Hi,
I have a simple printf-like function:
int print(const char *format, ...)
{
char buffer[1024];
va_list argptr;
int i;
va_start(argptr , format);
i = vsnprintf(buffe r, sizeof(buffer), format, argptr);
va_end(argptr);
buffer[sizeof(buffer)-1] = '\0';
printf("%s\n",b uffer); /* this bit just for the sake of testing */
return i;
}
If I call the function using something like:
char message[50];
strcpy(message, "hi there");
print("%s",mess age);
everything works, but if I do:
print(message);
it doesn't (program crashes with an abort).
Is there something wrong with what I'm doing, or should I be looking
elsewhere to work out the cause of my crash?
Thanks a lot,
Adam
Oct 15 '08
30 2759
On 20 Oct, 20:09, jameskuy...@ver izon.net wrote:
Reading and understanding
the documentation of the vsnprintf() function is the best way. The
standard's description of vsnprintf() cross-references snprintf(). The
description of snprintf() says "a null character is written at the end
of the characters actually written into the array." The documentation
that comes with your compiler should say roughly the same thing.
At the time I was doing the investigating I had trouble finding the
documentation. :( If someone can point me at exactly this item, given
I'm using GCC (what other information is needed?), it would probably
help me in future...
Thanks,
Adam
Adam wrote:
On 20 Oct, 20:09, jameskuy...@ver izon.net wrote:
Reading and understanding
the documentation of the vsnprintf() function is the best way. The
standard's description of vsnprintf() cross-references snprintf(). The
description of snprintf() says "a null character is written at the end
of the characters actually written into the array." The documentation
that comes with your compiler should say roughly the same thing.
At the time I was doing the investigating I had trouble finding the
documentation. :( If someone can point me at exactly this item, given
I'm using GCC (what other information is needed?), it would probably
help me in future...
On any unix-like platform where gcc has been installed, there's a good
chance that you'll find correct documentation by typing 'man
vsnprintf'. I'm not sure of the best approach on other platforms.
On 20 Oct, 19:07, Adam <n...@snowstone .org.ukwrote:
On 16 Oct, 00:50, Nate Eldredge <n...@vulcan.la nwrote:
Adam <n...@snowstone .org.ukwrites:
I have a simple printf-like function:
int print(const char *format, ...)
{
char buffer[1024];
va_list argptr;
int i;
va_start(argptr , format);
i = vsnprintf(buffe r, sizeof(buffer), format, argptr);
va_end(argptr);
buffer[sizeof(buffer)-1] = '\0';
printf("%s\n",b uffer); /* this bit just for the sake of testing */
return i;
}
If I call the function using something like:
char message[50];
strcpy(message, "hi there");
print("%s",mess age);
everything works, but if I do:
print(message);
it doesn't (program crashes with an abort).
I don't see a problem myself. I took your function and added a main
function:
<snip>
and it compiles and runs without problems on three different systems.
Actually, the bit I was wondering about was passing a (char *) to the
print function, rather than a literal string.
should be ok. *If* it contains a valid string. Possible problem
with % characters in the string?
char *message = "the %s are not as indicated\n"
printf (message);
I did find this
page:http://www.eskimo.com/~scs/cclass/int/sx11c.html
...which seems to suggest (in the first paragraph) this might be
wrong.
it says:
"When a function with a variable-length argument list is called, the
variable arguments are passed using C's old ``default argument
promotions.''
These say that types char and short int are automatically promoted to
int,
and type float is automatically promoted to double. Therefore,
varargs
functions will never receive arguments of type char, short int, or
float.
Furthermore, it's an error to ``pass'' the type names char, short int,
or
float as the second argument to the va_arg() macro. Finally, for
vaguely
related reasons, the last fixed argument (the one whose name is passed
as
the second argument to the va_start() macro) should not be of type
char,
short int, or float, either."
I can't see where the problem is. I don't see a mention of char*
There could be something that I'm missing, though, which doesn't show up
on those systems. Can you post a complete program that aborts, and tell
us the system and compiler you're using?
Well, the compiler is GCC. Trouble is, I can't replicate it in a
simple example (and a complex example would take me well out of
comp.lang.c territory). I though perhaps that some undefined behaviour
was causing problems in one case but not in another, but
I guess I need to look elsewhere for my problem.
what does your debugger say? Can you add diagnostic printouts?
