What does the extern keyword mean?

c objective-c extern
dontWatchMyProfile picture dontWatchMyProfile · May 2, 2010 · Viewed 42.9k times · Source

What does the extern keyword mean? I've seen that in front of an function declaration like

extern void DoFoo ...

Answer

Romain Hippeau picture Romain Hippeau · May 2, 2010

The extern keyword declares a variable or function and specifies that it has external linkage (its name is visible from files other than the one in which it's defined). When modifying a variable, extern specifies that the variable has static duration (it is allocated when the program begins and deallocated when the program ends). The variable or function may be defined in another source file, or later in the same file. Declarations of variables and functions at file scope are external by default.

You can find a more complete description here.

Related questions

3 questions about extern used in an Objective-C project

When I use the word extern before a method or variable declaration, am I making it global and therefore readable/writable/usable over the entire project ? If I use extern before a keyword, is there any chance it is still …

Read More
static const Vs extern const

I have been using static const in my header files as so: static NSString * const myString = @"foo"; But have read that this is not the 'safe' or correct way of doing this. Apparently, if I want my const strings to …

Read More
What does the question mark and the colon (?: ternary operator) mean in objective-c?

What does this line of code mean? label.frame = (inPseudoEditMode) ? kLabelIndentedRect : kLabelRect; The ? and : confuse me.

Read More