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String Literal Question

 
 
polas
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      10-08-2007
Good morning,

I have a quick question to clear up some confusion in my mind. I
understand that using a string literal in a declaration such as char
*p = "string literal" declares a pointer to memory holding the string
and the string might very well be held in read only memory.

However, I am sure that I read somewhere that the declaration char a[]
= "string literal", even though a is an array (and I understand the
differences between the two declarations), defines a such that it
might also be also held in read only memory and thus writing to
indexes of a might not work...

After some doubt about this popped into my mind, I have had a look
through the C faq and it seems to suggest that I am wrong - in the
second case one can use the array declared as above as normal, but I
would like to make sure.

Thanks,
Nick

 
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Richard Heathfield
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      10-08-2007
polas said:

> Good morning,
>
> I have a quick question to clear up some confusion in my mind. I
> understand that using a string literal in a declaration such as char
> *p = "string literal" declares a pointer to memory holding the string
> and the string might very well be held in read only memory.


Right. (Actually, we can say something stronger about p. It isn't merely
declared; it is actually defined, too.)

"string literal" is indeed a string literal, stored in (potentially)
read-only memory, and updating it invokes undefined behaviour (which means
it might work or it might do something obviously horrible, or it might do
something very subtle but gerharsterly). All this, you appear to know
already. So let's move on.

>
> However, I am sure that I read somewhere that the declaration char a[]
> = "string literal", even though a is an array (and I understand the
> differences between the two declarations), defines a such that it
> might also be also held in read only memory and thus writing to
> indexes of a might not work...


No, the string literal is still a string literal, but your definition of
the a array reserves fifteen bytes of storage, and ***copies*** the string
literal (including the null terminator) into that storage, which is
writeable.

> After some doubt about this popped into my mind, I have had a look
> through the C faq and it seems to suggest that I am wrong - in the
> second case one can use the array declared as above as normal, but I
> would like to make sure.


You may rest assured that the array is writeable, yes.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999
 
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polas
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Posts: n/a
 
      10-08-2007
On 8 Oct, 11:33, Richard Heathfield <(E-Mail Removed)> wrote:
> polas said:
>
> > Good morning,

>
> > I have a quick question to clear up some confusion in my mind. I
> > understand that using a string literal in a declaration such as char
> > *p = "string literal" declares a pointer to memory holding the string
> > and the string might very well be held in read only memory.

>
> Right. (Actually, we can say something stronger about p. It isn't merely
> declared; it is actually defined, too.)
>
> "string literal" is indeed a string literal, stored in (potentially)
> read-only memory, and updating it invokes undefined behaviour (which means
> it might work or it might do something obviously horrible, or it might do
> something very subtle but gerharsterly). All this, you appear to know
> already. So let's move on.
>
>
>
> > However, I am sure that I read somewhere that the declaration char a[]
> > = "string literal", even though a is an array (and I understand the
> > differences between the two declarations), defines a such that it
> > might also be also held in read only memory and thus writing to
> > indexes of a might not work...

>
> No, the string literal is still a string literal, but your definition of
> the a array reserves fifteen bytes of storage, and ***copies*** the string
> literal (including the null terminator) into that storage, which is
> writeable.
>
> > After some doubt about this popped into my mind, I have had a look
> > through the C faq and it seems to suggest that I am wrong - in the
> > second case one can use the array declared as above as normal, but I
> > would like to make sure.

>
> You may rest assured that the array is writeable, yes.
>
> --
> Richard Heathfield <http://www.cpax.org.uk>
> Email: -http://www. +rjh@
> Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
> "Usenet is a strange place" - dmr 29 July 1999


Thanks very much for the reply and information - that clears it up for
me. The fact that the array actually copies the string literal makes
sense to me and clears up what I was misunderstanding

Nick

 
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Army1987
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      10-08-2007
On Mon, 08 Oct 2007 03:21:38 -0700, polas wrote:

> However, I am sure that I read somewhere that the declaration char a[]
> = "string literal", even though a is an array (and I understand the
> differences between the two declarations), defines a such that it
> might also be also held in read only memory and thus writing to
> indexes of a might not work...

The literal in char a[] = "foo"; is a string literal just in order
not to clobber the syntax with another exception such as the one
about #include.
But in practice it is just syntactic sugar for
char a[] = {'f', 'o', 'o', 0};
--
Army1987 (Replace "NOSPAM" with "email")
A hamburger is better than nothing.
Nothing is better than eternal happiness.
Therefore, a hamburger is better than eternal happiness.

