Pointer and Functions - c

Question

I'm new in C programming. I've written this code from RTKLIB library.
extern void satno2id(int sat, char *id)
{
int prn;
switch (satsys(sat, &prn)) {
case SYS_GPS: sprintf(id,"G%02d",prn-MINPRNGPS+1); return;
case SYS_GLO: sprintf(id,"R%02d",prn-MINPRNGLO+1); return;
case SYS_GAL: sprintf(id,"E%02d",prn-MINPRNGAL+1); return;
case SYS_BDS: sprintf(id,"C%02d",prn-MINPRNBDS+1); return;
}
strcpy(id, "");
}
In this function first argument is Input and second is Output. Now the question is how I get the value of second argument in main() function?
I've written this block of code but it's getting error. What's wrong here?
int main(){
char *id;
satno2id(68, &id);
printf("satellite number is %s", *id);
}

Answer 1

In C, the name of an array degrades to a pointer to the first element of that array.
The id in the function expects a pointer to the first element of a character array (string). This is evident because sprintf() function, which is used to write to a character array is used in satno2id().
So send an char array instead like
char id[4];
satno2id(68, id);
I made the size of the array to be 4 as satno2() seems to be writing a string of length 3. The extra byte is to store the \0 terminator.

Answer 2

You need to pass an array to satno2id, not an uninitialized pointer. Also, to avoid buffer overflow you also need to pass the length of the array so the function can assert that the array is long enough.
#include <assert.h>
#define LEN(array) (sizeof (array) / sizeof (array)[0])
extern void satno2id(int sat, char id[], int idLen)
{
int prn;
assert(idLen >= 4 + 1);
switch (satsys(sat, &prn)) {
case SYS_GPS:
sprintf(id, "G%02d", prn - MINPRNGPS + 1);
break;
case SYS_GLO:
sprintf(id, "R%02d", prn - MINPRNGLO + 1);
break;
case SYS_GAL:
sprintf(id, "E%02d", prn - MINPRNGAL + 1);
break;
case SYS_BDS:
sprintf(id, "C%02d", prn - MINPRNBDS + 1);
break;
default:
strcpy(id, "");
}
}
int main(void)
{
char id[8];
satno2id(68, id, LEN(id));
printf("satellite number is %s\n", id);
return 0;
}