Hello all,
I used setjmp() in a recent of program of mine (it is not completed, so
I have not the chance to test it out yet). I am not very profocient in
C coding as are some of my co-workers. They (my co-workers) say (with
vehement ardor ;) ) that the usage of setjmp() emplyoyed in function"C"
that was called from function "B" that was called from function "A"
that was called form the main(), will cause havoc in the stack. And it
makes sense, since the longjmp() function/macro has no way of knowing
how high is the stack is, in order to pop out the excess elements in
order to render it as it was when the setjmp() function first set the
"jmpbuf" environment. However, if that were the case the existence of
the setjmp()/longjmp() function pair will not exist.
So, is setjmp()/longjmp() reliable as a "non-local goto", or will it
eventually cause a stack overflow or some other problem?
P.S. I am using the setjmp()/longjmp() only for error handling.
Regards,
-Roger
15  2431 
rover8898 wrote:
Hello all,
I used setjmp() in a recent of program of mine (it is not completed, so
I have not the chance to test it out yet). I am not very profocient in
C coding as are some of my co-workers. They (my co-workers) say (with
If you're not proficient with C then I definitely do not recommend playing
around with any *jmp() function. Even when you do become proficient, I'd find
another method. I personally never use them, but that doesn't necessarily
mean they are useless.
It's nice you have coworkers to help you. And they seem to be well
enough informed to realize there is a bit of fiddling required if
you're nested 33 levels deep and want to jump out to the mail program.
But setjmp/longjmp manage this just fine. The basic rigamarole is
setjmp() saves all the CPU registers, including the stack pointer, in
its save area parameter. Then longjmp() mostly just restores those
register values, including the old program counter. Quick and slick
and should probably be used sparingly. But in some cases it's almost
unavoidable.
In more refined languages, like old Pascal, there is no restriction on
goto's, and the compiler generates the right code to do the
inter-procedure jumps. And as a side-benefit, the compiler knows what
files are local to all the intermediate levels, and closes them. Poor
setjmp() doesnt have any way of knowing what files are open, so you'll
have to write your own code to clean up any open files.
Ancient_Hacker wrote:
It's nice you have coworkers to help you. And they seem to be well
enough informed to realize there is a bit of fiddling required if
you're nested 33 levels deep and want to jump out to the mail program.
But setjmp/longjmp manage this just fine. The basic rigamarole is
setjmp() saves all the CPU registers, including the stack pointer, in
its save area parameter. Then longjmp() mostly just restores those
register values, including the old program counter. Quick and slick
and should probably be used sparingly. But in some cases it's almost
unavoidable.
In more refined languages, like old Pascal, there is no restriction on
goto's, and the compiler generates the right code to do the
inter-procedure jumps. And as a side-benefit, the compiler knows what
files are local to all the intermediate levels, and closes them. Poor
setjmp() doesnt have any way of knowing what files are open, so you'll
have to write your own code to clean up any open files.
A.H. touches on one aspect of a larger problem: longjmp()
simply blasts past all the intermediate call levels back to the
setjmp() caller, without giving the skipped levels any chance to
clean up *anything* -- he mentions open files that don't get
closed, but it's the same problem with malloc()ed memory that
doesn't get free()d, painted windows that don't get erased, and
squitted wadgetts that don't get squatted.
In short, despite the word "long" in the name, longjmp() should
really only be used for "short" jumps within a tightly-coupled set
of functions. The functions between the setjmp() and the longjmp()
must be aware of their precarious position and must be written in a
way that requires no cleanup that longjmp() might cancel. Functions
can be written this way, and written usefully, but it is no small
task to write them *and* defend them against future changes. Hence
the advice: Avoid setjmp()/longjmp() most of the time, and when you
do use them, use them "locally." They are not general-purpose.
--
Eric Sosman es*****@acm-dot-org.invalid
Ancient_Hacker wrote:
>
.... snip ...
