C compiler ignores 'static' for declaration of struct

In C, if I declare a structure like so:

static struct thing
int number;
;


and compile it (with gcc in this case), the compiler prints this warning:

warning: 'static' ignored on this declaration

[-Wmissing-declarations]

Why is this?

My intention in making the struct static would be to keep thing out of the global namespace so that another file could declare its own thing if it wanted.

thing

thing

static

static struct timeval startw

startw

timeval

3 Answers
3

Struct tags (and typedef names) have no linkage, which means they are not shared across translation units. You might use the term "private" to describe this. It is perfectly fine for two different units to define their own struct thing.

struct thing

There would only be a problem if it was attempted to make a cross-unit call of a function with external linkage that accepts a struct thing or type derived from that. You can minimize the chance of this happening by ensuring that functions with external linkage are only called via prototypes in header files (i.e. don't use local prototypes).

struct thing

You cant define the storage without defining the actual object.

static struct thing
int number;
obj1,obj2;


is ok and:

struct thing
int number;
;

static struct thing x,y;


You can't use static on this way to control the linkage of a type like you could for a function or object, because in C types never have linkage anyway.

static

"Global namespace" isn't quite the term you want here. C describes names of objects and functions as having "external linkage" if the same name can be declared in different translation units to mean the same thing (like the default for functions), "internal linkage" if the same name can be redeclared within the same translation unit to mean the same thing (like declarations marked static), or "no linkage" when a declaration names a different object or function from any other declaration (like variables defined within a function body). (A translation unit, roughly speaking, is one *.c file together with the contents of the headers it includes.) But none of this applies to types.

static

So if you want to use a struct type that's essentially private to one source file, just define it within that source file. Then you don't need to worry about another usage of the same name colliding with yours, unless maybe somebody adds it to a header file that the source file was including.

(And just in case a C++ user comes across this Q&A, note the rules for this in C++ are very different.)

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