Invalid initializer when calling malloc in C?

Question

I keep getting an invalid initializer error in this line:

StackObject_t new = (StackObject_t*)malloc(sizeof(StackObject_t));

The StackObject_t type is defined as such:

typedef struct stackObject* pStackObject_t;
typedef struct stackObject
{
    void* obj;
    pStackObject_t next;
} StackObject_t;

and this is the exact error I get:

[Please see this discussion on why not to cast the return value of malloc() and family in C..](https://stackoverflow.com/q/605845/2173917) — Sourav Ghosh, Oct 20 '20 at 12:48
You're assigning a pointer to a thing (`StackObject_t *`) to a thing (`StackObject_t`). Also, recommend not using the variable name `new` because this is a reserved word in C++. This is fully legal in C, but it's a good habit to avoid C++ keywords just to make life easier if/when you step up to C++ — Steve Friedl, Oct 20 '20 at 12:49
@SteveFriedl Don't be so pessimistic. Not every C programmer has to step down to C++. — JeremyP, Oct 20 '20 at 12:56
Using `new` as identifier is a good way to get a warning if you accidentally put it in a C++ compiler. — Gerhardh, Oct 20 '20 at 12:59
None the less, syntax formatting IDEs tend to not make a difference between C and C++, as we can see from this SO post. So that alone is a good reason to avoid C++ keywords. — Lundin, Oct 20 '20 at 13:00

score 4 · Accepted Answer · answered Oct 20 '20 at 12:49

4

First of all, you should not cast the result of malloc as it can hide errors and C automatically casts void* to the correct pointer type. Secondly, your issue is because you have StackObject_t as the type of the variable but malloc returns a StackObject_t*. You can fix this by changing your line to StackObject_t* new = ... so the types match up.

answered Oct 20 '20 at 12:49

Aplet123

33,825
1
29
55

`malloc` returns a `void *`, not `StackObject_t*`. Also, C automatically **converts** `void *` to other pointer types. It is a conversion, which is an operation that is performed. It is not a **cast**, which is an explicit operator in source code. – Eric Postpischil Oct 20 '20 at 13:00

score 0 · Answer 2 · answered Oct 20 '20 at 13:06

It doesn't work for the same reason as int* ptr; int i = ptr; doesn't work. A pointer is not a valid initializer to an int.

The clang compiler gives a better diagnostic:

warning: incompatible pointer to integer conversion initializing 'StackObject_t' with an expression of type 'StackObject_t *'; dereference with * [-Wint-conversion]

Aha, StackObject_t is not compatible with StackObject_t *. You probably meant to write:

StackObject_t* ptr = malloc(sizeof *ptr);

Details: "Pointer from integer/integer from pointer without a cast" issues

Invalid initializer when calling malloc in C?

2 Answers2