Let's say there is a C header file:
//void test(void);
inline void test(void) {}
If we comment out the first line, MISRA C checker will complain
Type: MISRA C-2012 Declarations and Definitions (MISRA C-2012 Rule 8.4, Required) Triage unavailable. header.h:3:
- misra_c_2012_rule_8_4_violation: Function definition does not have a visible prototype.
I don't understand why inline functions also need prototypes. Thank you.
MISRA C enforces prototypes since it allows C90 where non-prototype function declarations were a huge problem. But also to avoid linker errors and avoid inconsistencies between declaration and definition. Consider for example inline void f() called as f(123) - valid C99 but leading to mysterious linker errors.
8.4 specifically is about enforcing a function declaration before writing a function declaration with external linkage. Most of the time you do not want an inline function to have external linkage. In fact another MISRA C rule 8.10 enforces inline functions to always have internal linkage by declaring them static - and the reason for that is to avoid undefined behavior, as stated by that rule's rationale.
Another feature and/or subtle safety problem is that in case you define a function inside a header, that function will exist in every translation unit that includes the header. So you end up with multiple functions with the same name - typically leading to a linker error. Unless you also give the function internal linkage, in which case the C compiler/linker will utilize name mangling to separate the different functions.
