Reading Memory

[Maxime Savard]

Hi there,

What I'm trying to do is quite simple and in fact I have found a way to
do it but I am not sure if it is OK. What I mean is sometime in c++ stuff
can "work" but not be "correct". Here is my code maybe you'll see something
wrong :

char test[4] = {'1','2','3','4'};
char get = '0';

int add = (int)&test;

add++;

memcpy(&get, (void*)add, sizeof(char));

printf("Second char in array is : %c\n", get);

My objective here was to read the '2' in the test array without using
test[1] as the starting point to read. Then I discovered that when using
&test+1 as the starting address I was in fact increasing it by 4 (maybe it
is basic stuff here but I don't know why, I'm kinda newb). Then I came up
with this technique. The final question is... Is it "correct" ?

Thanks for any info!
Max.

P.s. Sorry for any typo I'm french

 

[Ian Collins]

Hi there,

What I'm trying to do is quite simple and in fact I have found a way to
do it but I am not sure if it is OK. What I mean is sometime in c++ stuff
can "work" but not be "correct". Here is my code maybe you'll see something
wrong :

char test[4] = {'1','2','3','4'};
char get = '0';

int add = (int)&test;

Why are you casting to int? Naked casts are always a smell.

add++;

memcpy(&get, (void*)add, sizeof(char));

Again, why the casts? Also sizeof(char) is by definition 1.

printf("Second char in array is : %c\n", get);

My objective here was to read the '2' in the test array without using
test[1] as the starting point to read. Then I discovered that when using
&test+1 as the starting address I was in fact increasing it by 4 (maybe it
is basic stuff here but I don't know why, I'm kinda newb). Then I came up
with this technique. The final question is... Is it "correct" ?

Why didn't you just write

get = *(test+1);

 

[John Harrison]

Hi there,

What I'm trying to do is quite simple and in fact I have found a way to
do it but I am not sure if it is OK. What I mean is sometime in c++ stuff
can "work" but not be "correct".
Exactly.

Here is my code maybe you'll see something
wrong :

char test[4] = {'1','2','3','4'};
char get = '0';

int add = (int)&test;

add++;

memcpy(&get, (void*)add, sizeof(char));

I think it is undefined behaviour to cast a pointer to an integer then
back to a pointer again.

printf("Second char in array is : %c\n", get);

My objective here was to read the '2' in the test array without using
test[1] as the starting point to read. Then I discovered that when using
&test+1 as the starting address I was in fact increasing it by 4 (maybe it
is basic stuff here but I don't know why, I'm kinda newb).

test + 1 would work.

&test + 1 increments the pointer by the size of the array (which is
four) because &test is a pointer to the array, but test is a pointer to
the first element of the array. It's daft but its the way C and C++ have
always worked.

Then I came up

with this technique. The final question is... Is it "correct" ?

Not really, use 'test + 1' and don't use casts.

Thanks for any info!
Max.

P.s. Sorry for any typo I'm french

john

 

[Alf P. Steinbach]

* John Harrison:

I think it is undefined behaviour to cast a pointer to an integer then
back to a pointer again.

Technically implementation defined.

 

[Maxime Savard]

Neat! Works just fine, and good to know why my pointer increased by 4.

Thanks for the info John,
Max.

Hi there,

What I'm trying to do is quite simple and in fact I have found a way
to do it but I am not sure if it is OK. What I mean is sometime in c++
stuff can "work" but not be "correct".
Exactly.

Here is my code maybe you'll see something wrong :

char test[4] = {'1','2','3','4'};
char get = '0';

int add = (int)&test;

add++;

memcpy(&get, (void*)add, sizeof(char));

I think it is undefined behaviour to cast a pointer to an integer then
back to a pointer again.

printf("Second char in array is : %c\n", get);

My objective here was to read the '2' in the test array without using
test[1] as the starting point to read. Then I discovered that when using
&test+1 as the starting address I was in fact increasing it by 4 (maybe
it is basic stuff here but I don't know why, I'm kinda newb).

test + 1 would work.

&test + 1 increments the pointer by the size of the array (which is four)
because &test is a pointer to the array, but test is a pointer to the
first element of the array. It's daft but its the way C and C++ have
always worked.

Then I came up

with this technique. The final question is... Is it "correct" ?

Not really, use 'test + 1' and don't use casts.

Thanks for any info!
Max.

P.s. Sorry for any typo I'm french

john

 

[Default User]

Neat! Works just fine, and good to know why my pointer increased by 4.

Please don't top-post. Your replies belong following or interspersed
with properly trimmed quotes. See the majority of other posts in the
newsgroup, or the group FAQ list:
<http://www.parashift.com/c++-faq-lite/how-to-post.html>