Initializing a structure

Question

gcc 4.5.1 c89

compiling with the following flags: -Wall, -Wextra

I have the following structure that I am trying to initialize with default values. However, I get the following warnings:

static struct Device { char *name; char *config; } *app = { NULL, NULL };

Warnings:

warning: initialization from incompatible pointer type warning: excess elements in scalar initializer

However, if I do the following by declaring a non-pointer, I don't get any problems i.e.

 static struct Device { char *name; char *config; } app = { NULL, NULL };

Why is that?

Many thanks for any advice,

Pablo Santa Cruz · Accepted Answer · 2010-11-20 13:03:30Z

You can't initialize a pointer that way. You can do this though:

static struct Device { char *name; char *config; } *app = NULL;

Initialize a pointer to NULL. A pointer is nothing but a memory address. You can't assing { NULL, NULL } to a memory address.

icecrime · Accepted Answer · 2010-11-20 13:03:47Z

A pointer is not a structure but a simple type whose value is an address.

If your app variable is of pointer type, it may be initialized with NULL but not with the braces syntax you use
If the app variable is of type struct Device, you may use the brace initializer to provide an initial NULL value to the name and config fields

AndersK · Accepted Answer · 2010-11-20 13:04:06Z

because *app is a pointer so initializing it to { NULL, NULL } is wrong, you should initialize it to simply NULL.

in the case of app you have an actual variable instance of the struct so there you initialize the members of the struct to NULL, that is OK.

3 Answers 3