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   Nicolas Martyanoff (galdor@emacs.ch)'s status on Tuesday, 08-Aug-2023 20:16:36 JST Nicolas Martyanoff

   If you're writing C code and you are hiding pointer types with typedef, there is a special hell waiting for you.

   I'm looking at you libssh with "typedef struct ssh_key_struct* ssh_key". It's already bad enough to hide "struct".

   • Embed this notice
     Elias Mårtenson (loke@functional.cafe)'s status on Tuesday, 08-Aug-2023 20:16:35 JST Elias Mårtenson

     in reply to

     @galdor I never do that myself, but if the type is meant to be an opaque thing, then I don't really see the problem with it?

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     Elias Mårtenson (loke@functional.cafe)'s status on Tuesday, 08-Aug-2023 20:16:37 JST Elias Mårtenson

     in reply to

     @galdor A const ssh_key * is equivalent to struct ssh_key_struct **. In both cases it's a pointer to a pointer to a struct.

     The way to think about it is that the "pointer to ssh struct" is the object itself. The fact that it's a pointer is irrelevant. When you declare a ssh_key * all you do is create a pointer to this object type (which happens to be a pointer).

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     Nicolas Martyanoff (galdor@emacs.ch)'s status on Tuesday, 08-Aug-2023 20:16:39 JST Nicolas Martyanoff

     in reply to

     • Elias Mårtenson

     @loke It makes it hard to reason on what's happening.

     For example in libssh, some functions take a "const ssh_key *". So now you retain the explicit pointer type in some cases but not others.

     Especially annoying for people binding the library.

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     Nicolas Martyanoff (galdor@emacs.ch)'s status on Tuesday, 08-Aug-2023 20:17:16 JST Nicolas Martyanoff

     in reply to

     • Elias Mårtenson

     @loke I'm well aware of what a pointer is.

     By masking the pointer type in some cases but not others, you are making your code inconsistent.

     You also hide useful information: if a function accepts a parameter of type ssh_key, it is reasonable to wonder if it is a structure passed by copy, a pointer to something you must allocate yourself, or something more complicated altogether.

     One good thing with C is that what's happening is usually obvious. Hiding types because you cannot be bothered to write "struct ssh_key *" is a terrible idea.

     You may think it's ok because you know that "ssh_key" is a pointer type. But when you have worked with very large C codebases, you learn than you cannot expect to remember what is behind each typedef and must keep things explicit.

   • Embed this notice
     Elias Mårtenson (loke@functional.cafe)'s status on Tuesday, 08-Aug-2023 20:18:46 JST Elias Mårtenson

     in reply to

     @galdor

     By masking the pointer type in some cases but not others, you are making your code inconsistent.

     This one I disagree with. Unless they also expose ssh_key_struct of course. Could it be that you made a typo in your previous message and when you wrote: const ssh_key * you means to say const ssh_key_struct *?

     You also hide useful information: if a function accepts a parameter of type ssh_key, it is reasonable to wonder if it is a structure passed by copy, a pointer to something you must allocate yourself, or something more complicated altogether.

     This is much more valid argument in my opinion, and the main reason I never hide pointers behind typedefs. And if it's done, then the underlying struct that the typedef'ed pointer points to should be internal and not visible in the main header.

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