Re: translate C struct char array into D
On Friday, 30 July 2021 at 21:53:48 UTC, russhy wrote:
On Friday, 30 July 2021 at 14:05:58 UTC, workman wrote:
I get want to define this struct in D:
```c
struct test1 {
struct test1 *prev;
struct test1 *next;
size_t v1;
size_t v2;
size_t v3;
char data[];
};
```
```d
struct test1 {
test1 *prev;
test1 *next;
size_t v1;
size_t v2;
size_t v3;
char* data;
};
```
when I compare the size, test1.sizeof is 48 and sizeof(struct
test1) from C is 40.
Anyone can explain what should I do with this ?
If I use test1 as member of other struct, the total size will
not match.
```d
struct test1 {
test1 *prev;
test1 *next;
size_t v1;
size_t v2;
size_t v3;
char[0] data;
}
```
data.ptr to access its pointer
Thanks for you all for the explain and tips, I will try avoid
embed this struct into parent.
Re: translate C struct char array into D
On Friday, 30 July 2021 at 14:05:58 UTC, workman wrote:
I get want to define this struct in D:
```c
struct test1 {
struct test1 *prev;
struct test1 *next;
size_t v1;
size_t v2;
size_t v3;
char data[];
};
```
```d
struct test1 {
test1 *prev;
test1 *next;
size_t v1;
size_t v2;
size_t v3;
char* data;
};
```
when I compare the size, test1.sizeof is 48 and sizeof(struct
test1) from C is 40.
Anyone can explain what should I do with this ?
If I use test1 as member of other struct, the total size will
not match.
```d
struct test1 {
test1 *prev;
test1 *next;
size_t v1;
size_t v2;
size_t v3;
char[0] data;
}
```
data.ptr to access its pointer
Re: translate C struct char array into D
On Friday, 30 July 2021 at 14:05:58 UTC, workman wrote:
I get want to define this struct in D:
```c
struct test1 {
struct test1 *prev;
struct test1 *next;
size_t v1;
size_t v2;
size_t v3;
char data[];
};
```
The easy way: put a slice there instead of a fake array and
accept that data's contents will be allocated separately. Slices
will be more convenient to deal with in the language anyway.
The C89 way: add a zero-length array and deal with its .ptr in
appropriately sized allocations that you manage yourself (or get
from a C API).
A third way: something with
https://dlang.org/phobos/core_lifetime.html#.emplace
```d
import core.memory : pureMalloc, pureFree;
import std.conv : to;
struct Pascal {
ubyte len;
char[0] data;
static Pascal* alloc(string s) {
const len = s.length.to!ubyte;
auto res = cast(Pascal*) pureMalloc(Pascal.sizeof + len);
res.len = len;
res.data.ptr[0 .. len] = s[0 .. len];
return res;
}
char[] toString()() {
return data.ptr[0 .. len];
}
}
unittest {
auto s = Pascal.alloc("hello");
scope (exit) pureFree(s);
assert(s.toString == "hello");
assert((*s).sizeof == ubyte.sizeof);
}
```
Re: translate C struct char array into D
On Friday, 30 July 2021 at 15:51:12 UTC, Tejas wrote:
On Friday, 30 July 2021 at 14:40:17 UTC, Paul Backus wrote:
On Friday, 30 July 2021 at 14:05:58 UTC, workman wrote:
[...]
`char data[]` in the C struct is not a pointer, but actually a
[C99 flexible array member][1], and does not count towards the
struct's `sizeof`.
D does not have flexible array members, but you can simulate
one using a struct method:
```d
struct test1 {
// member variables...
char* data() {
return cast(char*) ( + 1);
}
}
```
[1]: http://port70.net/~nsz/c/c99/n1256.html#6.7.2.1p16
By the way how is this safe? And how exactly are we even
assigning the data to a variable/reference in this case? Can
you please show an example demonstrating this?
In order to use a flexible array member, you have to allocate
extra memory beyond the end of the struct, and it's entirely your
responsibility to ensure that you do not go out-of-bounds when
accessing that memory.
Re: translate C struct char array into D
On Fri, Jul 30, 2021 at 03:41:32PM +, Tejas via Digitalmars-d-learn wrote:
> On Friday, 30 July 2021 at 14:40:17 UTC, Paul Backus wrote:
[...]
> > ```d
> > struct test1 {
> > // member variables...
> >
> > char* data() {
> > return cast(char*) ( + 1);
> > }
> > }
> > ```
> >
> > [1]: http://port70.net/~nsz/c/c99/n1256.html#6.7.2.1p16
>
> B-but the performance penalty of having an extra level of indirection?
> Absolutely unacceptable!!
Premature optimization.
Any optimizer worth its salt would be able to inline such a trivial
function as .data above. Check the output of LDC -O, for example.
(Don't talk to me about dmd, if you care about performance you'd use LDC
instead.)
T
--
This is a tpyo.
Re: translate C struct char array into D
On Friday, 30 July 2021 at 14:40:17 UTC, Paul Backus wrote:
On Friday, 30 July 2021 at 14:05:58 UTC, workman wrote:
[...]
`char data[]` in the C struct is not a pointer, but actually a
[C99 flexible array member][1], and does not count towards the
struct's `sizeof`.
D does not have flexible array members, but you can simulate
one using a struct method:
```d
struct test1 {
// member variables...
char* data() {
return cast(char*) ( + 1);
}
}
```
[1]: http://port70.net/~nsz/c/c99/n1256.html#6.7.2.1p16
By the way how is this safe? And how exactly are we even
assigning the data to a variable/reference in this case? Can you
please show an example demonstrating this?
Re: translate C struct char array into D
On Friday, 30 July 2021 at 14:40:17 UTC, Paul Backus wrote:
On Friday, 30 July 2021 at 14:05:58 UTC, workman wrote:
[...]
`char data[]` in the C struct is not a pointer, but actually a
[C99 flexible array member][1], and does not count towards the
struct's `sizeof`.
D does not have flexible array members, but you can simulate
one using a struct method:
```d
struct test1 {
// member variables...
char* data() {
return cast(char*) ( + 1);
}
}
```
[1]: http://port70.net/~nsz/c/c99/n1256.html#6.7.2.1p16
B-but the performance penalty of having an extra level of
indirection? Absolutely unacceptable!!
Re: translate C struct char array into D
On Friday, 30 July 2021 at 14:05:58 UTC, workman wrote:
I get want to define this struct in D:
```c
struct test1 {
struct test1 *prev;
struct test1 *next;
size_t v1;
size_t v2;
size_t v3;
char data[];
};
```
```d
struct test1 {
test1 *prev;
test1 *next;
size_t v1;
size_t v2;
size_t v3;
char* data;
};
```
when I compare the size, test1.sizeof is 48 and sizeof(struct
test1) from C is 40.
Anyone can explain what should I do with this ?
If I use test1 as member of other struct, the total size will
not match.
`char data[]` in the C struct is not a pointer, but actually a
[C99 flexible array member][1], and does not count towards the
struct's `sizeof`.
D does not have flexible array members, but you can simulate one
using a struct method:
```d
struct test1 {
// member variables...
char* data() {
return cast(char*) ( + 1);
}
}
```
[1]: http://port70.net/~nsz/c/c99/n1256.html#6.7.2.1p16
