Hello. I have something that looks like this:

#include <stdio.h>
void foo(char **s) {
s=new char*[4];
for (int i=0;i<4;i++)
s[i]=new char[20];
}
void main() {
char **bar;
foo(bar);
for (int i=0;i<4;i++)
printf("%s\n",bar[i]);
}

Why do I get an error about memory not being able to be read? (Sorry if that's
not the exact message, it's a translation)
I also tried with bcc5.5, msvc6 and djgpp2.95, and djgpp was the only one not to
cause that error. What's wrong with the code?

You should write ***:

void foo(char ***s) {
*s=new char*[4];
for (int i=0;i<4;i++)
(*s)[i]=new char[20];
}
void main() {
char **bar;
foo(&bar);
}

What you did is initializing stack variable, which is not returned from
function. So, after function foo(...) executed, the parameter bar remained
unchanged.
DJPP should not differ from other compilers, because your code certainly
contains bug.

Nic Tiger.

"Carlos Santander B." <Carlos_member@pathlink.com> wrote in message news:bfk11b$29dm$1@digitaldaemon.com...
> Hello. I have something that looks like this:
>
> #include <stdio.h>
> void foo(char **s) {
> s=new char*[4];
> for (int i=0;i<4;i++)
> s[i]=new char[20];
> }
> void main() {
> char **bar;
> foo(bar);
> for (int i=0;i<4;i++)
> printf("%s\n",bar[i]);
> }
>
> Why do I get an error about memory not being able to be read? (Sorry if
that's
> not the exact message, it's a translation)
> I also tried with bcc5.5, msvc6 and djgpp2.95, and djgpp was the only one
not to
> cause that error. What's wrong with the code?
>
>

Thank you, very much.

In article <bfk1uq$2abo$1@digitaldaemon.com>, Nic Tiger says...
>
>You should write ***:
>
>void foo(char ***s) {
>*s=new char*[4];
>for (int i=0;i<4;i++)
>(*s)[i]=new char[20];
>}
>void main() {
>char **bar;
>foo(&bar);
>}
>
>What you did is initializing stack variable, which is not returned from
>function. So, after function foo(...) executed, the parameter bar remained
>unchanged.
>DJPP should not differ from other compilers, because your code certainly
>contains bug.
>
>Nic Tiger.
>

Carlos Santander B. schrieb...
> I have something that looks like this:
> 
> #include <stdio.h>
> void foo(char **s) {
  void foo(char **&s) {
                  ^
When compiled as C++ module you can declare s as a reference

> s=new char*[4];
> for (int i=0;i<4;i++)
> s[i]=new char[20];
you have allocated storage but not initialized it. Change the for-loop
to something like this:
    for(int i=0; i<4; ++i) {
      s[i] = new char[20];
      s[i][0] = '0';    // empty string
      // or
      sprintf(s[i], "Line %d", i); // Line 0 to Line 3
    }//for
> }
> void main() {
> char **bar;
> foo(bar);
> for (int i=0;i<4;i++)
> printf("%s\n",bar[i]);
> }
> 
> Why do I get an error about memory not being able to be read? (Sorry if that's not the exact message, it's a translation)
The storage allocated in your original program was lost because bar is passed by value. bar as a auto-variable is not initialized by default and can contain a garbage value. Accessing this pointer (printf does the access) in protected mode usually crashes the program.

> I also tried with bcc5.5, msvc6 and djgpp2.95, and djgpp was the only one not to cause that error.
The not initialized pointer might contain a valid address by chance. But still the printf can print garbage.


- Heinz