If I defined a variadic function:
#include <stdio.h>
#include <stdarg.h>
int f(char*s,...)
{
va_list ap;
int i=0;
va_start(ap, s);
while(s)
{
printf("%s ", s);
i++;
s=va_arg(ap,char*);
}
va_end(ap);
return i;
}
int main()
{
return f("a","b",0);
}
gcc (linux x64) compiles this and the exe runs and prints "a b ".
is there any need for a cast like:
return f("a","b",(char*)0)
on common systems?
The compiler can't auto promote pointer for variadic parameter, for example that why when you want to print a pointer in printf you must cast it to void *:
printf("%p", (void *)ptr);
The same rule applies to all variadic functions, compiler can't know that your function expect a char *, and 0 is by default just an integer so yes you need to cast it (char *)0.
Even NULL must be cast with your function (char *)NULL
so why does main() work and when will it fail?
Your code don't really "work". Your code invoke undefined behavior, anything can happen, so no one can answer this. Here you are lucky, but next time maybe not.
NULL must be cast because it's a void * and your function expect char *, like I said there is no auto promote, and standard is a little obscure about compatibility between char * and void *, in doubt always cast your variable with variable parameter function.return f("a","b",0); ? In this case is totally wrong. "it can be implicitly converted to any object pointer just fine", what do you mean ? that in the case case 0 is auto cast to char * ? This is totally wrong too. "Similarly, NULL is fine too without any cast, since there is already a manner of cast going on inside va_arg." => you answer yourself "Problems do however arise if a null pointer constant has different size than a pointer. In such cases, a conversion is necessary." so no NULL is NOT fine without a cast.Your code should work fine on any system that supports the SYSV x64 ABI standard. It may work on systems that use some other ABI standard that has similar requirements, and may fail on any system that uses some other ABI1. This is all undefined according to the C standard.
1In particular, Microsoft does NOT follow the standard x64 ABI, they have their own standard.
return f("a","b",(char*)0) on common systems?", for me common systems mean "in general", I'm very surprise the OP valid your answer, like if OP only want at any price valid that the code wasn't broken. That why Michael Walz and me didn't want to answer "why does main() work", we shouldn't encourage people that use undefined behavior.
sizeof(0)might be, and on a 64 bit system it is, less thansizeof((char*)0).execl()andexeclp())