Error in code, old C style

Hello everyone.
I am trying to build a custom call in FreeBSD 4.2 just as a proof of concept. I have no idea about C. Can anybody help me fixing these errors.

#include <sys/param.h>
#include <sys/system.h>

struct syshello_args
{
char *buf;
};

int sys_myhello(p, uap)
struct proc* p; struct syshello_args* uap;
{
sprintf(uap->buf, "Hello");
p->p_retval[0]=0;
return 0;
}

Thank you


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