three C tips

2010-09-03

This example says sizeof can be used on a dereferenced pointer, even if it is of type struct *, and doesn’t point to anything. This is because the size of the struct is known at compile time.
```
#include <stdio.h>

struct abc {
    int a;
    double b;
    char *c;
};

int main()
{
    struct abc *p;
    printf("%d\n",sizeof(*p));
    return 0;
}
```
Output:
```
16
```
Note that there is an alignment issue. If we change the definition of struct abc to:
```
struct abc {
    char a;
    double b;
    char *c;
};
```
The result will still be 16 since it’s 4-byte-aligned.

Note. This program is tested on a 32-bit machine.
This is a valid struct initialization. In this way you don’t care about the order of items.
```
struct abc {
    int a;
    double b;
    char *c;
};

struct abc item = {
    .b = 3.14,
    .c = "ccccc",
    .a = 10,
};
```
Also note that the compiler doesn’t care about the last comma. It’ll be OK whether you add it or not. I don’t know whether it’s a compiler-specific issue. But it looks fine.

Variables length array is a new feature in C99. In this case this is an execution-time sizeof.

#include <stdio.h>

int calc(int k)
{
    char buf[k+12][k+3];
    return sizeof(buf);
}

int main()
{
    printf("%d\n",calc(10));
    return 0;
}

Output: