The C99 standard describes snprintf() in an awkward way, so it's hard
(for me) to assume what the result will be in some situations.
1. The "Description" section states:
"... 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."
2. The "Returns" section states:
"The snprintf function returns the number of characters that would have
been written had n been sufficiently large, not counting the
terminating null character, or a negative value if an encoding error
occurred. Thus, the null-terminated output has been completely written
if and only if the returned value is nonnegative and less than n."
Consider the following code:
#include <stdio.h>
int main (void)
{
int r;
double d = -10;
char str[5];
r = snprintf (str, 5, "%f", d);
printf ("r=%d str=%s\n", r, (r 0 && r < 5) ? str : "<INVALID>");
return 0;
}
Since the precision is missing, it's taken as 6, but snprintf() further
limits it. My question is, what should be the return value in this
case:
a) 5, "str" contains "-10."
b) 5 (number of characters needed for the double to fit, since it
didn't fit into "str"), "str" contents invalid
c) -1 due to an encoding error, "str" contents invalid
d) something else that I've missed
My assumption would be 5, but it seems to be incorrect. The following
are the results from different systems:
gcc 3.3 (Mac OS X): 10
gcc 3.4.5 (Windows, mingw): -1
BC++ 5.5.1 (Windows): 10
MS VC++ 6.0 and 2005 (Windows, _snprintf() used): -1
DigitalMars 8.42n (Windows): -1
I'd also like to know what an "encoding error" means in snprintf()
case. Isn't it generally related to multibyte/wide characters?
--
WYCIWYG - what you C is what you get
14  13263 
matevzb <ma*****@gmail.comwrote:
The C99 standard describes snprintf() in an awkward way, so it's hard
(for me) to assume what the result will be in some situations.
1. The "Description" section states:
"... 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."
2. The "Returns" section states:
"The snprintf function returns the number of characters that would have
been written had n been sufficiently large, not counting the
terminating null character, or a negative value if an encoding error
occurred. Thus, the null-terminated output has been completely written
if and only if the returned value is nonnegative and less than n."
Consider the following code:
#include <stdio.h>
int main (void)
{
int r;
double d = -10;
char str[5];
r = snprintf (str, 5, "%f", d);
printf ("r=%d str=%s\n", r, (r 0 && r < 5) ? str : "<INVALID>");
return 0;
}
Since the precision is missing, it's taken as 6, but snprintf() further
limits it. My question is, what should be the return value in this
case:
a) 5, "str" contains "-10."
b) 5 (number of characters needed for the double to fit, since it
didn't fit into "str"), "str" contents invalid
c) -1 due to an encoding error, "str" contents invalid
d) something else that I've missed
The return value should be 10, since that's how many chars (without
the trailing '\0') are in "-10.000000", the string you get for an
unadorned "%f" with -10. The arry 'str' will only contain as many
of these 10 characters as fit into the string (including the trai-
ling '\0'), so it would be "-10." in your case. This allows you to
use snprintf() to find out how many characters you are going to
need - just call it with 0 as the second argument and it gives you
the strlen() of the string that would result for a fully success-
ful call of snprintf(). This number you can then be used to allocate
exactly as much memory as needed for the resulting string (and you
can then use the probably slightly faster sprintf();-)
My assumption would be 5, but it seems to be incorrect. The following
are the results from different systems:
gcc 3.3 (Mac OS X): 10
gcc 3.4.5 (Windows, mingw): -1
BC++ 5.5.1 (Windows): 10
MS VC++ 6.0 and 2005 (Windows, _snprintf() used): -1
DigitalMars 8.42n (Windows): -1
Looks as if some of these compilers use a non-compliant libc.
Some of the first implementations of sprintf() (before the
C99 standard, e.g. the GNU libc 5) did return -1 if the
resulting string did not fit into memory allowed for writing.
And this old behaviour does not seem to have been corrected
in all the implementations you tested.
I'd also like to know what an "encoding error" means in snprintf()
case. Isn't it generally related to multibyte/wide characters?
Yes. As far as I can see you will get an "encoding error" if you
ask snprintf() to print out a wchar_t string using "%ls", but
the string does contain data that are not valid wide characters.
Regards, Jens
--
\ Jens Thoms Toerring ___ jt@toerring.de
\__________________________ http://toerring.de
matevzb wrote:
The C99 standard describes snprintf() in an awkward way, so it's hard
(for me) to assume what the result will be in some situations.
1. The "Description" section states:
"... 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."
2. The "Returns" section states:
"The snprintf function returns the number of characters that would have
been written had n been sufficiently large, not counting the
terminating null character, or a negative value if an encoding error
occurred. Thus, the null-terminated output has been completely written
if and only if the returned value is nonnegative and less than n."