>
In more refined languages, like old Pascal, there is no restriction
on goto's, and the compiler generates the right code to do the
inter-procedure jumps. And as a side-benefit, the compiler knows
what files are local to all the intermediate levels, and closes
them. Poor setjmp() doesnt have any way of knowing what files are
open, so you'll have to write your own code to clean up any open
files.
I am very sympathetic with the original C decision to ban goto out
of functions. I did the same in my PascalP, because the end
objective was micro-computers (e.g. 8080) with limited memory. All
functions were packed into segments and consisted of position
independent code. The segments could be freely swapped in and out
of memory, i.e. I had virtual memory for code. The functions were
accessed by a segment jump table, which could hold up to 127 entry
points. Allowing extra-functional jumps would have meant including
rarely used exit points in the table, doubling its size. So I
settled for one terminate system function, the equivalent of a C
exit function.
The result was the ability to run a full one-pass compiler, almost
completely standard compliant, in as little as 5k of object code
space. The disk thrashed a bit by then.
Another thing longjmp can lose is malloced pointers, creating
memory leaks.
--
Some informative links:
<news:news.anno unce.newusers
<http://www.geocities.c om/nnqweb/>
<http://www.catb.org/~esr/faqs/smart-questions.html>
<http://www.caliburn.nl/topposting.html >
<http://www.netmeister. org/news/learn2quote.htm l>
<http://cfaj.freeshell. org/google/>
rover8898 wrote:
Hello all,
I used setjmp() in a recent of program of mine (it is not completed, so
I have not the chance to test it out yet). I am not very profocient in
C coding as are some of my co-workers. They (my co-workers) say (with
vehement ardor ;) ) that the usage of setjmp() emplyoyed in function"C"
that was called from function "B" that was called from function "A"
that was called form the main(), will cause havoc in the stack. And it
makes sense, since the longjmp() function/macro has no way of knowing
how high is the stack is, in order to pop out the excess elements in
order to render it as it was when the setjmp() function first set the
"jmpbuf" environment. However, if that were the case the existence of
the setjmp()/longjmp() function pair will not exist.
So, is setjmp()/longjmp() reliable as a "non-local goto", or will it
eventually cause a stack overflow or some other problem?
Using setjmp() when
you are a few levels down will not cause problems. Part of
the information setjmp() saves is how deep the stack is. All longjmp()
has to do is keep poping elements from the stack until the stack
depth is correct (this may be as simple as resetting the stack
pointer).
On the other hand why are you using setjmp()/longjmp() at all. While I
am told they have their uses, I am also told that they are difficult
to use correctly. I have never used them.
P.S. I am using the setjmp()/longjmp() only for error handling.
Try unwinding the stack yourself. Hijack the return value so
that every function returns SUCCESS of FAILURE. Check the
result of every function call. If the return is FAILURE and
there is something you can do, do it, otherwise clean
up and return FAILURE.
You will soon find yourself writing reams of virtually identical code.
You may ask yourself, "Why isn't the compiler handling this?"
You will learn to appreciate exceptions.
- William Hughes
"Eric Sosman" <es*****@acm-dot-org.invalidwrot e in message
news:1_******** *************** *******@comcast .com...
Ancient_Hacker wrote:
>It's nice you have coworkers to help you. And they seem to be
well
enough informed to realize there is a bit of fiddling required if
you're nested 33 levels deep and want to jump out to the mail
program.
But setjmp/longjmp manage this just fine. The basic rigamarole is
setjmp() saves all the CPU registers, including the stack pointer, in
its save area parameter. Then longjmp() mostly just restores those
register values, including the old program counter. Quick and slick
and should probably be used sparingly. But in some cases it's
almost
unavoidable.
In more refined languages, like old Pascal, there is no restriction
on
goto's, and the compiler generates the right code to do the
inter-procedure jumps. And as a side-benefit, the compiler knows
what
files are local to all the intermediate levels, and closes them.
Poor
setjmp() doesnt have any way of knowing what files are open, so
you'll
have to write your own code to clean up any open files.
