An Array and memory problem

[Daniel Smith]

Hi,

Two things confused me and I hope somebody can point out the wrong
part, thanks in advance

char * mytest ( const char src[])
{
char *ret;
ret = new char;
strncpy(ret, src, 5);
return ret;
}
in main( ), I use:
char *y = mytest("abcdef" );

The code works but I think there are problems. I used 'new' to allocate
some memory, how to release them?
Another one:

char * mytest ( const char src[])
{
char ret[10];
strncpy(ret, src, 5);
return ret;
}
in main ( ) : char *y = mytest("abcdef" );

After we use 'char rec[10]', I think the it is in stack and 'rec' will
be released after the function 'mytest' finishes. So I suppose I cannot
get anything in 'y', but why did I get 'abcde' in y?

Dan

[Victor Bazarov]

Re: An Array and memory problem

Daniel Smith wrote:

Hi,
>
Two things confused me and I hope somebody can point out the wrong
part, thanks in advance
>
char * mytest ( const char src[])
{
char *ret;
ret = new char;

You only allocate a _single_ char here. You need to check the length
of the "string" passed to this function and allocate _one_more_ than
the length:

char *ret = new char[strlen(src) + 1]();

strncpy(ret, src, 5);

And here you copy _five_ of them, stomping all over the memory you
don't even own. After that - undefined behaviour. You shouldn't be
using 'strncpy' either. Just copy the whole thing using 'strcpy':

strcpy(ret, src);

return ret;
}
in main( ), I use:
char *y = mytest("abcdef" );
>
The code works but I think there are problems. I used 'new' to
allocate some memory, how to release them?

Since the proper allocation is done using 'new[]', you need to do

delete[] y;

here.

Another one:
>
char * mytest ( const char src[])
{
char ret[10];
strncpy(ret, src, 5);
return ret;

Here you're returning a pointer to the local array. As soon as the
function returns, the pointer is _invalid_.

}
in main ( ) : char *y = mytest("abcdef" );
>
After we use 'char rec[10]', I think the it is in stack and 'rec' will
be released after the function 'mytest' finishes. So I suppose I
cannot get anything in 'y', but why did I get 'abcde' in y?

The behaviour is undefined. It could actually be (or pretend to be) what
you had expected.

V
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