Predeclaration of C stdint types

Question

When I tried to declare some types defined in stdint.h a conflict with /usr/include/bits/stdint-uintn.h:24:19 predeclaration occurs

typedef struct __uint8_t uint8_t;

Error message:

/usr/include/bits/stdint-uintn.h:24:19: error: conflicting types for ‘uint8_t’
   24 | typedef __uint8_t uint8_t;
      |                   ^~~~~~~
In file included from ../src/server/connection.c:1:
../src/server/connection.h:4:26: note: previous declaration of ‘uint8_t’ was here
    4 | typedef struct __uint8_t uint8_t;

How can I predeclare those types in my header files to avoid circular dependencies?

What compiler are you using? What are its command line arguments? — tstanisl, Jan 31 '21 at 17:08
`typedef struct __uint8_t uint8_t;` remove `struct`? What it's doing there? Anyway, why would you ever do that? Just `include `? `to avoid circular dependencies?` ? How are "circular dependencies" related? — KamilCuk, Jan 31 '21 at 17:26
Removing struct will just cause type to be not defined. Simply if you keep four `#include`s inside of headers you will get them included circular everywhere they occurs. To avoid this evaluate them inside of your *.c files. — R.M. Freeman, Jan 31 '21 at 17:30
as KamilCuk said, just include `stdint.h`, there will be no circular dependencies, because there's nothing circular about that. you may have multiple `#include` statements for the same stdint.h, but that's not a problem, it already has include guards — Stefan Riedel, Jan 31 '21 at 18:08

score 2 · Answer 1 · answered Jan 31 '21 at 18:07

2

This is because stdint.h includes bits/stdint-uintn.h, in which

typedef struct __uint8_t uint8_t;

is defined.

You should not redefine uint8_t. To me the solution is to remove

typedef struct __uint8_t uint8_t;

from your file ../src/server/connection.c.

Is there any reason you need to redefine it yourself ?

answered Jan 31 '21 at 18:07

Jessy SIMEON

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Yes, to avoid including `stdint.h` in my header. – R.M. Freeman Jan 31 '21 at 18:09
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@DanielFreeman but why would you want that? – Stefan Riedel Jan 31 '21 at 18:14
To avoid circular dependencies – R.M. Freeman Jan 31 '21 at 18:16
There are no circular dependencies. Circular means, well, circular. `stdint.h` is not going to include your headers. You want to use `uint8_t`, `uint16_t` and similar types, so you include `stdint.h`. That's its one and only purpose. That header exists just for those types – Stefan Riedel Jan 31 '21 at 18:17
Do you include other headers ? Those headers may include `stdint.h` as they need it, in which case you can't avoid `stdint.h` – Jessy SIMEON Jan 31 '21 at 18:22

score 1 · Accepted Answer · answered Jan 31 '21 at 20:19

1

#include <stdint.h> is the correct way to declare these types. Any other approach is undefined and/or non-portable.

There can never be a circular dependency issue with standard headers.

answered Jan 31 '21 at 20:19

M.M

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Ok, thank you, I didn't know that. But what makes them safe in this situation, are they locked controlled by some memory management? – R.M. Freeman Jan 31 '21 at 20:54
@DanielFreeman I don't understand your question . Declaring types does not involve any memory management – M.M Jan 31 '21 at 21:04
Yes, but I don't relly understand why is it correct to re-include `stdint` any where with the header file. I see that It most do something that not-standard types does not. Isn't this bad to let a `include` predecessor inside of a header file? I thought headers should be pure and restrict to the needful declarations. – R.M. Freeman Jan 31 '21 at 21:14
@DanielFreeman It's not bad to include a standard header that provides declarations that your header file requires . – M.M Jan 31 '21 at 21:15
Ok, I think I understand now. So the `stdint` does include.. macros or declaration but not definitions, as it is just a layer over standard compiler types? – R.M. Freeman Jan 31 '21 at 21:18
"declaration" and "definition" are the same thing for typedefs . – M.M Jan 31 '21 at 21:34
Ok, right. Thank you. I will remember to not override `typedef`ed stuff. – R.M. Freeman Jan 31 '21 at 21:42

Predeclaration of C stdint types

2 Answers2