Arrays in C bindings

[BobTheJanitor]

I have a c function with the parameters

> int argc
> const char *const *argv

I'm not too fluent in C, but I believe this represents a pointer to an array of strings. But the Crystal Compiler won't let me pass an array, or a pointer of an array

> only primitive types, pointers, structs, unions, enums and tuples are allowed in lib declarations, not Pointer(Array(String)

Please explain what I'm doing wrong.

[Tim Uckun]

No it doesn't represent an array of strings. 

Do this.

alias Char = LibC::Char

That's just to save some typing.

When you declare your function do this

fun blah (x : Char*)

Then just pass a string as normal

s="some string"

LibBlah.blah(s)

Crystal will magically call to_unsafe to turn the string into a Char*

If you have an out parameter you will need to cast it to string though so for example

LibBlah.blah(out out_string)

here the c lib created a const char* and you need to make it a string.

s2=String.new(out_string)

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[Ary Borenszweig]

Indeed, `const char *const *argv` is an array of C strings.

Tim Uckun reply would have been correct if just a single `*` appeared on the code, but there are two (so it's a pointer to a pointer).

A C char in Crystal lib is LibC::Char, or just UInt8. Adding two pointers you get LibC::Char**, or UInt8**.

If you have an `Array(String)`, then invoking `to_unsafe` on it (or just passing it to C, which will try to invoke `to_unsafe` for you) will give you something of type `Pointer(String)`, which is not `Pointer(UInt8*)` or `Pointer(Pointer(UInt8))`. This can be done with `map`:

strings.map(&.to_unsafe)

This will make it be `Array(Pointer(UInt8))`, and when invoking `to_unsafe` on that you'll get `Pointer(Pointer(UInt8))`. I don't know a better way of doing that.

Here's some code that compiles:

~~~

lib LibFoo

  fun foo(x : UInt8**)

end

strings = ["foo", "bar", "baz"]

LibFoo.foo(strings.map(&.to_unsafe))

~~~

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[BobTheJanitor]

Thank you

On Wednesday, January 4, 2017 at 4:59:54 AM UTC-7, aborenszweig wrote:

Indeed, `const char *const *argv` is an array of C strings.

Tim Uckun reply would have been correct if just a single `*` appeared on the code, but there are two (so it's a pointer to a pointer).

A C char in Crystal lib is LibC::Char, or just UInt8. Adding two pointers you get LibC::Char**, or UInt8**.

If you have an `Array(String)`, then invoking `to_unsafe` on it (or just passing it to C, which will try to invoke `to_unsafe` for you) will give you something of type `Pointer(String)`, which is not `Pointer(UInt8*)` or `Pointer(Pointer(UInt8))`. This can be done with `map`:

strings.map(&.to_unsafe)

This will make it be `Array(Pointer(UInt8))`, and when invoking `to_unsafe` on that you'll get `Pointer(Pointer(UInt8))`. I don't know a better way of doing that.

Here's some code that compiles:

~~~

lib LibFoo

  fun foo(x : UInt8**)

end

strings = ["foo", "bar", "baz"]

LibFoo.foo(strings.map(&.to_unsafe))

~~~

On Wed, Jan 4, 2017 at 2:21 AM, BobTheJanitor <0bobthe...@gmail.com> wrote:

I have a c function with the parameters

> int argc
> const char *const *argv

I'm not too fluent in C, but I believe this represents a pointer to an array of strings. But the Crystal Compiler won't let me pass an array, or a pointer of an array

> only primitive types, pointers, structs, unions, enums and tuples are allowed in lib declarations, not Pointer(Array(String)

Please explain what I'm doing wrong.

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