[MinGW-Notify] [mingw] #39512: v/snprintf don't null-terminate when truncating

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Tue Aug 27 04:45:54 JST 2019

#39512: v/snprintf don't null-terminate when truncating

  Open Date: 2019-08-26 19:45
Last Update: 2019-08-26 19:45

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Last Changes/Comment on this Ticket:
2019-08-26 19:45 Updated by: lazybumhorse
 * New Ticket "v/snprintf don't null-terminate when truncating" created

Ticket Status:

      Reporter: lazybumhorse
         Owner: (None)
          Type: Issues
        Status: Open
      Priority: 5 - Medium
     MileStone: (None)
     Component: WSL
      Severity: 5 - Medium
    Resolution: None

Ticket details:

Using MSYS2 on Windows 7, snprintf and vsnprintf are not C99+ standard
compliant because they do not null-terminate when truncating.

Relevant excerpt from the C99 standard:

    The snprintf function is equivalent to fprintf, except that the output is
    written into an array (specified by argument s) rather than to a stream. If
    n is zero, nothing is written, and s may be a null pointer. Otherwise,
    output characters beyond the n-1st are discarded rather than being written
    to the array, and a null character is written at the end of the characters
    actually written into the array.

    The vsnprintf function is equivalent to snprintf, with the variable
    argument list replaced by arg

The output of the following code:

 1.  #include <stdio.h>
 2.  #include <errno.h>
 4.  int main(void)
 5.  {
 6.     char buffer[3] = {0};
 7.     int ret = snprintf(buffer, sizeof(buffer), "%s", "abcd");
 9.     for (int i = 0; i < 3; i++)
10.        printf("%c", buffer[i]); /* should print "ab", does print "abc" */
11.     printf("ret = %d, errno = %d\n", ret, errno);
13.     return 0;
14.  }


ret = -1, errno = 34

This was independently confirmed on another Windows 7 machine.

errno is ERANGE and this seems to indicate it is using _vsnprintf (from
msvcrt.dll?), at least it is similar in behavior to _vsnprintf_s, minus the
null-termination: https://docs.microsoft.com/en-us/cpp/c-runtime-library/

If __USE_MINGW_ANSI_STDIO is defined as 1 before the stdio.h include, this
example does null-terminate, but I neither know what else that does entail, nor
does it help the case of non-conformity.

Out of curiosity, I installed an old mingw32 version from Sourceforge (which
seems to be from 2013?) and it did not have this issue.

A very simple but also very exhausting workaround is to always do:

 1.      int ret = snprintf(buffer, sizeof(buffer), "%s", "abcd");
 2.      if (ret < 0)
 3.          buffer[sizeof(buffer)-1] = '\0';

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