BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Access violation when allocating memory in windows          BorlandTalk.com Forum Index -> comp.lang.pascal.delphi.misc View previous topic :: View next topic   Author Message Guest Posted: Wed Feb 04, 2004 2:14 am    Post subject: Access violation when allocating memory in windows Hi, I am trying to allocate memory using New, Allocmem or GetMem. Once in while the function causes access violation message in debugger. If program runs outside delphi the application closes with no error or warning. When this problem happens it is repeatable and even rebooting the system won't help. Unfotunatelly, the try...except block won't catch it. The program simply vanishes! In debug mode where I can see the access violation message the CPU window shows instruction: 0040D24D push ebp I tried on different machines to make sure there is no hardware problem. The problem shows up on other machines as well. I just pass a pointer to a record of size 512byte to New function. Any idea to resolve this will be greatly appreciated. Thanks, RD. Back to top Dragon Lord Guest Posted: Wed Feb 04, 2004 2:29 am    Post subject: Re: Access violation when allocating memory in windows show the code Jeremy <insc199 (AT) sbcglobal (DOT) net> wrote Quote: Hi, I am trying to allocate memory using New, Allocmem or GetMem. Once in while the function causes access violation message in debugger. If program runs outside delphi the application closes with no error or warning. When this problem happens it is repeatable and even rebooting the system won't help. Unfotunatelly, the try...except block won't catch it. The program simply vanishes! In debug mode where I can see the access violation message the CPU window shows instruction: 0040D24D push ebp I tried on different machines to make sure there is no hardware problem. The problem shows up on other machines as well. I just pass a pointer to a record of size 512byte to New function. Any idea to resolve this will be greatly appreciated. Thanks, RD. Back to top Michael Brown Guest Posted: Wed Feb 04, 2004 7:44 am    Post subject: Re: Access violation when allocating memory in windows [email]insc199 (AT) sbcglobal (DOT) net[/email] wrote: Quote: Hi, I am trying to allocate memory using New, Allocmem or GetMem. Once in while the function causes access violation message in debugger. If program runs outside delphi the application closes with no error or warning. This is exactly what happens if you overflow an allocated block (eg: allocate a 100 byte buffer, then write 101 bytes into it), also known as heap corruption. They're a pain to track down as the AV usually occurs in a completely different area to where the problem is. Quote: When this problem happens it is repeatable and even rebooting the system won't help. Unfotunatelly, the try...except block won't catch it. It probably won't be catchable because the heap will be in an invalid state, and when it tries to allocate memory for the string it will fail again, causing another exception, and so on until it dies. Quote: The program simply vanishes! In debug mode where I can see the access violation message the CPU window shows instruction: 0040D24D push ebp The Delphi debugger is not good at catching this sort of thing. The exception is probably happening somewhere else altogether. You'll need to download a real assembly debugger (I recommend OllyDBG) to see where the exception is actually being thrown. Of course, you won't have the source code view, but it can help narrow things down a bit with a bit of skill. The best way to track down this sort of thing is to figure out how to repeat the AV, and then look through each procedure what's invlolved in that process and make sure you're not overflowing anything. Good luck :) [...] -- Michael Brown www.emboss.co.nz : OOS/RSI software and more Add michael@ to emboss.co.nz - My inbox is always open Back to top Ryan Guest Posted: Wed Feb 04, 2004 8:35 am    Post subject: Re: Access violation when allocating memory in windows Are you freeing the memory you allocated once you finished with it ? If not, try using FreeMem(yourBuffer) at the end of your procedure. "Dragon Lord" <invalid (AT) joke (DOT) com> wrote Quote: show the code Jeremy [email]insc199 (AT) sbcglobal (DOT) net[/email]> wrote in message news:8AYTb.19878$HA.16510 (AT) newssvr27 (DOT) news.prodigy.com... Hi, I am trying to allocate memory using New, Allocmem or GetMem. Once in while the function causes access violation message in debugger. If program runs outside delphi the application closes with no error or warning. When this problem happens it is repeatable and even rebooting the system won't help. Unfotunatelly, the try...except block won't catch it. The program simply vanishes! In debug mode where I can see the access violation message the CPU window shows instruction: 0040D24D push ebp I tried on different machines to make sure there is no hardware problem. The problem shows up on other machines as well. I just pass a pointer