Question

gcc 4.7.2
c89

Hello,

I have this function that assigns a pointer to an another pointer that is an element of a string structure.

The purpose of this string structure is to get and set the length of the actual string.

However, I don’t understand why they have used a ternary operator. It doesn’t make sense to me. There doesn’t seem to be a condition apart from str->len = src. Which only assigns and doesn’t compare anything.

typedef struct tag_str_t str_t;
struct tag_str_t {
    char *buf;
    size_t len;
};

static void string_set(str_t *str, const char *src)
{
    str->buf = (char*)src;
    str->len = src ? strlen(src) : 0;
}

Would it be just as simple and more readable by doing just this?

str->len = strlen(src);

Many thanks for any suggestions,

Answer 1

The code:

str->len = src ? strlen(src) : 0;

is equivalent to:

str->len = ((src != 0) ? strlen(src) : 0);

I’ve added the parentheses and the extra expression to indicate where the components of the expression are. Your comment seems to indicate that you think str->len = src is the condition in the ternary operator, but that is wrong; assignment has a very low precedence.

As Jesus Ramos also explained, the purpose of the test is to ensure that strlen() is not called with a NULL pointer, which would lead to a crash.