inline function means

alaios · 08-22-2005, 11:15 AM

Hi can u plz explain me what inline function means in c ?
Whats the difference with extern?
Can u provide me some examples when to use the one or the other?

Nylex · 08-22-2005, 11:27 AM

Hmm, I assume inline functions are the same in C as they are in C++. When you inline a function, wherever that function is called, the code is copied there as if you had written it yourself. For example:

Code:

int Double(int x)
{
      return 2*x;
}

int main()
{
    int x = 2;
    int y = Double(x);
    return 0;
}

If you had declared Double() to be inline, the compiler would change your main() method to look like:

Code:

int x = 2;
y = 2 * x;

Generally, you only declare functions as inline if they contain a small number of statements (1 or 2, possibly a few more).

Matir · 08-22-2005, 11:39 AM

Inline code is also important in functions that will get called quite frequently. You can get quite an advantage by avoiding the function calls (pushing flags, pointers on stack, creating new stack, jumping, reversing the procedure, etc.)

This is common in loops where the same function may be called in multiple ways... it avoids functions calls, but allows you to update the same code all in once place.

alaios · 08-22-2005, 03:42 PM

And how about extern? When we use it?

mehuljv · 08-26-2005, 02:47 AM

hi alaios,
extern can be used to declare a function whose definition is some where else. so that compiler will not give you any warning.

Mehul.

Matir · 08-26-2005, 01:08 PM

You should not use extern with functions (as far as I'm aware).

Externs should be used for VARIABLES defined in another c file.
For example:

Code:

/* file1.c */
#include <stdio.h>
int status;
void checkstatus();

int main(int argc,char **argv){
    status=argc;
    checkstatus();
    return 0;
}

Code:

/* file2.c */
#include <stdio.h>
extern int status;
void checkstatus(){
    printf("%d\n",status);
}

mehuljv · 08-29-2005, 11:44 AM

hi
there is nothing like you should not use extern for function, just traverse linux source code, u will find extern declartion of function at so many places.

Mehul.

Matir · 08-29-2005, 11:55 AM

Sorry, I should have been more clear.

The keyword 'extern' is mandatory when referring to variables (as a variable definition and a variable declaration are the same) but optional when referring to functions. Generally, extern is used for a function from another file, the extern keyword is left off if the function is in the same file.

aluser · 08-29-2005, 12:17 PM

functions can be defined as "extern inline", which has a special meaning: http://www.greenend.org.uk/rjk/2003/03/inline.html

It's also worth noting that, even when you declare a function inline, gcc *won't* inline it unless you give at least -O.

prasanram · 08-31-2005, 07:03 AM

alaios ,
Do remember that the member functions defined inside the class definition scope in c++, will be treated as a inline function Only !