How to convert an array to a cstring? - nim-lang

Question

I found (here) the source of a program to compute the MD5 of a file using Nim.
This program no longer compiles (nim 1.4) as the implicit conversion between array to cstring has been disabled.
How can this be fixed?
import md5
import os
proc calculateMD5Incremental(filename: string) : string =
const blockSize: int = 8192
var
c: MD5Context
d: MD5Digest
f: File
bytesRead: int = 0
buffer: array[blockSize, char]
byteTotal: int = 0
#read chunk of file, calling update until all bytes have been read
try:
f = open(filename)
md5Init(c)
bytesRead = f.readBuffer(buffer.addr, blockSize)
while bytesRead > 0:
byteTotal += bytesRead
md5Update(c, buffer, bytesRead) # <--- HERE buffer should be cstring
bytesRead = f.readBuffer(buffer.addr, blockSize)
md5Final(c, d)
except IOError:
echo("File not found.")
finally:
if f != nil:
close(f)
result = $d
if paramCount() > 0:
let arguments = commandLineParams()
echo("MD5: ", calculateMD5Incremental(arguments[0]))
else:
echo("Must pass filename.")
quit(-1)
Note: I'm more interested in the general question and not in MD5, this was the example that came to hand.

Answer 1

In your answer you turned array into string with $. This though does not return "ciao" but "['c', 'i', 'a', 'o']". Not sure if you want that but proper way to do the conversion is as follows:
const blockSize: int = 4
var
c: cstring
buffer: array[blockSize, char]
buffer = ['c','i','a','o']
c = cast[cstring](create(char, blockSize + 1))
moveMem(c[0].addr, buffer[0].addr, blockSize)
assert c == "ciao"
Mind that this is optimal unsafe solution. If you want to be safe but little slower the you can use this code:
const blockSize: int = 4
var
c: cstring
buffer: array[blockSize, char]
temp: string
buffer = ['c','i','a','o']
temp.setLen(blockSize)
for i in 0..<blockSize:
temp[i] = buffer[i]
c = temp
assert c == "ciao"

Answer 2

UPDATE In this context this is completely wrong. As pointed out in the accepted answer, $array will render it in a "screen friendly" way.
Correct implementation
The working procedure for md5(fromFile):
proc calculateMD5Incremental(filename: string) : string =
const blockSize: int = 8192
var
c: MD5Context
d: MD5Digest
f: File
s: cstring
bytesRead: int = 0
buffer: array[blockSize, char]
byteTotal: int = 0
#read chunk of file, calling update until all bytes have been read
try:
f = open(filename)
md5Init(c)
bytesRead = f.readBuffer(buffer.addr, blockSize)
while bytesRead > 0:
byteTotal += bytesRead
s = cast[cstring](create(char, blockSize + 1))
moveMem(s[0].addr, buffer[0].addr, blockSize)
md5Update(c, s, bytesRead)
bytesRead = f.readBuffer(buffer.addr, blockSize)
md5Final(c, d)
except IOError:
stderr.writeLine("ERROR: File not found: ", filename)
finally:
if f != nil:
close(f)
result = $d
Wrong conversion
~I found that a regular string is accepted so $buffer will convert the array to a valid string:~
const blockSize: int = 4
var
c: cstring
buffer: array[blockSize, char]
buffer = ['c','i','a','o']
c = $buffer