C Unix / Linux编程：md_addr_t是哪种类型？

Question

GCC gives me the following:

warning: passing argument 2 of ‘_panic’ discards qualifiers from pointer target type

misc.h:191: note: expected ‘char *’ but argument is of type ‘const char *’

The second argument is of type md_addr_t . How can I cast this to char* and what does md_addr_t meant to be? (Neither man-page could help me nor google)

Answer 1

The reason has been pointed out by larsmans correctly. This warning is seen when casting const away ie if a function take non-const argument but you pass const argument, then if the function modifies the argument passed you have undefined behaviour. md_addr_t which is typedef in the code has nothing to do with this. In the code you are getting these warning on using panic which is defined as follows (source from ss-ppc-little.tgz in your link):

#ifdef __GNUC__
/* declare a panic situation, dumps core */
#define panic(fmt, args...)     \
  _panic(__FILE__, __FUNCTION__, __LINE__, fmt, ## args)

void
_panic(char *file, char *func, int line, char *fmt, ...)
__attribute__ ((noreturn));
#else /* !__GNUC__ */
void 
panic(char *fmt, ...);
#endif /* !__GNUC__ */

On Unix/Linux system __GNUC__ is defined thus the first definition is used in which you are pre pending file name ( __FILE__ ), function name ( __FUNCTION__ ) & line number ( __LINE__ ) before printing out the message. (See this link for details).Here as you can see _panic expects char* as first & second arguments but arguments being passed are __FILE__ & __FUNCTION__ where __FUNCTION__ is static const char[] . You can change _panic(char *file, char *func, int line, char *fmt, ...) to _panic(char *file, const char func[], int line, char *fmt, ...) or to _panic(char *file, const char *func, int line, char *fmt, ...) - as the compiler is complaining fix your warning. As __FILE__ is also constant you can consider changing to _panic(const char *file, const char *func, int line, char *fmt, ...)
Hope this helps!

Answer 2

grep -R md_addr_t /usr/include does not return anything in a well-populated /usr/include , nor does it ring a bell. It must be something in your application/library.

In any case, casting to char * is just a matter of prefixing (char *) , which is also one of the few casts that are guaranteed to always work in C, although casting const away might mean that someone is going to write to read-only memory, which is not guaranteed to work...