c - Pass char* to method expecting unsigned char* -

i working on embedded device has sdk. has method like:

messagebox(u8*, u8*); // u8 typedefed unsigned char when checked 

but have seen in examples calling code like:

messagebox("hi","hello"); 

passing char pointer without cast. can well defined? asking because ran tool on code, , complaining above mismatch:

messagebox("status", "error calculating \rhash"); diy.c  89  error 64:  type mismatch (arg. no. 1) (ptrs signed/unsigned) diy.c  89  error 64:  type mismatch (arg. no. 2) (ptrs signed/unsigned) 

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different opinions on answer , confuses me more. sum up, using api way described above, problem? crash program?

and nice hear correct way pass string sdk methods expecting unsigned char* without causing constraint violation?

it constraint violation, technically not defined, in practice, not problem. yet should cast these arguments silence these warnings. alternative littering code ugly casts define inline function:

static inline unsigned char *ucstr(const char *str) { return (unsigned char *)str; } 

and use function wherever need pass strings apis (mistakenly) take unsigned char * arguments:

messagebox(ucstr("hi"), ucstr("hello")); 

this way not warnings while keeping type safety.

also note messagebox should take const char * arguments. sdk uses questionable conventions.