Thread: Pointer Initialization Within A Struct

Yes, string literals is a special case, because it is the C language's way to form a string.

--
Mats

Because a pointer is not the same thing as an array. You can have an array and initialize that

Code:

typedef struct {
    int a;
    int b;
    int array[3];
} thing;

thing g_thing = { 1, 2, { 3, 4, 5 } };

String literals are an exception. They already are somewhere in the memory and you just point a pointer to that.

To add to anon's suggestion: If you have a variable number of elements in your list of integers [array/pointer], then you may find that forming a temporary variable is the only option. If you have a compiler that supports variable number of arguements to macros (gcc does in recent versions at least, and I think the latest MS compilers also does), you could possibly use that.

--
Mats

Code:

/* First byte: Length. Following bytes: Opcode bytes. */
#define MAKE_INSTR(nm, ...) static const u8 OPCODE_##nm[] = { __VA_ARGS__ }
MAKE_INSTR(INVD,   2, 0x0f, 0x08);
MAKE_INSTR(WBINVD, 2, 0x0f, 0x09);
MAKE_INSTR(CPUID,  2, 0x0f, 0xa2);
MAKE_INSTR(RDMSR,  2, 0x0f, 0x32);
MAKE_INSTR(WRMSR,  2, 0x0f, 0x30);
MAKE_INSTR(VMCALL, 3, 0x0f, 0x01, 0xd9);
MAKE_INSTR(HLT,    1, 0xf4);
MAKE_INSTR(INT3,   1, 0xcc);
MAKE_INSTR(RDTSC,  2, 0x0f, 0x31);

static const u8 *opc_bytes[INSTR_MAX_COUNT] = 
{
    [INSTR_INVD]   = OPCODE_INVD,
    [INSTR_WBINVD] = OPCODE_WBINVD,
    [INSTR_CPUID]  = OPCODE_CPUID,
    [INSTR_RDMSR]  = OPCODE_RDMSR,
    [INSTR_WRMSR]  = OPCODE_WRMSR,
    [INSTR_VMCALL] = OPCODE_VMCALL,
    [INSTR_HLT]    = OPCODE_HLT,
    [INSTR_INT3]   = OPCODE_INT3,
    [INSTR_RDTSC]  = OPCODE_RDTSC
};

Here's a snippet of xen/arch/x86/hvm/svm/emulate.c that I originally wrote, which uses variable length array to describe differnet instruction formats.

--
Mats

Originally Posted by SMurf

Resolved by instead using:-

Code:

typedef struct {
    int a;
    int b;
    int *array;
} thing;

int g_thing1array[3] = { 3, 4, 5 };
thing g_thing1 = { 1, 2, g_thing1array };

One extra line isn't that bad I suppose.

Not a good very good solution.
The local array will go out of scope at the end of the function and then your pointer will point at undefined.

Originally Posted by cas

Thankfully C99 has added compound literals which allow you to do this the way you originally tried (although not with that precise syntax). Unfortunately, C99 isn't widespread. But if you can use it, here's what it looks like:

Code:

typedef struct {
  int a;
  int b;
  int *array;
} thing;

thing g_thing = { 1, 2, (int[]){ 3, 4, 5 } };

One thing to be careful of is that compound literals that are inside of blocks have automatic storage duration. So unlike string literals, pointers to them become invalid when the block is exited. This isn't an issue with file-scope (global) compound literals.

I doubt that's valid C. I only get syntax errors.

Originally Posted by Elysia

I doubt that's valid C. I only get syntax errors.

After I placed the last line in the main function, it compiled without any errors or warnings (besides a benign "unused variable" warning) for me. Perhaps you forgot to turn on C99 mode?

But Visual Studio does not support C99.
It doesn't matter what I do, this code...

Code:

typedef struct
{
	int a;
	int b;
	int* array;
} thing;

int main()
{
	thing g_thing = { 1, 2, (int[]){ 3, 4, 5 } };
}

...gives one syntax error in C and a slew of syntax errors in C++.

Originally Posted by Elysia

But Visual Studio does not support C99.

SMurf has not given any indication in this thread that he/she must use a Microsoft compiler or some other compiler that lacks support for C99, while cas explicitly stated that the code was written with respect to C99.

Of course. I was referring to your...

Perhaps you forgot to turn on C99 mode?

...quote. Since VS does not support C99, this is something I cannot do.

Still, I am skeptical, because so far as I know, casting to an array of any type is illegal.
Are you sure this is allowed and not some exception?

Not sure I know of any good reference for preprocessor stuff.

This described the Visual Studio preprocessor stuff.
http://msdn.microsoft.com/en-us/library/y4skk93w.aspx

If you have SPECIFIC questions on how to do something in particular, then I suggest you ask.

--
Mats