"need `this` for `s` of type `char*`" error message

[Mathias LANG]

On Thursday, 11 November 2021 at 07:04:57 UTC, rempas wrote:
> After not being able to use ImportC to automatically compile a 
> C library, now I'm trying to do it myself. The library I want 
> to use is [tomlc99](https://github.com/cktan/tomlc99) which is 
> a small C library to parse toml files. Now I compiled the 
> library and I copied the "config.h" file and modified to make 
> it a D file that will contain the declaration so D knows which 
> symbols to call. I resulted with a "toml.d" file and a "main.d" 
> file. The first one contains the decorations and the second one 
> contains the code from the example in the 
> [usage](https://github.com/cktan/tomlc99#usage) section. 
> However, when I try to compile, I'm getting the following error 
> message:
>
> ```
> main.d(51): Error: need `this` for `s` of type `char*`
> Deprecation: argument `__error` for format specification `"%s"` 
> must be `char*`, not `_error_`
> main.d(57): Error: need `this` for `i` of type `long`
> main.d(63): Error: need `this` for `s` of type `char*`
> ```
>
> I uploaded the modified files so someone is able to look at 
> them and explain me what I'm doing wrong so I can properly 
> learn. Links:
>
> [toml.d](https://gist.github.com/rempas/8aaab43b71e3da720ce298ef472f0673)
> [main.d](https://gist.github.com/rempas/07dfb15295c5f6142c6a8dfab669c40e)

     Your type definition is wrong:
     ```D
     struct toml_datum_t {
       int ok;
       union u {
         toml_timestamp_t* ts; /* ts must be freed after use */
         char*   s; /* string value. s must be freed after use */
         int     b; /* bool value */
         int64_t i; /* int value */
         double  d; /* double value */
       }
     }
     ```

     If you check the size of this struct, it's going to be 4, 
because `u` is a type definition. What you want is either:
     ```D
     struct toml_datum_t {
       int ok;
       union {
         toml_timestamp_t* ts; /* ts must be freed after use */
         char*   s; /* string value. s must be freed after use */
         int     b; /* bool value */
         int64_t i; /* int value */
         double  d; /* double value */
       }
     }
     ```

     Which you access via `host.s` or:
     ```D
     struct toml_datum_t {
       int ok;
       /// This is the type definition
       union U {
         toml_timestamp_t* ts; /* ts must be freed after use */
         char*   s; /* string value. s must be freed after use */
         int     b; /* bool value */
         int64_t i; /* int value */
         double  d; /* double value */
       }
       /// This is the field
       U u;
     }
     ```

Note that instead of doing this work yourself, I would highly 
recommend the excellent 
[dstep](https://github.com/jacob-carlborg/dstep).