(This is in addition to what other posters say about cutting
down your program in stages).
<snip>
--
Nick Keighley
A good designer must rely on experience, on precise, logic thinking;
and on pedantic exactness. No magic will do.
(Wirth)
On 21 Oct, 08:45, Nick Keighley <nick_keighley_ nos...@hotmail. com>
wrote:
On 20 Oct, 19:07, Adam <n...@snowstone .org.ukwrote:
>
On 16 Oct, 00:50, Nate Eldredge <n...@vulcan.la nwrote:
Adam <n...@snowstone .org.ukwrites:
I have a simple printf-like function:
int print(const char *format, ...)
{
* char buffer[1024];
* va_list argptr;
* int i;
* va_start(argptr , format);
* i = vsnprintf(buffe r, sizeof(buffer), format, argptr);
* va_end(argptr);
* buffer[sizeof(buffer)-1] = '\0';
* printf("%s\n",b uffer); /* this bit just for the sake of testing*/
* return i;
}
If I call the function using something like:
char message[50];
strcpy(message, "hi there");
print("%s",mess age);
everything works, but if I do:
print(message);
it doesn't (program crashes with an abort).
I don't see a problem myself. *I took your function and added a main
function:
<snip>
and it compiles and runs without problems on three different systems.
Actually, the bit I was wondering about was passing a (char *) to the
print function, rather than a literal string.
should be ok. *If* it contains a valid string. Possible problem
with % characters in the string?
Bingo :) That's exactly what it was. The input to my function was
coming from the GUI element of the app and I hadn't considered
checking for "%" in the string - and that's what was there!
Thanks all,
Adam
Nick Keighley <ni************ ******@hotmail. comwrites:
On 20 Oct, 19:07, Adam <n...@snowstone .org.ukwrote:
[...]
>I did find this page:http://www.eskimo.com/~scs/cclass/int/sx11c.html ...which seems to suggest (in the first paragraph) this might be wrong.
it says:
"When a function with a variable-length argument list is called, the
variable arguments are passed using C's old ``default argument
promotions.'' These say that types char and short int are
automatically promoted to int, and type float is automatically
promoted to double. Therefore, varargs functions will never receive
arguments of type char, short int, or float. Furthermore, it's an
error to ``pass'' the type names char, short int, or float as the
second argument to the va_arg() macro. Finally, for vaguely related
reasons, the last fixed argument (the one whose name is passed as
the second argument to the va_start() macro) should not be of type
char, short int, or float, either."
What's the basis for that last sentence? I'm fairly sure that, as far
as the standard is concerned, using a char, short, or float as the
last fixed argument is perfectly ok. Perhaps some early compilers got
this wrong?
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Keith Thompson wrote:
Nick Keighley <ni************ ******@hotmail. comwrites:
>Finally, for vaguely related reasons, the last fixed argument (the one whose name is passed as the second argument to the va_start() macro) should not be of type char, short int, or float, either."
What's the basis for that last sentence? I'm fairly sure that, as far
as the standard is concerned, using a char, short, or float as the
last fixed argument is perfectly ok.
From the description of va_start:
If the parameter parmN is declared (...) with a type that is not
compatible with the type that results after application of the default
argument promotions, the behavior is undefined.
Perhaps some early compilers got this wrong?
Big-endian machine usually get this wrong: the data is passed in the
"wrong" part a word. To compensate the compiler offsets the value
returned by the & operator.
--
Huibert
"Hey! HEY! Curious cat, here!" -- Krosp I (GG)
Huibert Bol <hu*********@qu icknet.nlwrites :
Keith Thompson wrote:
>Nick Keighley <ni************ ******@hotmail. comwrites:
>>Finally, for vaguely related reasons, the last fixed argument (the one whose name is passed as the second argument to the va_start() macro) should not be of type char, short int, or float, either."
What's the basis for that last sentence? I'm fairly sure that, as far as the standard is concerned, using a char, short, or float as the last fixed argument is perfectly ok.
From the description of va_start:
If the parameter parmN is declared (...) with a type that is not
compatible with the type that results after application of the default
argument promotions, the behavior is undened.
>Perhaps some early compilers got this wrong?