 
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Barry Schwarz
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      10-10-2007
On Mon, 08 Oct 2007 10:33:31 +0000, Richard Heathfield
<(E-Mail Removed)> wrote:

>polas said:
>
>> Good morning,
>>
>> I have a quick question to clear up some confusion in my mind. I
>> understand that using a string literal in a declaration such as char
>> *p = "string literal" declares a pointer to memory holding the string
>> and the string might very well be held in read only memory.

>
>Right. (Actually, we can say something stronger about p. It isn't merely
>declared; it is actually defined, too.)
>
>"string literal" is indeed a string literal, stored in (potentially)
>read-only memory, and updating it invokes undefined behaviour (which means
>it might work or it might do something obviously horrible, or it might do
>something very subtle but gerharsterly). All this, you appear to know
>already. So let's move on.
>
>>
>> However, I am sure that I read somewhere that the declaration char a[]
>> = "string literal", even though a is an array (and I understand the
>> differences between the two declarations), defines a such that it
>> might also be also held in read only memory and thus writing to
>> indexes of a might not work...

>
>No, the string literal is still a string literal, but your definition of
>the a array reserves fifteen bytes of storage, and ***copies*** the string
>literal (including the null terminator) into that storage, which is
>writeable.


It is probably an implementation detail as to whether the string
literal actually exists to be copied into the array or the compiler
uses another initialization technique to cause the array to contain
the specified value.

>
>> After some doubt about this popped into my mind, I have had a look
>> through the C faq and it seems to suggest that I am wrong - in the
>> second case one can use the array declared as above as normal, but I
>> would like to make sure.

>
>You may rest assured that the array is writeable, yes.



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Richard Heathfield
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      10-10-2007
Barry Schwarz said:

(in reference to: char arr[] = "initialiser string"

> It is probably an implementation detail as to whether the string
> literal actually exists to be copied into the array or the compiler
> uses another initialization technique to cause the array to contain
> the specified value.


Yes. As far as I can see, a strictly conforming program would not be able
to tell the difference anyway.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999
 
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karthikbalaguru
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      10-11-2007
On Oct 8, 3:21 pm, polas <(E-Mail Removed)> wrote:
> Good morning,
>
> I have a quick question to clear up some confusion in my mind. I
> understand that using a string literal in a declaration such as char
> *p = "string literal" declares a pointer to memory holding the string
> and the string might very well be held in read only memory.
>
> However, I am sure that I read somewhere that the declaration char a[]
> = "string literal", even though a is an array (and I understand the
> differences between the two declarations), defines a such that it
> might also be also held in read only memory and thus writing to
> indexes of a might not work...
>
> After some doubt about this popped into my mind, I have had a look
> through the C faq and it seems to suggest that I am wrong - in the
> second case one can use the array declared as above as normal, but I
> would like to make sure.
>


Example -
char *i_reg_fname = "none";

String constants are sequences of characters enclosed in double
quotes.
String literal is the formal term for a double-quoted string in C
source. 'i_reg_fname' is a pointer to characters.

Here,
Note that any attempt to modify the string that 'i_reg_fname' points
to will result in undefined behaviour.
that is, i_reg_fname[0]='q'; // not allowed . will cause undefined
behaviour.

Some compilers have a switch controlling whether string literals are
writable or not (for compiling old code),
and some may have options to cause string literals to be formally
treated as arrays of const char (for better error catching).

But irrespective of theat, if you have declared as below , then it is
possible to change.
char i_reg_fname[] = "none";
that is, i_reg_fname[0]='q'; // allowed .

i_reg_fname is a non-const pointer to char.
It may point to a string literal, but it isn't declared const.

An attempt to modify it will cause undefined behaviour .
Attempting to modify a string literal invokes undefined
behavior, because the C standard defines that attempting to modify a
string literal invokes undefined behavior.
It is because of the C standard and it is not 'const'.

In actual practice, the behaviour depends on where the compiler
decides to put its string constants. Some compilers have a switch
controlling whether
string literals are writable or not (for compiling old code),and some
may have options to
cause string literals to be formally treated as arrays of const char
(for better
error catching).

Earlier C didn not have the 'const' keyword, so if you wanted to
pass
a string literal to a particular function( In sucha a way that the
string will
not be modified inside the
function), then that particular function must take a 'char*'
argument.
Thats all.

Lot of information is available regarding this String Literal in
internet and groups. Also read the C-Faq (question 1.32 will give you
enough info).

Refer :- http://groups.google.co.in/group/com...3a3e04/?hl=en#

Karthik Balaguru

 
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