Are char* and signed char* compatible (SCM_BYTEVECTOR_CONTENTS)?

Are char* and signed char* compatible (SCM_BYTEVECTOR_CONTENTS)?

[Vivien Kraus via General Guile related discussions]

Hello,

I’m trying to use a bytevector from C.

1. According to an example in the manual, SCM_BYTEVECTOR_CONTENTS can
be assigned to a char*.

2. Also from the manual, it is a signed char*.

3. I've found this question online saying they are not compatible (
https://stackoverflow.com/questions/12769500/why-is-char-not-compatible-with-signed-char-or-unsigned-char
).

As I understand it, at least one of the above 3 affirmations is
incorrect. Could someone clarify this to me?

Re: Are char* and signed char* compatible (SCM_BYTEVECTOR_CONTENTS)?

[Mike Gran]

On Fri, 2021-02-26 at 10:33 +0100, Vivien Kraus via General Guile
related discussions wrote:
> Hello,
> 
> I’m trying to use a bytevector from C.
> 
> 1. According to an example in the manual, SCM_BYTEVECTOR_CONTENTS can
> be assigned to a char*.
> 
> 2. Also from the manual, it is a signed char*.
> 
> 3. I've found this question online saying they are not compatible (
> https://stackoverflow.com/questions/12769500/why-is-char-not-compatible-with-signed-char-or-unsigned-char
> ).
> 
> As I understand it, at least one of the above 3 affirmations is
> incorrect. Could someone clarify this to me?

Depending on your compiler and OS, char is either signed char (-128 to
127) or unsigned char (0 to 255).  It is probably signed char.

You are free to cast the contents of a bytevector as signed char *,
unsigned char *, or char * (which is probably signed char).  It depends
on what you want.  When you use a  bytevector from within Guile, it
will appear as unsigned 8=bit integers. But from within guile you could
use bytevector-s8-ref if you wanted to pull a byte out as a signed 8-
bit integer.

-Mike