to a record of size 512byte to New function. Any idea to resolve this will be greatly appreciated. Thanks, RD. Back to top Bruce Roberts Guest Posted: Wed Feb 04, 2004 5:34 pm    Post subject: Re: Access violation when allocating memory in windows "Michael Brown" <see (AT) signature (DOT) below> wrote Quote: code view, but it can help narrow things down a bit with a bit of skill. The best way to track down this sort of thing is to figure out how to repeat the AV, and then look through each procedure what's invlolved in that process and make sure you're not overflowing anything. Good luck Ideally one should make use of Delphi's strong typing and rich type definition features so that these sorts of errors are not even possible. IOW try, when ever possible, to avoid use of direct memory allocation (AllocMem, etc.). Back to top Dragon Lord Guest Posted: Wed Feb 04, 2004 6:50 pm    Post subject: Re: Access violation when allocating memory in windows Yes, in fact, i've never really found a need to call the raw memory allocators in delphi or the windows api. The furthest I get is to, New and Dispose of Pointer typed records, and even then its rare. Jeremy ----- Original Message ----- From: "Bruce Roberts" <ber (AT) bounceitattcanada (DOT) xnet> Newsgroups: comp.lang.pascal.delphi.misc Sent: Wednesday, February 04, 2004 12:34 PM Subject: Re: Access violation when allocating memory in windows Quote: "Michael Brown" <see (AT) signature (DOT) below> wrote in message news:H82Ub.33676$9k7.716334 (AT) news (DOT) xtra.co.nz... code view, but it can help narrow things down a bit with a bit of skill. The best way to track down this sort of thing is to figure out how to repeat the AV, and then look through each procedure what's invlolved in that process and make sure you're not overflowing anything. Good luck :) Ideally one should make use of Delphi's strong typing and rich type definition features so that these sorts of errors are not even possible. IOW try, when ever possible, to avoid use of direct memory allocation (AllocMem, etc.). Back to top Guest Posted: Wed Feb 04, 2004 7:13 pm    Post subject: Re: Access violation when allocating memory in windows Thanks for the pointer. You are correct. The problem was a few lines before. I was over writing the stack by four bytes! Appreciate the help. "Michael Brown" <see (AT) signature (DOT) below> wrote Quote: insc199 (AT) sbcglobal (DOT) net wrote: Hi, I am trying to allocate memory using New, Allocmem or GetMem. Once in while the function causes access violation message in debugger. If program runs outside delphi the application closes with no error or warning. This is exactly what happens if you overflow an allocated block (eg: allocate a 100 byte buffer, then write 101 bytes into it), also known as heap corruption. They're a pain to track down as the AV usually occurs in a completely different area to where the problem is. When this problem happens it is repeatable and even rebooting the system won't help. Unfotunatelly, the try...except block won't catch it. It probably won't be catchable because the heap will be in an invalid state, and when it tries to allocate memory for the string it will fail again, causing another exception, and so on until it dies. The program simply vanishes! In debug mode where I can see the access violation message the CPU window shows instruction: 0040D24D push ebp The Delphi debugger is not good at catching this sort of thing. The exception is probably happening somewhere else altogether. You'll need to download a real assembly debugger (I recommend OllyDBG) to see where the exception is actually being thrown. Of course, you won't have the source code view, but it can help narrow things down a bit with a bit of skill. The best way to track down this sort of thing is to figure out how to repeat the AV, and then look through each procedure what's invlolved in that process and make sure you're not overflowing anything. Good luck :) [...] -- Michael Brown www.emboss.co.nz : OOS/RSI software and more Add michael@ to emboss.co.nz - My inbox is always open Back to top Dragon Lord Guest Posted: Wed Feb 04, 2004 7:44 pm    Post subject: Re: Access violation when allocating memory in windows "Michael Brown" <see (AT) signature (DOT) below> wrote Quote: insc199 (AT) sbcglobal (DOT) net wrote: Hi, I am trying to allocate memory using New, Allocmem or GetMem. Once in while the function causes access violation message in debugger. If program runs outside delphi the application closes with no error or warning. This is exactly what happens if you overflow an allocated block (eg: allocate a 100 byte buffer, then write 101 bytes into it), also known as heap corruption. They're a pain to track down as the AV usually occurs in a completely different area to where the problem is. When this problem happens it is repeatable and even rebooting the system won't help. Unfotunatelly, the try...except block won't catch it. It probably won't be catchable because the heap will be in an invalid state, and when it