Big-endian machine usually get this wrong: the data is passed in the
"wrong" part a word. To compensate the compiler offsets the value
returned by the & operator.
You know, I was thinking just after I wrote the above, and just before
I sent it, that maybe I should check the standard first. I really
should pay more attention to myself when I think things like that.
It would have been nice if the language had some type-safe built-in
mechanism for dealing with variadic functions. Instead, <stdarg.his
designed to be compatible with each of the various ugly kludges that
are necessary on different platforms. It's not too surprising that it
has little inconsistencies like this. Well, not quite
inconsistencies , but things that would undoubtedly be more consistent
if they were designed from scratch.
We'll know better next time we invent C from scratch. 8-)}
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Harald van =?UTF-8?b?RMSzaw==?= <tr*****@gmail. comwrites:
On Thu, 16 Oct 2008 08:18:38 +0000, Kenny McCormack wrote:
In article
<51************ *************** *******@l33g200 0pri.googlegrou ps.com>,
Peter Nilsson <ai***@acay.com .auwrote:
>There's no upper or lower limit on automatic storage, but I prefer not to allocate more than 256 bytes in a single function.
(CLC pedant mode)
I would imagine that the lower limit is 0 in most cases.
Are you aware of any implementations that require at least one automatic
object?
(Continued pedant mode)
I think whatever location is used to store the return address qualifies as
an object, [...]
A return address is not a C value; hence storage for it is not
an object.
On Sun, 09 Nov 2008 23:21:56 -0800, Tim Rentsch wrote:
Harald van =?UTF-8?b?RMSzaw==?= <tr*****@gmail. comwrites:
>On Thu, 16 Oct 2008 08:18:38 +0000, Kenny McCormack wrote:
Are you aware of any implementations that require at least one
automatic object?
(Continued pedant mode)
I think whatever location is used to store the return address qualifies as an object, [...]
A return address is not a C value; hence storage for it is not an
object.
The storage for the return address is usually an object:
object:
region of data storage in the execution environment, the contents of
which can represent values
Note how it says "can" represent values. If the same register or memory
location is ever used to store a definite C value, then that location is
an object. I'll admit that a dedicated return address register probably
doesn't qualify, though.
It doesn't matter whether the return address itself qualifies as a value,
and I think you may be right that it doesn't.
Harald van =?UTF-8?b?RMSzaw==?= <tr*****@gmail. comwrites:
On Sun, 09 Nov 2008 23:21:56 -0800, Tim Rentsch wrote:
Harald van =?UTF-8?b?RMSzaw==?= <tr*****@gmail. comwrites:
On Thu, 16 Oct 2008 08:18:38 +0000, Kenny McCormack wrote:
Are you aware of any implementations that require at least one
automatic object?
(Continued pedant mode)
I think whatever location is used to store the return address qualifies
as an object, [...]
A return address is not a C value; hence storage for it is not an
object.
The storage for the return address is usually an object:
object:
region of data storage in the execution environment, the contents of
which can represent values
Note how it says "can" represent values. If the same register or memory
location is ever used to store a definite C value, then that location is
an object. I'll admit that a dedicated return address register probably
doesn't qualify, though.
Hmmm. Interesting argument.
Despite 3.14 being worded the way it is, it seems clear that the term
"object" as it is used in the standard should be read as not including
return-address memory. Considering the requirements of 6.2.4, for
example, it needn't have a constant address (or any address at all, in
the C sense), nor does it have a well-defined lifetime. Also, just
because a certain memory location was an object in the past doesn't
mean it's an object now. Machines with segmented addressing, for
example, can have portions of the virtual address space disappear,
which removes any object-ness of that memory; similarly, automatic
variables past the end of their lifetimes may have their storage
locations cease being objects at any time, either because of page
invalidation, address range checking, or being overlaid with
differently sized objects of another function. Considering all these
aspects, the quoted reasoning given above isn't convincing.
So, I still find the proposition that "the term 'object' doesn't
include return-address memory" to be more consistent with all that the
standard says about objects, and therefore more compelling. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
by: Andrew Chalk |
last post by:
I am a raw beginner to Python. I just read in "Learning Python" that
assigning to a string argument inside a function does not change the string
in the caller. I want an assignment in the function to alter the passed
string in the caller. Is there any way to do this?
