GCC: __attribute__((malloc))

Question

Quoting from GCC documentation (emphasis mine):

The malloc attribute is used to tell the compiler that a function may be treated as if any non-NULL pointer it returns cannot alias any other pointer valid when the function returns and that the memory has undefined content. This often improves optimization. Standard functions with this property include malloc and calloc. realloc-like functions do not have this property as the memory pointed to does not have undefined content.

I have the following code:

struct buffer {
    size_t alloc;  // Allocated memory in bytes
    size_t size;   // Actual data size in bytes
    char data[];   // Flexible array member
};


#define ARRAY_SIZE <initial_value>

buffer *buffer_new(void) __attribute__((malloc))
{
    struct buffer *ret;

    ret = malloc(sizeof(struct buffer) + ARRAY_SIZE);
    if (!ret)
        fatal(E_OUT_OF_MEMORY);

    ret->alloc = ARRAY_SIZE;
    ret->size = 0;

    return ret;
}

Now I'm a bit puzzled here: though I didn't initialize the data member, I still set the alloc and size fields to their respective values. Can I still consider this allocated segment to be of "undefined content" and use the malloc attribute?

Answer 1

It is safe to mark your buffer_new function with __attribute__((malloc)), because the block it returns contains no pointers.

The latest GCC documentation clarifies the meaning of __attribute__((malloc)): the block returned by a function so marked must not contain any pointers to other objects. The intention is to help the compiler estimate which pointers might possibly point into the same object: the attribute tells GCC it needn't worry that the object your function returns might include pointers to something else it's tracking.