Consider the following code:
#include <stdio.h>
int main (void)
{
int r;
double d = -10;
char str[5];
r = snprintf (str, 5, "%f", d);
printf ("r=%d str=%s\n", r, (r 0 && r < 5) ? str : "<INVALID>");
return 0;
}
Since the precision is missing, it's taken as 6, but snprintf() further
limits it. My question is, what should be the return value in this
case:
a) 5, "str" contains "-10."
b) 5 (number of characters needed for the double to fit, since it
didn't fit into "str"), "str" contents invalid
c) -1 due to an encoding error, "str" contents invalid
d) something else that I've missed
My assumption would be 5, but it seems to be incorrect. The following
are the results from different systems:
gcc 3.3 (Mac OS X): 10
gcc 3.4.5 (Windows, mingw): -1
BC++ 5.5.1 (Windows): 10
MS VC++ 6.0 and 2005 (Windows, _snprintf() used): -1
DigitalMars 8.42n (Windows): -1
10 is the correct result. Put simply, Microsoft's version of snprintf is
broken (Their documentation even claims that snprintf returns a negative
value if the number of characters to be written exceeds the buffer
size). Likely, either for compatibility with Microsoft's implementation,
or because they actually use Microsoft's implementation, mingw and
DigitalMars return the same result.
--
Clark S. Cox III cl*******@gmail.com
Clark S. Cox III wrote:
matevzb wrote:
The C99 standard describes snprintf() in an awkward way, so it's hard
(for me) to assume what the result will be in some situations.
1. The "Description" section states:
"... 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."
2. The "Returns" section states:
"The snprintf function returns the number of characters that would have
been written had n been sufficiently large, not counting the
terminating null character, or a negative value if an encoding error
occurred. Thus, the null-terminated output has been completely written
if and only if the returned value is nonnegative and less than n."
Consider the following code:
#include <stdio.h>
int main (void)
{
int r;
double d = -10;
char str[5];
r = snprintf (str, 5, "%f", d);
printf ("r=%d str=%s\n", r, (r 0 && r < 5) ? str : "<INVALID>");
return 0;
}
Since the precision is missing, it's taken as 6, but snprintf() further
limits it. My question is, what should be the return value in this
case:
a) 5, "str" contains "-10."
b) 5 (number of characters needed for the double to fit, since it
didn't fit into "str"), "str" contents invalid
c) -1 due to an encoding error, "str" contents invalid
d) something else that I've missed
My assumption would be 5, but it seems to be incorrect. The following
are the results from different systems:
gcc 3.3 (Mac OS X): 10
gcc 3.4.5 (Windows, mingw): -1
BC++ 5.5.1 (Windows): 10
MS VC++ 6.0 and 2005 (Windows, _snprintf() used): -1
DigitalMars 8.42n (Windows): -1
10 is the correct result. Put simply, Microsoft's version of snprintf is
broken (Their documentation even claims that snprintf returns a negative
value if the number of characters to be written exceeds the buffer
size). Likely, either for compatibility with Microsoft's implementation,
or because they actually use Microsoft's implementation, mingw and
DigitalMars return the same result.
I think your latter guess is correct. They call the Microsoft supplied
'C runtime library.'
On Jan 20, 5:54 pm, j...@toerring.de (Jens Thoms Toerring) wrote:
matevzb <mate...@gmail.comwrote:
Since the precision is missing, it's taken as 6, but snprintf() further
limits it. My question is, what should be the return value in this
case:
a) 5, "str" contains "-10."
b) 5 (number of characters needed for the double to fit, since it
didn't fit into "str"), "str" contents invalid
c) -1 due to an encoding error, "str" contents invalid
d) something else that I've missed
The return value should be 10, since that's how many chars (without
the trailing '\0') are in "-10.000000", the string you get for an
unadorned "%f" with -10. The arry 'str' will only contain as many
of these 10 characters as fit into the string (including the trai-
ling '\0'), so it would be "-10." in your case.
Ah stupid me, should've paid more attention to "...returns the number
of characters that would have been written had n been sufficiently
large".
This allows you to use snprintf() to find out how many characters you are going to
need - just call it with 0 as the second argument and it gives you
the strlen() of the string that would result for a fully success-
ful call of snprintf(). This number you can then be used to allocate
exactly as much memory as needed for the resulting string (and you
can then use the probably slightly faster sprintf();-)
I remember doing this (non-portably) through the use of FILE pointers
back when snprintf() wasn't widely available. Since snprintf() seems to
be working incorrectly on Windows, I might be going back to this anyhow
=)
<snip>
I'd also like to know what an "encoding error" means in snprintf()
case. Isn't it generally related to multibyte/wide characters?
Yes. As far as I can see you will get an "encoding error" if you
ask snprintf() to print out a wchar_t string using "%ls", but
the string does contain data that are not valid wide characters.
