How to create separate character pointers?

[Lambda]

I'd like to create separate character pointers,
pass them to a function to assign each one different value,
and assign the pointers to an array.

But when I try:

#include <stdio.h>
#include <string.h>

int main(void)
{
int i;

for (i = 0; i < 5; ++i)
{
char *str = "test";
printf("%p:%s\n ", str, str);
}

return 0;
}

All the pointers refer to the same address.
When the 'char *str = "test"' is run, no new pointer is created?

[Eric Sosman]

Lambda wrote:

I'd like to create separate character pointers,
pass them to a function to assign each one different value,
and assign the pointers to an array.

But when I try:

#include <stdio.h>
#include <string.h>

int main(void)
{
int i;

for (i = 0; i < 5; ++i)
{
char *str = "test";
printf("%p:%s\n ", str, str);
}

return 0;
}

All the pointers refer to the same address.
When the 'char *str = "test"' is run, no new pointer is created?

Each execution of the loop creates a new `str' variable.
Each time, that variable is initialized to point to the same
"test" string. If your telephone number is written on five
pieces of paper, it doesn't mean you have five different
telephones.

--
Eric Sosman
es*****@ieee-dot-org.invalid

[James Kuyper]

Lambda wrote:

I'd like to create separate character pointers,
pass them to a function to assign each one different value,
and assign the pointers to an array.

But when I try:

#include <stdio.h>
#include <string.h>

int main(void)
{
int i;

for (i = 0; i < 5; ++i)
{
char *str = "test";
printf("%p:%s\n ", str, str);
}

return 0;
}

All the pointers refer to the same address.
When the 'char *str = "test"' is run, no new pointer is created?

A new pointer object is created each time, though on many
implementations that new pointer object is likely to be created in
exactly the same location. The key problem isn't the pointer object,
it's the pointer value that it's initialized with. When you write a
string literal such as "test" in contexts like this one, what happens is
that an unnamed array of 5 characters is created and filled in with 't',
'e', 's', 't', and '\0'. The string literal is treated as a pointer to
that array. Therefore, every time you go through the loop, you
initialize your pointer object with a pointer value that points at the
same unnamed array.

The right way to fix this depends very much on what it is that you're
trying to do. The simplest approach that matches your description would
be as follows:

#include <stdio.h>
int main(void)
{
char str1[] = "test";
char str2[] = "test";
char str3[] = "test";
char str4[] = "test";
char str5[] = "test";
char *array[5] = {str1, str2, str3, str4, str5};
int i;

for(i=0; i<5; ++i)
printf("%p:%s\n ", array[i], array[i]);
// Use array.

return 0;
}

Note that in this case the string literals are used to initialize an
array, not a pointer in an array. In this case, no unnamed array is
created; instead, each definition creates a separate array and
initializes that array

While the above code matches your description, I doubt that it's what
you really want. The following more complicated case is probably closer
to what you're looking for:

#include <stdio.h>
#include <stdlib.h>
#define STRINGS 5

int main(void)
{
char *array[STRINGS];
int i;
for(i=0; i<STRINGS; i++)
{
array[i] = malloc(5);
if(array[i])
{
strcpy(array[i], "test");
printf("%p:%s\n ", array[i], array[i]);
}
else
{
printf("%p: failed allocation\n", array[i]);
}
}

for(i=0; i<STRINGS; i++)
free(array[i]);
return 0;
}

[CBFalconer]

James Kuyper wrote:

>

.... snip ...

>
for(i=0; i<5; ++i)
printf("%p:%s\n ", array[i], array[i]);
// Use array.

To avoid awkward line wraps, avoid using the // comments in posted
material. They are also illegal under C90.

--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.

--
Posted via a free Usenet account from http://www.teranews.com

[James Kuyper]

CBFalconer wrote:

James Kuyper wrote:
... snip ...

> for(i=0; i<5; ++i)
printf("%p:%s\n ", array[i], array[i]);
// Use array.

To avoid awkward line wraps, avoid using the // comments in posted
material.

They are also illegal under C90.

I write for the current standard, not the old one. It's been many years
since I last worked on a system that didn't have a compiler which would
support at least that much of C99.

[Mark Bluemel]

James Kuyper wrote:

CBFalconer wrote:

>James Kuyper wrote:
... snip ...

>> for(i=0; i<5; ++i)
printf("%p:%s\n ", array[i], array[i]);
// Use array.

To avoid awkward line wraps, avoid using the // comments in posted
material.

>They are also illegal under C90.

I write for the current standard, not the old one. It's been many years
since I last worked on a system that didn't have a compiler which would
support at least that much of C99.

Fair enough, but Chuck's first point still applies. "/* ... */" works
fine when wrapped by a newsreader, while "//" doesn't

It's not a problem in the specific code you posted, but in general I'd
agree with Chuck that "/* ... */" is a better choice for newsgroup postings.

[Willem]

Eric wrote:
) <snip>
) for (i = 0; i < 5; ++i)
) {
) char *str = "test";
) printf("%p:%s\n ", str, str);
) }
)
) Each execution of the loop creates a new `str' variable.
) Each time, that variable is initialized to point to the same
) "test" string. If your telephone number is written on five
) pieces of paper, it doesn't mean you have five different
) telephones.

I bet that even if you wrote printf("%p->%p:%s\n", &str, str, str);
that you can't find a compiler that wouldn't output the same each
iteration, although I'm sure the Standard would allow it.
SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

[ymuntyan]

On Nov 8, 6:43 am, CBFalconer <cbfalco...@yah oo.comwrote:

James Kuyper wrote:

... snip ...

for(i=0; i<5; ++i)
printf("%p:%s\n ", array[i], array[i]);
// Use array.

To avoid awkward line wraps, avoid using the // comments in posted
material. They are also illegal under C90.

Nice example of why you got to have very thick skin to read this
group. You don't even need to cast return value of malloc!

>
--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.

--
Posted via a free Usenet account fromhttp://www.teranews.co m

Just wonderful. Pure wisdom from Chuck.

--
Posted via groups.google.c om. It's so much worse than teranews. Yeah!

[Coos Haak]

Op Thu, 08 Nov 2007 09:50:54 -0800 schreef ym******@gmail. com:

On Nov 8, 6:43 am, CBFalconer <cbfalco...@yah oo.comwrote:

>James Kuyper wrote:

... snip ...

>> for(i=0; i<5; ++i)
printf("%p:%s\n ", array[i], array[i]);
// Use array.

To avoid awkward line wraps, avoid using the // comments in posted
material. They are also illegal under C90.

Nice example of why you got to have very thick skin to read this
group. You don't even need to cast return value of malloc!

Of course not, C isn't C++
--
Coos

[CBFalconer]

James Kuyper wrote:

CBFalconer wrote:

>James Kuyper wrote:
... snip ...

>> for(i=0; i<5; ++i)
printf("%p:%s\n ", array[i], array[i]);
// Use array.

To avoid awkward line wraps, avoid using the // comments in posted
material.

>They are also illegal under C90.

I write for the current standard, not the old one. It's been many
years since I last worked on a system that didn't have a compiler
which would support at least that much of C99.

/* .. */ is valid under all systems, and is immune to usenet line
wrap.

--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.

--
Posted via a free Usenet account from http://www.teranews.com