Comments on question "Why do a lot of #defines in the kernel use do { ... }
while(0)?" in the FAQ:
IMHO the important point of the question is missing. As both Miller and
Collins point out, you want a block statement so you can have several lines
of code and declare local variables. But then the natural thing would be to
just use for example
#define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }
However that wouldn't work in some cases. The following code is meant to be
an if-statement with two branches:
if(x>y)
exch(x,y); // Branch 1
else
do_something(); // Branch 2
but it would be interpreted as an if-statement with only one branch. :
if(x>y) { // Single-branch if-statement!!!
int tmp; // The one and only branch consists
tmp = x; // of the block.
x = y;
y = tmp;
}
; // empty statement
else // ERROR!!! "parse error before else"
do_something();
The problem is the semi-colon (;) coming directly after the block.
The solution for this is to sandwich the block between "do" and "while(0)".
Then we have a single statement with the capabilities of a block, but not
considered as being a block statement by the compiler.
Our if-statement now becomes:
if(x>y)
do {
int tmp;
tmp = x;
x = y;
y = tmp;
} while(0);
else
do_something();
Sincerely,
Per Persson
per.persson@gnosjo.pp.se
-
Kernelnewbies: Help each other learn about the Linux kernel.
Archive: http://mail.nl.linux.org/kernelnewbies/
IRC Channel: irc.openprojects.net / #kernelnewbies
Web Page: http://www.kernelnewbies.org/