I thought so. Also, another word of warning to Windows _snprintf()
users - in case it returns -1, the string may not be null-terminated (I
don't know what should happen in a correct implementation though).
Thanks to all for explanations.
--
WYCIWYG - what you C is what you get br***************@gmail.com writes:
OpenWatcom returns 10.
For what?
Pleaes provide context when you post a followup. Google Groups has
corrected the bug that made this difficult.
Read <http://cfaj.freeshell.org/google/>.
--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
Jens Thoms Toerring wrote:
My assumption would be 5, but it seems to be incorrect. The following
are the results from different systems:
gcc 3.3 (Mac OS X): 10
gcc 3.4.5 (Windows, mingw): -1
BC++ 5.5.1 (Windows): 10
MS VC++ 6.0 and 2005 (Windows, _snprintf() used): -1
DigitalMars 8.42n (Windows): -1
Looks as if some of these compilers use a non-compliant libc.
Some of the first implementations of sprintf() (before the
C99 standard, e.g. the GNU libc 5) did return -1 if the
resulting string did not fit into memory allowed for writing.
And this old behaviour does not seem to have been corrected
in all the implementations you tested.
Well, VC++ 6.0 doesn't claim to support C99, so it is OK for it
to return -1. I wonder if the OP was invoking Digital Mars
and GCC in C99 mode or not.
Old Wolf wrote:
Jens Thoms Toerring wrote:
>>My assumption would be 5, but it seems to be incorrect. The following
are the results from different systems:
gcc 3.3 (Mac OS X): 10
gcc 3.4.5 (Windows, mingw): -1
BC++ 5.5.1 (Windows): 10
MS VC++ 6.0 and 2005 (Windows, _snprintf() used): -1
DigitalMars 8.42n (Windows): -1
Looks as if some of these compilers use a non-compliant libc.
Some of the first implementations of sprintf() (before the
C99 standard, e.g. the GNU libc 5) did return -1 if the
resulting string did not fit into memory allowed for writing.
And this old behaviour does not seem to have been corrected
in all the implementations you tested.
Well, VC++ 6.0 doesn't claim to support C99, so it is OK for it
to return -1. I wonder if the OP was invoking Digital Mars
and GCC in C99 mode or not.
IIRC, the return value was the same in C90/C89. But, I could be wrong as
I don't have a copy in front of me.
--
Clark S. Cox III cl*******@gmail.com
Clark S. Cox III <cl*******@gmail.comwrote:
Old Wolf wrote:
Well, VC++ 6.0 doesn't claim to support C99, so it is OK for it
to return -1. I wonder if the OP was invoking Digital Mars
and GCC in C99 mode or not.
IIRC, the return value was the same in C90/C89. But, I could be wrong as
I don't have a copy in front of me.
As far as I can see there was no snprintf() in C89/90.
Regards, Jens
--
\ Jens Thoms Toerring ___ jt@toerring.de
\__________________________ http://toerring.de
On Jan 22, 6:36 pm, j...@toerring.de (Jens Thoms Toerring) wrote:
Clark S. Cox III <clarkc...@gmail.comwrote:
Old Wolf wrote:
Well, VC++ 6.0 doesn't claim to support C99, so it is OK for it
to return -1. I wonder if the OP was invoking Digital Mars
and GCC in C99 mode or not.
IIRC, the return value was the same in C90/C89. But, I could be wrong as
I don't have a copy in front of me.
As far as I can see there was no snprintf() in C89/90.
I'd say Clark probably meant the (gcc) extension in this case.
Here is some additional info that I missed the first time:
- gcc was invoked in C99 mode (both mingw and Mac versions).
- BC++ 5.5 doesn't provide C99, it has its own snprintf() extension,
which is a bit different though.
- DMC has some C99 features but no switches for it AFAIK. It has
_snprintf() which doesn't conform to C99.
- VC++ 2005 only supports some C99 features, snprintf() not included.
_snprintf() doesn't conform to C99 - same as VC++ 6.0.
I might try some other compilers, but it all seems pretty useless.
--
WYCIWYG
On 22 Jan 2007 11:04:04 -0800, "matevzb" <ma*****@gmail.comwrote:
>I might try some other compilers, but it all seems pretty useless.
Why? AFAICS, you can detect an error on all implementations with
soething like:
int n = snprintf (...);
if (n < 0 || !(n < bufsize)) {
// error
}
On Jan 22, 9:11 pm, rpbg...@yahoo.com (Roland Pibinger) wrote:
On 22 Jan 2007 11:04:04 -0800, "matevzb" <mate...@gmail.comwrote:
I might try some other compilers, but it all seems pretty useless.
