Access violation error for a simple code

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Hi,

I came across the following condition . It may be a simplest solution for you but i will appreciate if you help me in this regard.

char *p="hello";
*(p+1) = 'A';

This program gives Access violation error for second line.

It gives the same error even if i use

p[1]='A';

Posted 4-Jul-11 20:10pm

PrafullaVedante

Updated 4-Jul-11 20:23pm

v2

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5 solutions

Solution 2

Change string declaration to char p[]="hello";. This should allow you to access p elements by index; and you can always turn it into char* by taking first character address

Posted 4-Jul-11 20:35pm

Timberbird

Updated 4-Jul-11 20:38pm

v2

Comments

Christian Graus 5-Jul-11 2:42am

Interesting, but, is it correct behaviour ? The VS debugger can find and set the values just fine. Would it blow up in a release build, or just in debug ?

Timberbird 5-Jul-11 2:58am

Yes, in release version it can - and I think it will - cause an error. See my comment below

PrafullaVedante 5-Jul-11 2:43am

It works. :)

But why the earlier case fails ? :(

Timberbird 5-Jul-11 2:57am

For example, see this. Strings declared like char* p="Smth"; should not be modified - consider them constants. The problem is that the compiler can place your string in read-only memory, and I guess that's what happens in release version :)

Stefan_Lang 5-Jul-11 6:08am

The problem is that string literals are technically constants, and the compiler is allowed to place them in read-only memory. Therefore, at runtime, trying to modify that memory will lead to undefined behaviour!

The suggestion works because it creates an array on the stack, rather than just a pointer into read-only memory.

Rajesh R Subramanian 5-Jul-11 6:48am

Wow, that is the answer. I should have read your comment before adding my answer. :)

Solution 1

Why would you do this ? It looks like a circus trick to me, not a programming problem. You should access the variables inside p by index, not like this.

Posted 4-Jul-11 20:12pm

Christian Graus

Comments

PrafullaVedante 5-Jul-11 2:22am

It gives the same error for p[1]='A' also.

Christian Graus 5-Jul-11 2:26am

Interesting, it works in the debug window. I don't know why it does that, I would not have expected it to.

Sergey Alexandrovich Kryukov 5-Jul-11 3:31am

This is a known sloppiness of Microsoft -- there was the discussion. It should throw exception in all cases.
--SA

PrafullaVedante 5-Jul-11 2:32am

:(

Mohibur Rashid 10-Jul-11 1:45am

I do not agree with Christian Graus at all

here is an example of using Rajesh R Subramanian way

a simple strcpy function

void strcpy(char *dest, char *src)
{
while((*src)!=0){(*dest)=(*src);src++;}
}

and another solution

void strcpy(char *dest, char *src)
{
for(int i;dest[i]!=0;i++)
{
src[i]=dest[i];
}
}

which one is better?

if you know how to handle pointer you dont need to define another pointless variable to track down your index

John R. Shaw 13-Jul-11 18:06pm

Strange I do not see what strcpy has to do with the question. But the while loop would be more efficient.

Solution 4

The string "hello" is stored in a read-only area of the memory. It is a constant string value.

Posted 9-Jul-11 13:58pm

hbprotoss

Updated 9-Jul-11 13:59pm

v2

Solution 5

As a sidenote to the answer by Rajesh:
Put something like this in a dll and you're able to share data between different instance of same dll.

#pragma data_seg (".MYSEG")
    int g_nInteger = 0;
    char g_szString[] = "hello world";
#pragma data_seg()
// Tell the linker that the section
// .MYSEG is readable, writable and is shared.
#pragma comment(linker, "/SECTION:.MYSEG,RWS")

Sometimes it's quite useful :)

Best regards
Espen Harlinn

Posted 9-Jul-11 14:26pm

Espen Harlinn

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Rajesh R Subramanian · Accepted Answer · 2011-07-05T00:46:00

char *p = "hello";
char pArr[] = "Hello";

The above two are not the same. When you declare it as a pointer and initialize it with a value, the compiler puts it in the Data Segment[^], AND in the "Initialised and read only" area. When you try to change contents stored in this section, it should blow up (that's what you're experiencing).

However, the latter one is just an array (the contents are stored in the code executing thread's stack) and therefore the contents can be changed as you wish.