Just out of curiosity this seems to work for me:
If I change COUNT_TO to 53 then the prog quits before getting input. Both should be errors, so why dosent my compiler complain about it?Code:#include <stdio.h> #define COUNT_TO 52 int main() { int i, a[10]; for(i=1; i<COUNT_TO; i++) { a[i]=i; printf("%i\n", a[i]); } getchar(); }
Accessing any values you haven't allocated storage for is undefined behavior. This means anything can happen, including appearing to work normally.
Edit: In general the compiler can't determine whether indexes will be valid at runtime, so it isn't trying, even though in your case it's possible to determine that.
Last edited by robatino; 07-13-2007 at 05:23 PM.
Oh, I guess the number it could count to before crashing then would be dependant on whats used where in the memory. Thanks.
Depending on what operating system you are using, you can get software which will detect that sort of thing (buffer overruns). For example, I think Valgrind would do that (for Linux only).
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I don't think valgrind will be able to detect that according to wikipedia
In addition to the performance penalty an important limitation of valgrind is its inability to detect errors in stack usage, even trivial ones like the below access of an array outside its bounds. Please note the below code example does not apply/work if compiler optimisation is enabled.
Code:int main(void) { /* Declare and define a variable to always be 0 */ const int retval = 0; /* Declare an array with one element */ int one[1]; one[1] = 1; /* Write outside its bounds */ /* The return value has been changed from 0 to 1 */ return retval; }
BoundsChecker will catch this kind of errors...
PCLint - probably will if the array declaration and out-of bounds access is in the same function...
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Mmm, I thought that Valgrind might only work for dynamically allocated memory. It would be able to detect
But not for automatic variables.Code:int *x = malloc(sizeof(*x)); x[1] = 1;
I think you can use some functions built in to MSVC to detect buffer overruns. I forget the function name -- do a search for it. I don't think Dev-C++ or any other compilers support this particular function.
dwk
Seek and ye shall find. quaere et invenies.
"Simplicity does not precede complexity, but follows it." -- Alan Perlis
"Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
"The only real mistake is the one from which we learn nothing." -- John Powell
Other boards: DaniWeb, TPS
Unofficial Wiki FAQ: cpwiki.sf.net
My website: http://dwks.theprogrammingsite.com/
Projects: codeform, xuni, atlantis, nort, etc.
At work we turn on page-heap allocation and get a crash dump file at the exact line the overrun occurred. This only works for heap allocations though, not stack. Works nicely when using the debugger too, of course.
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