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Today's Topics:
1. [mingw] #39512: v/snprintf don't null-terminate when
truncating (MinGW Notification List)
----------------------------------------------------------------------
Message: 1
Date: Mon, 26 Aug 2019 19:45:54 +0000
From: MinGW Notification List <[email protected]>
To: OSDN Ticket System <[email protected]>
Subject: [MinGW-Notify] [mingw] #39512: v/snprintf don't
null-terminate when truncating
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8
#39512: v/snprintf don't null-terminate when truncating
Open Date: 2019-08-26 19:45
Last Update: 2019-08-26 19:45
URL for this Ticket:
https://osdn.net//projects/mingw/ticket/39512
RSS feed for this Ticket:
https://osdn.net/ticket/ticket_rss.php?group_id=3917&tid=39512
---------------------------------------------------------------------
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>
3.
4. int main(void)
5. {
6. char buffer[3] = {0};
7. int ret = snprintf(buffer, sizeof(buffer), "%s", "abcd");
8.
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);
12.
13. return 0;
14. }
is
abc
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/
reference/snprintf-s-snprintf-s-l-snwprintf-s-snwprintf-s-l
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';
--
Ticket information of MinGW - Minimalist GNU for Windows project
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URL for this Ticket:
https://osdn.net/projects/mingw/ticket/39512
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