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1353 :: BUG with sprintf?

• mr_july (orginal) [2015-06-22T10:06:20+02:00] view original

  Hello, the following code

  
  import strutils
  
  proc sprintf(buf, frmt: cstring) {.header: "<stdio.h>",
                                    importc: "sprintf",
                                    varargs, noSideEffect.}
  
  let
    n = 1
    format = "%2.2d"
  var buf = ""
  buf.sprintf(format, 1)
  echo buf
  echo "buf='" & buf & "'"
  echo "buf='$1'" % buf
  Run

  produces

  
  01
  buf=''
  buf=''
  Run

  why buf produces an output only in first case?

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• Araq (orginal) [2015-06-22T10:53:36+02:00] view original

  var buf = "" doesn't allocate a buffer that's large enough and you need to call setLen(buf, len(cstring(buf)) afterwards to create a proper string. Also don't use sprintf, it's unsafe garbage.

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• def (orginal) [2015-06-22T11:07:41+02:00] view original

  In this case the buffer actually is large enough as Nim (currently) makes the string at least 7 chars big. Instead echo just uses printf, which ignores the length of the Nim string and just prints until the \0 is encountered. Meanwhile & is written in Nim and respects the length field, which is still 0.

  This should work:

  import strutils
  
  proc sprintf(buf, frmt: cstring) {.header: "<stdio.h>",
                                    importc: "sprintf",
                                    varargs, noSideEffect.}
  
  let
    n = 1
    format = "%2.2d"
  var buf = newString(8)
  buf.sprintf(format, 1)
  buf.setLen(buf.cstring.len)
  echo "buf='" & buf & "'"
  echo "buf='$1'" % buf
  Run

  Which you can of course abstract away in a proc, but sprintf is often unsafe as you don't get to specify the size of buf. snprintf fixes this.

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• mr_july (orginal) [2015-06-22T13:08:51+02:00] view original

  I'm trying to rewrite it with snprintf:

  
  import strutils
  
  proc snprintf(buf, size: int, frmt: cstring) {.header: "<stdio.h>",
                                    importc: "snprintf",
                                    varargs, noSideEffect.}
  
  let
    n = 1
    fmt = "%2.2d"
    maxFmtLen = 5
  var buf = newString(maxFmtLen)
  buf.snprintf(maxFmtLen, fmt.cstring, n)
  buf.setLen(buf.cstring.len)
  echo buf
  echo "buf='" & buf & "'"
  echo buf
  echo "buf='$1'" % buf
  echo buf
  Run

  but I get the error:

  
  bug.nim(12, 4) Error: type mismatch: got (string, int, cstring, int)
  but expected one of:
  bug_fixed_snprintf.snprintf(buf: int, size: int, frmt: cstring)
  Run

  what is wrong?

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• def (orginal) [2015-06-22T13:13:01+02:00] view original

  It should be buf: cstring not buf: int in snprintf.

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• mr_july (orginal) [2015-06-22T14:03:57+02:00] view original

  That's it! :-) Thank you!

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