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Full Discussion: fgets problems
Top Forums Programming fgets problems Post 302397063 by CRGreathouse on Saturday 20th of February 2010 04:33:59 PM
Old 02-20-2010
Quote:
Originally Posted by Corona688
Of all the faith-based initiatives Bush funded, faith-based software maintenance has to be the worst. Smilie Bottom line is, you don't know why it crashed, don't know why it stopped crashing, and don't know if the problem is truly solved yet or not. All we know is that something trashed some memory somewhere somehow. Time for a memory debugger.
I'm just saying that in these functions, the only memory I manage myself is in a single array, char line[]. All other memory management is done by the library. And since library-related memory troubles will be problems *within* the Pari stack rather than overflowing the Pari stack*, the particular tool recommended here seem powerless to help me.

What I would need is something that could help me debug memory use within the Pari stack.

* I recognize that you haven't seen the library code, so this you're going to have to trust... or not.

Quote:
Originally Posted by Corona688
All this odd stack rearranging makes me more and more suspicious of memory trashing, not less!
Agreed. It's error-prone, to be sure. The biggest problem (in my experience with this library) is leaving garbage behind, but I have had occasional problems of corrupted data -- say, moving the pointer too far so that some members of a live structure can be overwritten later.

Quote:
Originally Posted by Corona688
When it works perfectly it works perfectly, but one mistake and you're mangling your own stack frame. It could also be a disguised array-bounds problem, these macros are using hidden stack variables and writing beyond them could smash your stack too.
The testing behind the library code is quite solid; I'm not concerned about it going past the end of the stack or overwriting things in the 'valid data' portion. My code, on the other hand, is comparatively suspect: I could move the pointer to the wrong place, so that the (correct) library code would overwrite my data.

There are, as you suggest, several stack variables, but the only one that I use regularly is avma, the "available memory area" pointer.
 

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LEARN ABOUT OPENSOLARIS

sigstack

sigstack(3UCB)					     SunOS/BSD Compatibility Library Functions					    sigstack(3UCB)

NAME

       sigstack - set and/or get signal stack context

SYNOPSIS

       /usr/ucb/cc [ flag ... ] file ...
       #include <signal.h>

       int sigstack(nss, oss)
       struct sigstack *nss, *oss;

DESCRIPTION

       The sigstack() function allows users to define an alternate stack, called the "signal stack", on which signals are to be processed.  When a
       signal's action indicates its handler should execute on the signal stack (specified with a sigvec(3UCB) call), the system checks to see	if
       the  process  is  currently  executing on that stack.  If the process is not currently executing on the signal stack, the system arranges a
       switch to the signal stack for the duration of the signal handler's execution.

       A signal stack is specified by a sigstack() structure, which includes the following members:

	 char  *ss_sp;	      /* signal stack pointer */
	 int   ss_onstack;    /* current status */

       The ss_sp member is the initial value to be assigned to the stack pointer when the system switches the process to the  signal  stack.  Note
       that,  on  machines  where  the	stack  grows  downwards in memory, this is not the address of the beginning of the signal stack area.  The
       ss_onstack member is zero or non-zero depending on whether the process is currently executing on the signal stack or not.

       If nss is not a null pointer, sigstack() sets the signal stack state to the value in the sigstack() structure pointed to by nss. If nss	is
       a  null	pointer,  the signal stack state will be unchanged.  If oss is not a null pointer, the current signal stack state is stored in the
       sigstack() structure pointed to by oss.

RETURN VALUES

       Upon successful completion, 0 is returned.  Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

       The sigstack() function will fail and the signal stack context will remain unchanged if one of the following occurs.

       EFAULT	 Either nss or oss points to memory that is not a valid part of the process address space.

SEE ALSO

       cc(1B), sigaltstack(2), sigvec(3UCB), signal(3C)

WARNINGS

       Signal stacks are not "grown" automatically, as is done for the normal stack.  If the stack overflows unpredictable results may occur.

NOTES

       Use of these interfaces should be restricted to only applications written on BSD platforms.  Use of these interfaces with any of the system
       libraries or in multi-threaded applications is unsupported.

SunOS 5.11							    30 Oct 2007 						    sigstack(3UCB)
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