Why? AFAICS, you can detect an error on all implementations with
soething like:
int n = snprintf (...);
if (n < 0 || !(n < bufsize)) {
// error
}
Because for n >= bufsize the string is still valid and properly
terminated in a conforming implementation - i.e. it's not an error. In
the example
int r;
double d = -10;
char str[5];
r = snprintf (str, 5, "%f", d);
I explicitly specify the string will contain up to 5 characters
(4+NUL), even if it means chopping away the double's zeroes. The result
however is -1 in non-conforming implementations plus the string isn't
null-terminated (at least in VC++).
--
WYCIWYG
Why? AFAICS, you can detect an error on all implementations with
soething like:
int n = snprintf (...);
if (n < 0 || !(n < bufsize)) {
// error
}
That's the right way.
shorter:
int n = snprintf(...);
if (n < 0 || n >= bufsize) {
// error
}
Although not perfect - the best libc reference known to me: http://www.delorie.com/djgpp/doc/libc
Regards,
Stefan
Jens Thoms Toerring wrote:
Clark S. Cox III <cl*******@gmail.comwrote:
>Old Wolf wrote:
>>Well, VC++ 6.0 doesn't claim to support C99, so it is OK for it
to return -1. I wonder if the OP was invoking Digital Mars
and GCC in C99 mode or not.
>IIRC, the return value was the same in C90/C89. But, I could be wrong as
I don't have a copy in front of me.
As far as I can see there was no snprintf() in C89/90.
Indeed, I misremembered.
--
Clark S. Cox III cl*******@gmail.com This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
by: Dave |
last post by:
Hi,
I tried something with 'return value' of a function and i got two different
behaviours.
My question is: why does method 1 not work?
Thanks
Dave
method 1: here, whatever i choose (ok or...
|
by: tshad |
last post by:
I am trying to set up a class to handle my database accesses.
I can't seem to figure out how to get the return value from my dataReader
from these routines (most of which I got elsewhere). They...
|
by: Dmitriy Lapshin [C# / .NET MVP] |
last post by:
Hi all,
I think the VB .NET compiler should at least issue a warning when a function
does not return value. C# and C++ compilers treat this situation as an error
and I believe this is the right...
|
by: vashwath |
last post by:
Hi all,
To test the return value of system I wrote 2 programs which are shown
below.
Program 1:
#include <stdio.h>
#include <stdlib.h>
int main(void)
|
by: lovecreatesbeauty |
last post by:
Hello experts,
Is the following code snippet legal? If it is, how can exit() do the
keyword return a favor and give a return value to the main function?
Can a function call (or only this...
|
by: Eric Lilja |
last post by:
Hello, consider this complete program:
#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;
class Hanna
{
|
by: Tim Roberts |
last post by:
I've been doing COM a long time, but I've just come across a behavior
with late binding that surprises me. VB and VBS are not my normal
milieux, so I'm hoping someone can point me to a document...
|
by: philip |
last post by:
hello, i am new to asp.net and sql server, and i have 3 questions for
asking:
1. i am writing a store procedure of login validation for my asp.net
application and wondering what the different...
|
by: Terry Olsen |
last post by:
How do I get this to work? It always returns False, even though I can see
"This is True!" in the debug window. Do I have to invoke functions
differently than subs?
Private Delegate Function...
|
by: erikbower65 |
last post by:
Using CodiumAI's pr-agent is simple and powerful. Follow these steps:
1. Install CodiumAI CLI: Ensure Node.js is installed, then run 'npm install -g codiumai' in the terminal.
2. Connect to...
|
by: linyimin |
last post by:
Spring Startup Analyzer generates an interactive Spring application startup report that lets you understand what contributes to the application startup time and helps to optimize it. Support for...
|
by: erikbower65 |
last post by:
Here's a concise step-by-step guide for manually installing IntelliJ IDEA:
1. Download: Visit the official JetBrains website and download the IntelliJ IDEA Community or Ultimate edition based on...
|
by: DJRhino1175 |
last post by:
When I run this code I get an error, its Run-time error# 424 Object required...This is my first attempt at doing something like this. I test the entire code and it worked until I added this -
If...
|
by: Rina0 |
last post by:
I am looking for a Python code to find the longest common subsequence of two strings. I found this blog post that describes the length of longest common subsequence problem and provides a solution in...
|
by: DJRhino |
last post by:
Private Sub CboDrawingID_BeforeUpdate(Cancel As Integer)
If = 310029923 Or 310030138 Or 310030152 Or 310030346 Or 310030348 Or _
310030356 Or 310030359 Or 310030362 Or...
|
by: lllomh |
last post by:
Define the method first
this.state = {
buttonBackgroundColor: 'green',
isBlinking: false, // A new status is added to identify whether the button is blinking or not
}
autoStart=()=>{
|
by: lllomh |
last post by:
How does React native implement an English player?
|
by: Mushico |
last post by:
How to calculate date of retirement from date of birth
| |
