Bounded interfaces in C

Take the buffer and its capacity together, and report what was needed.

size_t format_label(char *out, size_t cap, const char *lang, int stars) {
    static const char *filled = "***";
    size_t needed = strlen(lang) + 1 + (size_t) stars;

    if (out != NULL && cap > needed) {
        size_t n = strlen(lang);
        memcpy(out, lang, n);
        out[n] = ' ';
        memcpy(out + n + 1, filled, (size_t) stars);
        out[needed] = '\0';
    }
    return needed;
}

How it works

1. Passing a buffer without its size is how overflows happen.
2. Returning the required length lets the caller retry bigger.
3. This is the shape of snprintf, and it is a good shape.

Keywords and builtins used here

• NULL
• char
• const
• format_label
• if
• int
• return
• size_t
• static

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Step 3 of 4 in Signatures, step 3 of 11 in APIs, errors & testing.

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