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How to pass arguments to functions

Updated September 2026 · Beginner-friendly guide

Pass values by value

C copies each argument into its parameter. Changing a scalar parameter does not change the caller's variable.

int twice(int value)
{
    value *= 2;
    return value;
}

Use a pointer to modify

Pass a valid object address when a function must update caller-owned data. Check a pointer before dereferencing when null is permitted.

bool increment(int *value)
{
    if (value == NULL) {
        return false;
    }
    ++*value;
    return true;
}

Pass array length separately

An array parameter becomes a pointer; the function cannot recover the array length with sizeof. Always pass the element count.

int sum(const int values[], size_t count)
{
    int total = 0;
    for (size_t i = 0; i < count; ++i) {
        total += values[i];
    }
    return total;
}

Express read-only access

Use a pointer to const when the function reads caller data without modifying it. State who owns memory and how long it must remain valid.

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