In article <f4**************************@posting.google.com >,
candy <ca********@yahoo.com> wrote:
Consider the following C code:
void funct(){
}
int main(){
int x;
x=funct;
printf("%x\n",x);
return 0;
}
It prints the address of the function funct().
No; it does not even compile:
% strictcc t.c
t.c: In function `main':
t.c:6: error: assignment makes integer from pointer without a cast
t.c:7: error: implicit declaration of function `printf'
%
[no executable produced]
On the IA64, for instance, funct has a 64-bit address, but x is
only 32 bits long. What happens when you put 64 bits into a 32-bit
holder?
It means that the function funct() is in the process stack even
before it is called.
No. Even where there is a "process stack", and even where function
addresses happen to fit in "int"s (e.g., IA32), there is no activation
record for funct() on that stack at that point.
So,it should be safe to call setjmp in the function funct() in the
following way:
jmp_buf env;
int funct(){
if(setjmp(env)==1)
printf("called by longjmp\n");
return 0;
}
int main(void){
funct();
longjmp(env,1);
return 0;
}
On execution,the above program crashes.Can anybody please tell me
that what is the problem.
The call to longjmp() attempts to jump into a function activation
that has already returned. This is not allowed in Standard C; the
effect is undefined. You are lucky that it crashed, rather than
doing some sort of by pretending to work.
(Some people sometimes attempt to [ab]use longjmp() to implement
thread context switchers. Whether this works is up to the
implementation: some implementations actually unwind the stack
inside longjmp(), looking for the corresponding setjmp(), and since
different threads have separate stacks, the unwound stack eventually
reaches its top and the program simply aborts or crashes. In
general, you are better off writing your thread-context-switch code
in machine-specific assembly-language -- you usually need some
anyway to save registers, for instance.)
--
In-Real-Life: Chris Torek, Wind River Systems
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