A.H. touches on one aspect of a larger problem: longjmp()
simply blasts past all the intermediate call levels back to the
setjmp() caller, without giving the skipped levels any chance to
clean up *anything* -- he mentions open files that don't get
closed, but it's the same problem with malloc()ed memory that
doesn't get free()d, painted windows that don't get erased, and
squitted wadgetts that don't get squatted.
In short, despite the word "long" in the name, longjmp() should
really only be used for "short" jumps within a tightly-coupled set
of functions. The functions between the setjmp() and the longjmp()
must be aware of their precarious position and must be written in a
way that requires no cleanup that longjmp() might cancel.
.... unless, of course, you've called setjmp() just before calling
longjmp(), so that the jumped-to section of code can jump back. This
is, I believe, a standard mechanism for implementing user-mode threads
on platforms that don't natively support them.
Functions can be written this way, and written usefully, but it is no
small task to write them *and* defend them against future changes.
Hence the advice: Avoid setjmp()/longjmp() most of the time, and
when you do use them, use them "locally." They are not general-
purpose.
That sounds like a variation on the rule against using goto. The
problems with set/longjmp() are worse, but in general one should not use
either unless you're savvy enough to explain _why_ the rule doesn't
apply in a particular case and how there's no better (or even possible)
technique. There's virtually always a safer, more elegant way of doing
whatever needs doing.
S
--
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking
--
Posted via a free Usenet account from http://www.teranews.com
Stephen Sprunk wrote:
"Eric Sosman" <es*****@acm-dot-org.invalidwrot e in message
news:1_******** *************** *******@comcast .com...
>[...]
In short, despite the word "long" in the name, longjmp() should
really only be used for "short" jumps within a tightly-coupled set
of functions. The functions between the setjmp() and the longjmp()
must be aware of their precarious position and must be written in a
way that requires no cleanup that longjmp() might cancel.
... unless, of course, you've called setjmp() just before calling
longjmp(), so that the jumped-to section of code can jump back. This
is, I believe, a standard mechanism for implementing user-mode threads
on platforms that don't natively support them.
It might be "standard" in the sense of "widespread ," but it's
not "Standard." 7.12.2.1, paragraph 2, with footnote:
[...] if the function containing the invocation of the
setjmp macro has terminated execution _208) in the interim,
[...] the behavior is undefined.
208) For example, by executing a return statement or because
another longjmp call has caused a transfer to a setjmp
invocation in a function earlier in the set of nested calls.
So the "ping-pong" scheme you describe only works reliably if both
pairs of setjmp()/longjmp() are in the same function -- and if they
are in the same function, why not use goto?
--
Eric Sosman es*****@acm-dot-org.invalid
In article <45************ ***********@fre e.teranews.com> ,
Stephen Sprunk <st*****@sprunk .orgwrote:
>This
is, I believe, a standard mechanism for implementing user-mode threads
on platforms that don't natively support them.
s/standard mechanism/ancient hack/
-- Richard
rover8898 wrote:
Hello all,
I used setjmp() in a recent of program of mine (it is not completed, so
I have not the chance to test it out yet). I am not very profocient in
C coding as are some of my co-workers. They (my co-workers) say (with
vehement ardor ;) ) that the usage of setjmp() emplyoyed in function"C"
that was called from function "B" that was called from function "A"
that was called form the main(), will cause havoc in the stack. And it
makes sense, since the longjmp() function/macro has no way of knowing
how high is the stack is, in order to pop out the excess elements in
order to render it as it was when the setjmp() function first set the
"jmpbuf" environment. However, if that were the case the existence of
the setjmp()/longjmp() function pair will not exist.
So, is setjmp()/longjmp() reliable as a "non-local goto", or will it
eventually cause a stack overflow or some other problem?
P.S. I am using the setjmp()/longjmp() only for error handling.
You may have a look at: http://cern.ch/laurent.deniau/html/e...exception.html http://cern.ch/laurent.deniau/html/o...tml#Exceptions
I haven't really used these codes in my projects, it was more a didactic
approach of the concept. But I am using the same techniques with success
in a much more advanced OO framework.
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