Memory write error, exported procedure has memory write error if the input buffer is larger than aound 20 or more kilobytes

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AvidStudent

Extremely Active Member

Group: Members
Posts: 234
Member No.: 17238
Joined: 10-January 09

I was writing a find and replace procedure to be exported in a DLL project. The memory write error was caused by an invalid buffer size. I had allocated space for a string buffer without adding some padding to the end as well as space for the zero terminator. This caused a page fault that was reported as a memory write error. When you get this type of error I suggest that you CHECK YOUR BUFFERS FOR PROPER SIZE and read Shoorick's comments below!

On another note, how am I supposed to free a local buffer that must be passed to another program? When the buffer is freed the data no longer exists. Perhaps the program receiving the data should free it?

Here is the updated and working code.

CODE

FindAndReplace proc lpSzFind:LPSTR, lpSzReplace:LPSTR, lpSzText:LPSTR

LOCAL szBuffer:BYTE
LOCAL lpBuffer:DWORD
LOCAL dwBufferSize:DWORD

mov eax, lpSzFind
cmp eax, 0
jz no_handle

mov eax, lpSzReplace
cmp eax, 0
jz no_handle

mov eax, lpSzText
cmp eax, 0
jz no_handle

lea eax, szBuffer
mov lpBuffer, eax

push lpSzText
push lpSzReplace
push lpSzFind
call szRepGetBSize
mov dwBufferSize, eax

mov lpBuffer, alloc(dwBufferSize)

cmp eax, 0
jz no_memory

invoke szRep, lpSzText, lpBuffer, lpSzFind, lpSzReplace

;STDCALL VERSION
mov eax, lpBuffer

;Visual Basic 6 Version using AvidStudent's BSTR.inc
;push lpBuffer
;call BSTR

ret 12

no_handle:

ret 12

no_memory: ;<?> How can we raise an error in this event <?>

ret 12

FindAndReplace endp

;Calling PROC must check for valid handles to increase code performance.
szRepGetBSize proc lpSzFind:DWORD, lpSzReplace:DWORD, lpSzText:DWORD

LOCAL BSIZE:DWORD
LOCAL IWRDS:DWORD

push lpSzText
call StrLen
;This line did not exist and was the source of memory error
add eax, 5 ;4 BYTES Padding + NULL CHAR
mov BSIZE, eax

push lpSzFind
call StrLen
mov ecx, eax

push lpSzFind
push lpSzText
call szWcnt
mov IWRDS, eax

mul ecx

sub BSIZE, eax

push lpSzReplace
call StrLen
mov ecx, eax

mov eax, IWRDS

mul ecx

add BSIZE, eax

mov eax, BSIZE

ret 12

szRepGetBSize endp

If you are going to add this to your project make sure you take note of shoorick's comments below. As well as ragdog's code posting


ragdog
Extremely Active Member Group: Moderators Posts: 873 Member No.: 5019 Joined: 13-May 07	Hi Avid Can your upload the complete source?


shoorick
Extremely Active Member Group: Admins Posts: 2301 Member No.: 160 Joined: 22-June 04	it is necesary to study subject starting from memory allocation methods. i do not know how "alloc()" works exactly, but there are some prefered methods with winapi: HeapAlloc and VirtualAlloc. HeapAlloc will let you allocate any size of memory block on NT systems, but recomended below 4Mb (look msdn better), while VirtualAlloc lets you allocate huge memory areas, but in anyway you will not be able to allocate 2048 Mb with this way. there also one method exists with pages which lets allocation of huge memory blocks, but i even do not remember names of these functions on stack you may easy allocate memory block with fast access, but process usually gets 4Mb stack, and also has other data, so, not of all 4Mb are available to allocate without stack overflow. finally, such operations, like you suggest, better provide via streams: load part, process, save, then load next part - it will be faster, less resource consumping and not too more complex then plain memory addressing.

ragdog

Extremely Active Member

Group: Moderators
Posts: 873
Member No.: 5019
Joined: 13-May 07

For Alloc memory use i this

CODE

Local @lpBuffer:DWORD
Local @hSize :DWORD
invoke lstrLen,addr lpszText
mov @hSize,eax
invoke VirtualAlloc,0,eax,MEM_COMMIT,PAGE_EXECUTE_READWRITE
test eax,eax
jz @Error
mov @lpBuffer, eax
...
..
.

invoke VirtualFree,@lpBuffer,hSize,MEM_RELEASE
@Error:

I´m not sure how fast it is you can make a speed test
For my works is this ok


AvidStudent
Extremely Active Member Group: Members Posts: 234 Member No.: 17238 Joined: 10-January 09	Hey ragdog, I was gonna upload the source but I found out I had a huge debug listing in the files so I deleted it and recompiled everything, then the whole app was broken. I am guessing this had something to do with the errors I was getting. I am rewriting the FindAndReplace proc line by line to see where the error rises. Hopefully I get it taken care of. if I don't I'll post the files here later.

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