On 6/14/23 11:29 PM, Pat Maddox wrote:
Hi there, I want to call a C function that upcases a string. I have
something working, I just want to check in here to see if there's a
better approach that I'm missing. I ask because `std.string.toStringZ()`
returns an `immutable char *`.
As far as I can tell, I have two options:
1. Make the extern definition accept immutable.
2. Cast to `char *`.
I opted for 2 because it seems that 1 would be confusing - the
definition says immutable, but it mutates the string.
Anyway, is this the D way to mutate a string from C, or is there another
approach I'm unaware of?
```
extern (C) void upcase(char *);
import std.stdio;
import std.string;
void main() {
auto s = "hello d";
auto cs = cast (char *) std.string.toStringz(s);
upcase(cs);
writeln(std.string.fromStringz(cs));
}
```
So interestingly enough, toStringz is pure, and returns an unrelated
type, so you shouldn't need to cast. However, for some reason, it does
require a cast. That seems like a bug to me.
It also works with:
```
extern (C) void upcase(immutable char *);
import std.stdio;
import std.string;
void main() {
auto s = "hello d";
auto cs = std.string.toStringz(s);
upcase(cs);
writeln(std.string.fromStringz(cs));
}
```
but it seems that "immutable" is a lie in that case
Yeah, don't do it this way. just matching the type for a C prototype is
very dangerous, especially if the C function doesn't obey the type
constraints.
Can I ask if this is a specific case of trying to use a C function to do
"upper casing", or if this is a general question about C functions and
modifying strings?
Because we do have upper-casing facilities in both std.ascii and std.uni.
But in general, if you want a mutable character array that's zero
terminated, you need to make a copy with a zero terminator, but type it
as mutable. I'm surprised there isn't a way to do this easily in the
library.
But if you need something that isn't wasteful, I would do something like
this:
```d
pure char *toMutStringz(const(char)[] str)
{
char[] result = str ~ "\0";
return result.ptr;
}
```
-Steve
