start from, not the offset from zero. if we did
      # this with years, then 2000 would be 1999.  no y2k bug!
      # brilliant.
      (( offs = $remst[1] + 1 ))
      if [[ $tailtype = c ]]; then
	tail +${offs}c $loc | zftp append $rem || stat=1
      else
	tail --bytes=+$offs $loc | zftp append $rem || stat=1
      fi
    fi
  fi
done
return $stat
# }
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