reassigning value of a pointer

[bhalicki]

Hi all,

In the following code I am trying to change the contents of a string:

int main()
{
char *string="testin g";
rename(string);

return 0;
}

void rename(char *s)
{
printf("Charact er 1: %c\n",*s);
*s='a';
printf("Charact er 1 now: %c\n",*c);
}

Anyway, I am sure this is possible but for some reason I'm getting
segmentation faults at line *s='a'; when I attempt to change the value
in which *s is currently pointing to, being the first character in the
string.

Any ideas?

Thanks in advance,

Ben.

[Walter Roberson]

In article <11************ **********@g14g 2000cwa.googleg roups.com>,
<bh******@dodo. com.au> wrote:

In the following code I am trying to change the contents of a string: int main()
{
char *string="testin g";

The standard allows string literals to be stored in read-only memory.
An assignment of a string literal to a pointer sets the pointer value
to the address of that {possibly read-only} memory.

In particular, char *string="testin g"; usually does not allocate some
memory somewhere and copy the string into it at runtime. That's
one of the possible behaviours, but it isn't the only possible
behaviour.

Another thing to note is that it is allowed for the compiler
to merge all string literals -- so for example if you also had

char *anotherstring= "testing";

then anotherstring could end up as the same pointer value as
your string variable. Furthermore, if you had

char *thirdstring="j ust testing";

then string and anotherstring could end up pointing at the terminal
"testing" substring of the "just testing".

If you need a modifiable string, allocate the memory (somehow) and
copy the appropriate contents into it.
--
"It is important to remember that when it comes to law, computers
never make copies, only human beings make copies. Computers are given
commands, not permission. Only people can be given permission."
-- Brad Templeton

[bhalicki]

Thanks for that information Walter. I managed to get it working, by
allocating the memory then using strcpy to load the string.

Regards,

Ben.

[Martin Ambuhl]

bh******@dodo.c om.au wrote:

Hi all,

In the following code I am trying to change the contents of a string:
Then don't use string literals.

int main()
{
char *string="testin g";

If you want to change the string, don't make 'string' point to a literal
constant; use an array:
char string[] = "testing";

Always check the FAQ before posting. In this case, you might have found
<http://www.eskimo.com/~scs/C-faq/q1.32.html> useful.

[hemalatha.gopalakrishnan]

in the above program what is "c", not declared in the program, first u
declare that and assign a value and then print it ok. i think that
there is no problem in the line *s='a';
so try it and send me the result.

[pete]

Martin Ambuhl wrote:

Then don't use string literals. If you want to change the string,
don't make 'string' point to a literal constant;
There is no such thing as a "literal constant" in C.
char string[] = "testing";

You just used a string literal to initialize an array.

--
pete

[Richard Bos]

bh******@dodo.c om.au wrote:

int main()
{
char *string="testin g";
rename(string);

Your immediate problem is as others have explained it. However, you have
another bug: rename() is already a Standard function, and you cannot
redefine it, let alone redefine it with an incompatible declaration.

Richard

[David G. Hong]

It becomes a standard function iff (if and only if) you include
<stdio.h>. Although dubbed as *Standard* Input Ouput - not really a
standard. Any programmer could redefine functions already defined in
*Standard* headers. Other than that, what the original author intended
to do clearly shouldn't be named as rename(..); rather,
change_sptr(... ); Or something simliar.

[pete]

David G. Hong wrote:

It becomes a standard function iff (if and only if) you include
<stdio.h>.

How do you figure that?

N869
7.1.3 Reserved identifiers
[#1]
-- All identifiers with external linkage in any of the
following subclauses (including the future library
directions) are always reserved for use as identifiers
with external linkage.

--
pete

[Lawrence Kirby]

On Wed, 28 Sep 2005 06:34:39 +0000, pete wrote:

Martin Ambuhl wrote:

Then don't use string literals.

If you want to change the string,
don't make 'string' point to a literal constant;

There is no such thing as a "literal constant" in C.

It is a resonable description of a string literal object.

char string[] = "testing";

You just used a string literal to initialize an array.

which solves the primary problem in the original code. I'm not clear what
point you are trying to make here.

Lawrence