For example
def SafeAdd(self, Variable, Value):
if self.form.has_key( Value ):
Variable = self.form.value
|
by: Lyn |
last post by:
Hi,
I have a Search input form which collects from the user a person's name. I
am using LIKE with a "%" suffix in the SQL so that the user does not have to
type in the full name. When they hit the Search button, a query is run to
search the Person table for a match. This produces a recordset (I am using
ADO).
If the RecordCount is zero, they get a No Match message.
If the RecordCount is 1, a DoCmd.OpenForm is performed to open the...
|
by: Stuart McGraw |
last post by:
I am looking for a VBA "format" or "template" function, that is,
a function that takes a format string and a varying number of
arguments, and substitutes the argument values into the format
string as specified in the format string. For example
fmt("this is $1 format string. arg2 is $2", "short", 3)
would return the string
"this is short format string. arg2 is 3"
Now, the above is easy to write and I have done so. What I want
is...
|
by: al |
last post by:
char s = "This string literal";
or
char *s= "This string literal";
Both define a string literal. Both suppose to be read-only and not to be
modified according to Standard. And both have type of "const char *".
Right? But why does the compiler I am using allow s to be modified, instead
of generating compile error?
|
by: Nascimento |
last post by:
Hello,
How to I do to return a string as a result of a function.
I wrote the following function:
char prt_tralha(int num)
{
int i;
char tralha;
| |
by: michael.casey |
last post by:
The purpose of this post is to obtain the communities opinion of the
usefulness, efficiency, and most importantly the correctness of this
small piece of code. I thank everyone in advance for your time in
considering this post, and for your comments.
I wrote this function to simplify the task of combining strings using
mixed sources. I often see the use of sprintf() which results in
several lines of code, and more processing than really...
|
by: AT |
last post by:
Can someone please explain me the following:
quote from documentation
"Unlike other intrinsic data types, String is a reference type. When a
variable of reference type is passed as an argument to a function or
subroutine, a reference to the memory address where the data is stored is
passed instead of the actual value of the string. "
Code
|
by: robert maas, see http://tinyurl.com/uh3t |
last post by:
I'm working on examples of programming in several languages, all
(except PHP) running under CGI so that I can show both the source
files and the actually running of the examples online. The first
set of examples, after decoding the HTML FORM contents, merely
verifies the text within a field to make sure it is a valid
representation of an integer, without any junk thrown in, i.e. it
must satisfy the regular expression: ^ *?+ *$
If the...
|
by: phpCodeHead |
last post by:
Code which should allow my constructor to accept arguments:
<?php
class Person {
function __construct($name)
{
$this->name = $name;
}
function getName()
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However, people are often confused as to whether an ONU can Work As a Router. In this blog post, well explore What is ONU, What Is Router, ONU & Routers main usage, and What is the difference between ONU and Router. Lets take a closer look !
Part I. Meaning of...
|
by: Hystou |
last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can effortlessly switch the default language on Windows 10 without reinstalling. I'll walk you through it.
First, let's disable language synchronization. With a Microsoft account, language settings sync across devices. To prevent any complications,...
| |
by: jinu1996 |
last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven tapestry of website design and digital marketing. It's not merely about having a website; it's about crafting an immersive digital experience that captivates audiences and drives business growth.
The Art of Business Website Design
Your website is...
|
by: Hystou |
last post by:
Overview:
Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows Update option using the Control Panel or Settings app; it automatically checks for updates and installs any it finds, whether you like it or not. For most users, this new feature is actually very convenient. If you want to control the update process,...
|
by: isladogs |
last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM).
In this session, we are pleased to welcome a new presenter, Adolph Dupr who will be discussing some powerful techniques for using class modules.
He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms.
Adolph will...
|
by: conductexam |
last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one.
At the time of converting from word file to html my equations which are in the word document file was convert into image.
Globals.ThisAddIn.Application.ActiveDocument.Select();...
|
by: TSSRALBI |
last post by:
Hello
I'm a network technician in training and I need your help.
I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs.
The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols.
I succeeded, with both firewalls in the same network. But I'm wondering if it's possible to do the same thing, with 2 Pfsense firewalls...
|
by: 6302768590 |
last post by:
Hai team
i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
| |
by: bsmnconsultancy |
last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...
| |