tries to allocate memory for the string it will fail again, causing another exception, and so on until it dies. The program simply vanishes! In debug mode where I can see the access violation message the CPU window shows instruction: 0040D24D push ebp The Delphi debugger is not good at catching this sort of thing. The exception is probably happening somewhere else altogether. You'll need to download a real assembly debugger (I recommend OllyDBG) to see where the exception is actually being thrown. Of course, you won't have the source code view, but it can help narrow things down a bit with a bit of skill. The best way to track down this sort of thing is to figure out how to repeat the AV, and then look through each procedure what's invlolved in that process and make sure you're not overflowing anything. Good luck :) [...] -- Michael Brown www.emboss.co.nz : OOS/RSI software and more Add michael@ to emboss.co.nz - My inbox is always open I use my delphi source code in Ollydbg all the time. 1) Set project options Under Compiler: Unchecking: Optimization, sometimes helps but can be checked Check All Debugging options, exception Definitions only Check "Use Debug DCUs" if you want debugging symbols for the system units, if you want source for some of the units, copy over the unit you want source to, into your project directory Under Linker: This is the most important. Check include TD32 debug Info. 2) Compile your application, and make sure the exe is in your projects directory 3) start up ollydbg, then start your application through it, to view your source code in olly, just goto view->source or view->source files. You can set breakpoints in your source code, or follow the source when you step through the assembly. I for one, like the ollydbg debugger interface, more than the one that comes with delphi. If you want it to follow your source code to different source files, i recommend setting in the options under the CPU tab. "Synchronize source with CPU". Also, just like the delphi debugger, it breakpoints on exceptions, which there are some defaults in the delphi debugger to ignore, you may have to include those in the "Debug" Tab. Jeremy Back to top Michael Brown Guest Posted: Wed Feb 04, 2004 11:35 pm    Post subject: Re: Access violation when allocating memory in windows Bruce Roberts wrote: Quote: "Michael Brown" <see (AT) signature (DOT) below> wrote in message news:H82Ub.33676$9k7.716334 (AT) news (DOT) xtra.co.nz... code view, but it can help narrow things down a bit with a bit of skill. The best way to track down this sort of thing is to figure out how to repeat the AV, and then look through each procedure what's invlolved in that process and make sure you're not overflowing anything. Good luck :) Ideally one should make use of Delphi's strong typing and rich type definition features so that these sorts of errors are not even possible. IOW try, when ever possible, to avoid use of direct memory allocation (AllocMem, etc.). It's not so much direct memory allocation that the problem is, but more for arrays. Any time when you have a dynamic array you potentially face this problem. For example, if you do SetLength(AArray, NumEntries); // Some more code in here, or possibly a completely different function for ALoop := 0 to NumEntries do AArray[ALoop] := 1; I do quite a bit of porting between C and Pascal, and more often that I should I forget to subtract 1 off the high bound, causing problems like this. Fortunately, I know I do this, so finding such problems is not too much trouble :) -- Michael Brown www.emboss.co.nz : OOS/RSI software and more Add michael@ to emboss.co.nz - My inbox is always open Back to top Rob Kennedy Guest Posted: Wed Feb 04, 2004 11:55 pm    Post subject: Re: Access violation when allocating memory in windows Michael Brown wrote: Quote: It's not so much direct memory allocation that the problem is, but more for arrays. Any time when you have a dynamic array you potentially face this problem. That's what the range-checking compiler option is for. -- Rob Back to top Bruce Roberts Guest Posted: Fri Feb 06, 2004 5:41 pm    Post subject: Re: Access violation when allocating memory in windows "Rob Kennedy" <me (AT) privacy (DOT) net> wrote Quote: That's what the range-checking compiler option is for. Which I always keep on, even in release code. Another good idea is to use the Low and High functions whenever possible for array bounds. Back to top J French Guest Posted: Sat Feb 07, 2004 8:25 am    Post subject: Re: Access violation when allocating memory in windows On Fri, 6 Feb 2004 12:41:25 -0500, "Bruce Roberts" <ber (AT) bounceitattcanada (DOT) xnet> wrote: Quote: "Rob Kennedy" <me (AT) privacy (DOT) net> wrote in message news:bvs0ps$102i7e$1 (AT) ID-220940 (DOT) news.uni-berlin.de... That's what the range-checking compiler option is for. Which I always keep on, even in release code. Another good idea is to use the Low and High functions whenever possible for array bounds. 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