Safely printing all attributes of struct in C - c

Question

I'm writing a toString method that prints out all attributes of an inputted pointer to a struct. In reading safe methods for handling strings, I finally created the below solution:
note a Person struct has attributes name, weight, height, and age (all ints except for name, which is a char array).
char* toString(struct Person* inputPerson)
{
// Allocate memory to return string
char* ret = malloc(sizeof (char) * 100);
// copy "Name: " into string
strcpy(ret, "Name: ");
// safely copy name, at most leaving enough room for the other params (leaving 50 bytes)
strncat(ret, inputPerson->name, 100-50);
// copy "Age: " into string
strcat(ret, "\n\tAge: ");
// create tmp char to allow us to convert ints age, weight, and height into strings
char tmp[4];
// safely convert string to int (max number of digits being 3)
snprintf(tmp, sizeof(tmp), "%d", inputPerson->age);
// safely copy at most 3 characters, plus a null terminating char
strcat(ret, tmp); // the previous snprintf makes sure that tmp is not too large.
// repeat previous 2 steps for both weight and height attributes
strcat(ret, "\n\tWeight: ");
snprintf(tmp, sizeof(tmp), "%d", inputPerson->weight);
strcat(ret, tmp);
strcat(ret, "\n\tHeight: ");
snprintf(tmp, sizeof(tmp), "%d", inputPerson->height);
strcat(ret, tmp);
// Return a pointer to the string
return ret;
}
My question is: is this overkill? All I want to do is print out each attribute safely and securely. For each string, I have to make sure it is a maximum size before appending. For each integer, I have to snprintf it into a string (ensuring a maximum allowed length) and then append that string to my return string. Is there a simpler way? Also my heebie jeebies are growing every time I look at the "100-50" code portion: how can I specify "size allocated to ret" instead of 100?

Answer 1

Since you are mallocing the return buffer anyway, why not make life easier on yourself by allocating a buffer of the correct size? As Joachim suggests, you can even do that with snprintf:
char* toString(struct Person* inputPerson)
{
size_t space_required =
snprintf(0, 0,
"Name: %s\n"
"\tAge: %d\n"
"\tWeight: %d\n"
"\tHeight: %d\n",
inputPerson->name,
inputPerson->age,
inputPerson->weight,
inputPerson->height);
// space_required excludes the terminating NUL
// sizeof(char) == 1 *by definition*
char *ret = malloc(space_required+1);
if (!ret)
return 0;
snprintf(ret, space_required+1,
"Name: %s\n"
"\tAge: %d\n"
"\tWeight: %d\n"
"\tHeight: %d\n",
inputPerson->name,
inputPerson->age,
inputPerson->weight,
inputPerson->height);
return ret;
}
That probably seems awful repetitive. If your C library has asprintf, you can avoid the repetition:
char* toString(struct Person* inputPerson)
{
char *ret;
if (asprintf(&ret,
"Name: %s\n"
"\tAge: %d\n"
"\tWeight: %d\n"
"\tHeight: %d\n",
inputPerson->name,
inputPerson->age,
inputPerson->weight,
inputPerson->height) == -1)
return 0;
return ret;
}
If you don't have asprintf, you can implement it using malloc and vsnprintf. I'll leave that as an exercise ;-)